Review ArticleMechanical Alloying A Novel Technique to SynthesizeAdvanced Materials

Challapalli Suryanarayana⋆

Department of Mechanical and Aerospace Engineering University of Central Florida Orlando FL 32816-2450 USA

⋆Correspondence should be addressed to Challapalli Suryanarayana surya orlandohotmailcom

Received 7 April 2019 Accepted 4 May 2019 Published 30 May 2019

Copyright copy 2019 Challapalli Suryanarayana Exclusive Licensee Science and Technology Review Publishing House Distributedunder a Creative Commons Attribution License (CC BY 40)

Mechanical alloying is a solid-state powder processing technique that involves repeated cold welding fracturing and reweldingof powder particles in a high-energy ball mill Originally developed about 50 years ago to produce oxide-dispersion-strengthenedNi- and Fe-based superalloys for aerospace and high temperature applications it is now recognized as an important technique tosynthesizemetastable and advancedmaterials with a high potential for widespread applicationsThemetastablematerials producedinclude supersaturated solid solutions intermediate phases quasicrystalline phases amorphous alloys and high-entropy alloysAdditionally nanocrystalline phases have been produced in virtually every alloy system Because of the fineness of the powderstheir consolidation to full density without any porosity being present is a challenging problem Several novel methods have beendeveloped to overcome this issue Powder contamination during milling and subsequent consolidation constitutes another issuethis can be resolved though expensive A number of applications have been developed for these novel materialsThis review articlepresents an overview of the process of mechanical alloying mechanism of grain refinement to nanometer levels and preparationof materials such as nanocomposites and metallic glasses The application of mechanical alloying to synthesize some advancedmaterials such as pure metals and alloys hydrogen storage materials and energy materials is described The article concludes withan outlook on future prospects of this technique

1 Introduction

Advanced materials have been defined as those where firstconsideration is given to the systematic synthesis and controlof the crystal structure and microstructure of materials toprovide precisely tailored properties for demanding applica-tions [1] Even though different materials have been in usefor several millennia with rapid advances taking place inthe technology sectors there has been a great demand formaterials that are stronger stiffer lighter corrosion-resistantand usable at higher temperatures than the existing andavailable materials Thus the high-tech industries have givenan immense fillip to efforts in developing novel materials thatwill perform better for demanding applications In responseto these demands materials scientists have been constantlyon the lookout for novel methods to produce such materials

An ideal and most useful material should have an excel-lent combination of properties It should have a high strengthto withstand heavy loads and good ductility to be easilydeformed into different shapes But traditional wisdom tellsus that these two properties work in opposite directions the

ductility decreases when the strength increases Additionallythe material should have good stiffness so that it does notdeflect too much under load and high fracture toughness toresist easy and quick fracture It will bemost convenient if it ismade up of light metals when it can be used more effectivelyin the aerospace industry Further we also wish to have goodcorrosion resistance Added to this excellent combination ofproperties we would like the material to be easily available(to avoid geopolitical conflicts) and also inexpensive Sucha material does not exist at the moment and therefore letus call that imaginary ideal metal utopium (Figure 1) Thiswill be like utopia the perfect world But since the names ofmost metals end with ldquo umrdquo like aluminum magnesiumtitanium uranium etc let us call that perfect (and elusive)metal utopium

It has long been realized that the material properties ingeneral and more importantly the mechanical propertiescan be improved only by processing them under far-fromequilibrium (or nonequilibrium) conditions [2] Novel mate-rials such as metallic glasses [3 4] quasicrystalline alloys[5 6] nanostructured materials [7 8] high-entropy alloys

AAASResearchVolume 2019 Article ID 4219812 17 pageshttpsdoiorg103413320194219812

2 Research

[9] high-temperature superconductors superhard carbo-nitrides and thin-film diamond materials were unheard ofprior to 1960s Synthesis of martensite by quenching ofsteel to room temperature from an elevated temperaturewhere it exists in the austenitic condition is an acceptedmethod to strengthen steel The mechanical properties canbe further modified through tempering by reheating themartensitic steel to higher temperatures Development ofaircraft aluminum alloys through precipitation hardening inthe 1910sndash1930s laid the foundation for the development ofhigh strength aluminum alloys These principles are nowfurther exploited to increase the strength of usually softaluminum and magnesium alloys Development of rapidsolidification processing methods to achieve high solidifica-tion rates (about 106 Ks) produced fine-grained alloys andmetallic glasses with high mechanical strength attractivesoft magnetic properties and vastly improved corrosionresistance Synthesis of bulk metallic glasses at much slowersolidification rates (ltabout 102 Ks) and with larger sectionthicknesses (presently one can produce 80-mmdiameter rodsof fully glassy alloys) provided a much wider range of glassymaterials The appearance of nanostructured materials andhigh-entropy alloys on the scene has significantly enhancedthe range of materials available to the materials scientistsIn most of these cases the academic curiosity of motivatedresearchers resulted in the synthesis of novel materials someof which have subsequently found commercial applications

Some of these ldquononequilibrium processingrdquo techniquesinclude rapid solidification from the melt mechanical alloy-ing laser processing plasma processing spray formingphysical and chemical vapor deposition techniques and ionmixing The relative advantages and disadvantages of thesetechniques have been discussed in a relatively recent publi-cation [2] However in the present article we will focus onthe technique of mechanical alloying describe the techniqueand its potential in synthesizing and developing advancedmaterials and explore and exploit them for commercialapplications

2 The Process of Mechanical Alloying

Nonequilibrium processing of materials starts with energiz-ing the material and using that excited (high-energy) stateto process the materials to obtain the desired microstructureand properties Researchers have been traditionally usingtemperature pressure light or electricity to transform theequilibrium material into an energized condition But aneasy and unconventional method to energize a material isto use ldquobrute forcerdquo (or mechanical stress) The so-calledsevere plastic deformation processes (note that this name andthe different variants under this category have come aroundmuch later than the technique ofmechanical alloying) subjectthe material to high impact shear and torsional forces toenergize the material

The technique of mechanical alloying (MA) was devel-oped in the 1960s in response to an industrial necessity Facedwith the question of identifying a material that has adequatestrength both at intermediate and elevated temperatures

High Ductility

Low CostHighStrength

HighFracture

Toughness

HighTemperature

CapabilityHigh

Stiffness

HighCorrosionResistance

Light Weight(Low Density)

UTOPIUM

Figure 1 Typical properties expected of an ideal metal for present-day industrial applications include high strength good ductilityexcellent corrosion resistance high fracture toughness and theability to be used at elevated temperatures Since such a metalcurrently does not exist let us call it utopium (not utopia since thenames of most metals end with um eg aluminum magnesiumtitanium uranium)

John Benjamin of INCO developed this method to uniformlydisperse fine yttria (Y

2O3) particles in a complex nickel-

based alloy matrix [10] Such alloys are now being referred toas oxide-dispersion strengthened (ODS) superalloys [11ndash14]The ldquosciencerdquo of the MA technique was developed much laterin the late 1980s and is continuing to be expanded further

MA involves loading of the individual elemental powdersor prealloyed powders along with the grinding mediumin a high-energy ball mill typically maintaining a ball-to-powder weight ratio of 101 or higher The process involvesrepeated cold welding fracturing and rewelding of powderparticles During this process the resulting powder sizecan be controlled by balancing the fracturing and weldingevents About 1-2 wt of a process control agent (PCA)is used especially when ductile metals are fabricated ThePCA is adsorbed on the surfaces of the powder particles andminimizes excessive cold welding among themselves andorto the milling container and the grinding medium thisinhibits agglomeration of powder particles The particle andgrain sizes decrease progressively with milling time reachingnanometer levels

The process of MA can be carried out in small capacityand high-energy mills (SPEX mills) to produce about 10ndash20g per run for alloy screening purposes in medium-energyplanetary mills to produce about 200ndash500 g at a time orin low-energy much larger mills (attritors) to produce kgquantities Currently attritors capable of producing about40ndash50 kg of powder in 24ndash48 hours are available in Russia forlarge-scale commercial applications High-energy and high-speed mills that can produce about 5000 kg of powder perhour are also available In recent years cryogenic milling(milling at liquid nitrogen temperatures) has become apopular method [15] Facilities to cool the powders to lowtemperatures or heat them to high temperatures andmonitorthe pressure and temperature during milling are some of theattachments that are currently available

Research 3

Metal A

Metal B

Intermetallic

Dispersoid

20 m 5 m 20 m 5 m

Typical Starting Powders

Ball-Powder-Ball Collision

After Single Collision

(a)

(b)

Figure 2 (a) Deformation characteristics of representative constituents of starting powders in mechanical alloying Note that the ductilemetal powders (metals A and B) get flattened while the brittle intermetallic and dispersoid particles get fragmented into smaller particles(b) Ball-powder-ball collision of powder mixture during mechanical alloying

Themilling time decreases with an increase in the energyof the mill As a rule of thumb it can be estimated that aprocess that takes only a few minutes in the SPEX mill maytake hours in an attritor and a few days in a commercial low-energy mill even though the actual details can be differentdepending on the efficiency of the different mills and thepowder characteristics Full details of the process includingthe effect of different variables on the alloying behavior maybe found in two recent references [16 17]

21 Mechanism of Alloying The effects of a single collisionon each type of constituent powder particle are shown inFigure 2(a) The initial impact of the grinding ball causes theductile metal powders to flatten and work hardenThe severeplastic deformation increases the surface-to-volume ratio

of the particles and ruptures the surface films of adsorbedcontaminants The brittle intermetallic powder particles getfractured and are refined in size The oxide dispersoidparticles are comminuted more severely

Whenever two grinding balls collide a small amountof the powder being milled is trapped in between themTypically around 1000 particles with an aggregate weight ofabout 02 mg are trapped during each collision (Figure 2(b))During this process the powdermorphology can bemodifiedin two different ways depending on whether one is dealingwith ductilendashductile ductilendashbrittle or brittlendashbrittle powdercombinations If the starting powders are softmetal particlesthe flattened layers overlap and form cold weldsThis leads toformation of layered composite powder particles consistingof various combinations of the starting ingredients The

4 Research

more brittle constituents tend to become occluded by theductile constituents and trapped in the compositeThe work-hardened elemental or composite powder particles may frac-ture at the same timeThese competing events of coldwelding(with plastic deformation and agglomeration) and fracturing(with size reduction) continue repeatedly throughout themilling period Eventually at the steady-state condition arefined and homogenized microstructure is obtained andthe composition of the powder particles is the same as theproportion of the starting constituent powders

Along with the cold welding event described above somepowder may also coat the grinding medium andor the innerwalls of the container A thin layer of the coating is beneficialin preventing wear and tear of the grinding medium and alsoin preventing contamination of the milled powder with thedebris But a too thick layer will result in lower yield and alsopossible compositional inhomogeneity of the powder whichshould be avoided

The generally accepted explanation for alloying to occurfrom blended elemental powders and formation of differenttypes of phases is that a very fine and intimate mixture ofthe components (often lamellar if the constituent elementsare sufficiently ductile) is formed after milling if not thefinal product Alloy formation is facilitated through crys-talline defects (grain boundaries dislocations stacking faultsvacancies and others) introduced into thematerial due to theintense cold working operation which act as fast diffusionpaths and a slight rise in the powder temperature duringmilling as a result of frictional forces and impact of thegrinding balls against other balls and the surfaces of thecontainer If the final desired phase had not been formeddirectly by MA then a short annealing treatment at anelevated temperature was found to promote diffusion andconsequently alloy formation Accordingly it was shown thatby a proper choice of the process parameters during MA andby choosing an appropriate alloy composition it is possibleto produce a variety of alloys starting from metal powdersThe thermodynamic criterion that the phase with the lowestfree energy under the continuous deformation conditionswould be the most ldquostablerdquo phase has been found to be validA significant attribute of MA has been the ability to alloymetals even with positive heats of mixing that are difficult orimpossible to alloy otherwise [18 19]

The effects of MA are generally categorized under twogroups(1) Constitutional Changes Milling of the blended ele-

mental powders could lead to the formation of solid solu-tions (both equilibrium and supersaturated) intermetallicphases (equilibrium metastable (high temperature andorhigh pressure) and quasicrystalline) and amorphous phasesin alloy systems The actual type of phase formed could bedifferent depending on the alloy system its composition andthe milling conditions employed A number of metastablealloys have been produced this way(2) Microstructural Changes As a result of MA the

processed materials could develop ultrafine-grained andnanostructured phases In addition uniform dispersion ofa large volume fraction of very fine oxide or other ceramicparticles can be achieved in different matrices which is

either impossible or difficult by other processes These novelnanocomposites can exhibit properties far superior to thosethat have conventional grain sizes and sometimes evencompletely new behavior [16 17 20]

In many cases chemical reactions have also been foundto occur leading to the production of pure metals from theirores synthesis of alloys and formation of novel materialsand microstructures Such a process has been referred to asmechanochemical processing [21] All these effects have beenwell documented in the literature andTable 1 summarizes theimportant attributes of MA

Before discussing the synthesis of advanced materialsand their applications let us first describe the constitutionalchanges that can take place in MA materials

3 Constitutional Changes

The literature on MA has several examples of the differentequilibrium and nonequilibrium phases produced in differ-ent alloy systems An exhaustive list of the results (up to dateat the time of compilation) is available in [16 17] and the listis constantly growingWewill however briefly highlight heresome of the more recent and salient features in the followingparagraphs

31 Solid Solutions Solid solution alloys exhibit highstrength changes in density and suppression or eliminationof undesirable second phases Increasing the strength ofalloys through precipitation of a second phase from asupersaturated solid solution during aging (precipitationhardening) has been known to materials scientists for overa hundred years Further since the strengthening effect isrelated to the size and volume fraction of the second phaseparticles and since both of them could be controlled bythe extent of supersaturation and the aging parametersachievement of extended solid solubility limits in alloysystems is very desirable Solid solutionsmdashboth equilibriumand supersaturatedmdashare still being produced [22ndash24]

Rules similar to those developed by Hume-Rothery forsolid solution formation in binary noble-metal alloy systemsunder equilibrium conditions also seem to be generallyapplicable to supersaturated solutions obtained by MA eventhough exceptions have been noted in some cases [25 26]

As will be described later the mechanically alloyed mate-rials exhibit very fine grain sizes often reaching nanometerlevels Since these nanocrystalline materials contain a largevolume fraction of atoms in the grain boundaries thediffusivity of solute atoms is significantly enhanced [27] andthis would lead to increases in the solid solubility levelsAs a general rule of thumb solute elements which exhibitlimited solubility under equilibrium conditions exhibit highsolubility levels under the nonequilibrium conditions of MAEven though solid solubility extensions have been achievedby other nonequilibrium processing techniques as well (egrapid solidification processing vapor deposition and laserprocessing of materials) there appear to be differences in thesolid solubility extensions achieved by MA and other meth-ods [26] An important observation made in MA materials

Research 5

Table 1 Attributes of mechanical alloying

(1) Production of fine dispersion of second phase (usually oxide) particles(2) Extension of solid solubility limits(3) Refinement of grain sizes down to nanometer range(4) Synthesis of novel crystalline and quasicrystalline phases(5) Development of amorphous (glassy) phases(6) Disordering of ordered intermetallics(7) Possibility of alloying of difficult to alloy elementsmetals

(8)Inducement of chemical (displacement) reactions at low temperatures for (a) Mineral and Wasteprocessing (b) Metals refining (c) Combustion reactions and (d) Production of discrete ultrafineparticles

(9) Scaleable process

is that the solute atoms segregate to grain boundaries fromwhere they could diffuse into grains and form solid solutions

32 Intermetallic Phases Intermetallic phases (or inter-metallics for short) are typically ordered compounds existingat simple ratios of the two ormore elements involved Becauseof their ordered nature they are strong and hard stiffhave high melting points and also exhibit other interestingbehaviors such as excellent corrosionoxidation resistanceHowever they usually suffer from low ambient temperatureductility again due to the ordered nature of the compoundsA new family of intermetallics showing high ductility atroom temperature and based on rare-earth elements hasbeen recently developed [28 29] Since MA is known to (a)disorder the ordered lattices (b) refine grain sizes down tonanometer levels and (c) also alter the crystal structures allthese effects could induce some amount of plasticity into thematerial In fact the potential to ductilize the normally brittleintermetallics has been the driver for accelerated researchinto the MA of intermetallics

A number of intermetallics (both equilibrium andmetastable) and high-temperature or high-pressure phaseshave been synthesized byMAstarting fromelemental powderblends [16 17 24 30] More interestingly quasicrystallinephases with the forbidden crystal symmetries first reportedin rapidly solidified Al-Mn alloys have also been synthesizedby MA in a number of alloy systems [5 31] Applicationsfor these exotic materials have been limited Based ontheir high hardness low friction and low surface reactivityquasicrystalline materials have been used to coat nonstickfrying pans Also steels reinforcedwith small quasicrystallineparticles are very strong and have been used for acupunctureand surgery dental instruments and razor blades [32]

The most significant change that occurs in MA materialsis the formation of amorphous phases and this will bediscussed in the next section

4 Metallic Glasses

Amorphous alloys are materials that lack crystallinity andhave a random arrangement of atoms Because of thisextreme disorder these materials exhibit a very interestingcombination of high strength good corrosion resistance

and useful electrical and magnetic properties These novelmaterials have been traditionally produced starting from thevapor or liquid states Amorphous or noncrystalline alloysbased onmetals are commonly referred to as metallic glassesAt present there is worldwide activity in the area of metallicglasses and bulk metallic glasses [4 33]

Metallic glasses were first produced by rapid solidificationprocessing (RSP) in 1960 in the form of thin ribbons bysolidifying the alloy melt at rates gt106 Ks [34] Subsequentdevelopments include the synthesis of bulk metallic glasseswith a large section thickness of a few centimeters This ispossible in multicomponent alloys which could be solidifiedinto the glassy condition at low critical cooling rates of lt102Ks [4] The current record is an 80-mm diameter glassy rodproduced by solidification in a Pd

425Cu30Ni75P20

alloy (thesubscripts represent the atomic percentage of the elementsin the alloy) [35] While solidification methods produce themetallic glasses either in thin ribbon or bulk form directlyfrom the liquid the amorphous phases are in powder formbyMAmethodsThese need to be consolidated to bulk shapefor subsequent characterization and applications

Amorphous alloys were synthesized by MA first in anY-Co intermetallic compound [36] and later in a blendedelemental Ni-Nb powder mixture [37] Amorphous phaseshave now been synthesized in a very large number ofalloy systems starting from either blended elemental powdermixtures intermetallic compounds or mixtures of them[38] Comparisons have been frequently made between theamorphous alloys produced by MA and RSP techniquesand both similarities and differences have been noted [26]Further the mechanism of glass formation appears to besignificantly different depending uponwhether it is producedby MA or RSP methods

Several different criteria have been proposed to explainthe glass-forming ability of alloys produced by solidificationmethodsThese include the reduced glass transition tempera-ture119879rg =119879g119879l where119879g and119879l represent the glass transitionand liquidus temperature of the alloy respectively presenceof deep eutectics in the phase diagrams and significantdifferences in the atomic sizes of the constituent elementsleading to development of strain energy in the solid solutionWhen the strain energy exceeds a critical value determinedby the size and amount of the solute atoms it destabilizes the

6 Research

crystal lattice and results in the formation of an amorphousphase With the introduction of bulk metallic glasses nearlythirty years ago a very large number of new criteria mostlybased on the glass transformation temperatures (119879g 119879x and119879l where 119879x represents the crystallization temperature of theamorphous alloy) have been proposed In spite of a very largenumber of criteria proposed [4] it has not been possible yetto accurately predict the glass-forming ability of alloys [39]

There are some significant differences in the formation ofmetallic glasses by the MA and RSP methods For examplemetallic glasses are obtained by MA in almost every alloysystem provided that sufficient energy has been stored in thepowder But metallic glasses are obtained by RSP methodsonly in certain composition rangesmdashabout 20 at metalloidcontent in metal-metalloid systems and different composi-tions in metal-metal systems mostly at compositions cor-responding to deep eutectics Additionally glass formationby MA occurs by the accumulation of crystal defects whichraises the free energy of the crystalline phase above that of thehypothetical glassy phase under which conditions the crys-talline phase becomes destabilized On the other hand forglass formation by the RSP methods the critical cooling ratefor glass formation needs to be exceeded so that formationof the crystalline nuclei is completely suppressed Since MAis a completely solid-state powder processing technique thecriteria for amorphous phase formation should be differentfrom those of solidification methods A new criterion wasproposed that is the number of intermetallics present inthe constituent phase diagrams determines the glass-formingability of alloys processed by MAmdashthe more the number ofintermetallics in the constituent phase diagrams the easier itis to amorphize the powder blend [40]

Amorphization by MA occurs when the free energy ofthe hypothetical amorphous phase 119866A is lower than that ofthe crystalline phase G119862 ie 119866A lt 119866C A crystalline phasenormally has a lower free energy than the amorphous phaseBut its free energy can be increased by introducing a varietyof crystal defects such as dislocations grain boundaries andstacking faults If an intermetallic has formed then additionalenergy can be introduced by disordering the crystal lattice Bythis approach it is then possible to obtain a situation when

119866A lt 119866C + 119866D (1)

where 119866D is the free energy increase due to introductionof crystal defects It has been shown that the presence ofintermetallics in an alloy system can significantly increasethe energy contributing to amorphization This is due totwo important effects Firstly disordering of intermetallicscontributes an energy of about 15 kJmol to the systemSecondly a slight change in the stoichiometry of the inter-metallic increases the free energy of the system drasticallyAdditionally grain size reduction contributes about 5 kJmolFurther disordering of intermetallics has also been shownto be possible by heavy deformation Since MA reduces thegrain size to nanometer levels and also disorders the usuallyordered intermetallics the energy of the milled powders issignificantly raised In fact it is raised to a level above that ofthe hypothetical amorphous phase and thus formation of theamorphous phase is favored over the crystalline phase

41 Mechanical Crystallization It was reported that an amor-phous phase formed by milling transforms into a crystallinephase on continued milling and that this phenomenonhas been referred to as mechanical crystallization [41 42]The process of mechanical crystallization is very unusualand offers many opportunities to design novel alloys withimproved properties For example it will be possible toproduce the alloy in either a fully amorphous amorphous +nanocrystalline composite or a completely crystalline statewith different grain sizes depending upon the time ofmillingor the temperature and time of subsequent annealing of theamorphous alloy

Figure 3(a) shows the XRD patterns of the blended ele-mental powder mixture of the composition Fe42Ge28Zr10B20mechanically alloyed for 10 and 60 h [42] While the pat-tern at 10 h of milling clearly shows the presence of anamorphous phase the pattern at 60 h of milling showsthe presence of a crystalline phase superimposed over thebroad and diffuse halo representing the amorphous phaseThe crystalline phase has been identified as the 120572-Fe solidsolution containing the solute elements present in the blendThis 120572-Fe phase is the first phase to crystallize from theamorphous alloy (as in primary crystallization) and externalannealing of the milled powder showed that the amount ofthe 120572-Fe increased along with the presence of other inter-metallics Eutectic crystallization (simultaneous formation oftwo crystalline phases) has also been observed in other alloysystems

Mechanical crystallization has also been observed inseveral other alloy systems that formed the amorphous phaseon MA In fact it is now becoming apparent that thisphenomenon of mechanical crystallization is not as rare asit was once thought to be

Many reasons have been suggested for the formation of acrystalline phase after the formation of an amorphous phaseduring MA These include (a) rise in temperature to a levelabove that of the crystallization temperature of the amor-phous alloy (b) powder contamination due to which a stablecrystalline impurity phase forms (c) phenomenon of inversemelting and (d) basic thermodynamic considerations It nowappears clear that the basic thermodynamic stabilities of thedifferent phases under the different conditions of milling areresponsible for mechanical crystallization Such a situationcan be explained with reference to a free energy versuscomposition diagram

Figure 3(b) is a schematic diagram showing the variationof free energy with composition for the Fe42X28Zr10B20 (X =AlGe andNi) alloy systems investigatedThepossible (stableand metastable) constitution in this system is (a) blendedelemental (BE) powder mixture (b) 120572-Fe the solid solutionof all the alloying elements in Fe (c) an amorphous phase (d)intermetallic phases and (e) different combinations of thesephases The stability of any phase will be determined by itsrelative position in the free energy vs composition plotmdashthelower the free energy the more stable the phase is Since it ispossible to have a large number of intermetallic phases in thismulticomponent system and since it is difficult to indicateeach of them separately for simplicity all of them are groupedtogether as ldquointermetallicsrdquo

Research 7Fr

ee E

nerg

y

Solute Concentration (at)20 40 60 80Fe 100

BE powder

amorphous

-Fe

Intermetallics

1

23

45

Inte

nsity

30 40 502-Theta

(a)

(b)

10 h

60 h

60 70 80 90

1

Figure 3 (a) X-ray diffraction patterns of the Fe42Ge28Zr10B20

powder blend milled for 10 and 60 h While the powder milledfor 10 h shows the presence of an amorphous phase the powdermilled for 60 h shows the presence of a crystalline phase coexistingwith the amorphous phase suggesting that the amorphous phase hascrystallized (b) Hypothetical free energy vs composition diagramto explain the mechanism of mechanical crystallization in theFe42X28Zr10B20systemNote that point ldquo1rdquo represents the free energy

of the blended elemental powders Similarly point ldquo2rdquo representsformation of the 120572-Fe solid solution containing all the alloyingelements in Fe point ldquo3rdquo formation of the homogeneous amorphousphase point ldquo4rdquo a mixture of the amorphous phase with a differentsolute content (amorphous) from that at ldquo3rdquo and the solid solution120572-Fe with different solute content and point ldquo5rdquo the equilibriumsituation when the solid solution and intermetallic phases coexist

The free energy of the BE powder mixture is indicated bypoint ldquo1rdquo which represents the point of intersection of thecomposition vertical with the line joining the free energiesof pure Fe and the ldquosoluterdquo On milling this powder a solidsolution phase containing all the solute elements in Fe (ora mixture of intermetallics and a solid solution in somecases) is seen to form and its free energy is indicated bypoint ldquo2rdquoThe solid solution forms because it has a lower free

energy than the BE powder mixture Since MA introduces avariety of crystal defects the crystalline phases in the milledpowders will contain excess energyThis energy will continueto increase with milling time and reach a value which isabove that of the metastable amorphous phase Thus theamorphous phase gets stabilized (point ldquo3rdquo) On primarycrystallization of the amorphous phase the new constitutionwill be a mixture of the 120572-Fe solid solution (or intermetallics)and the amorphous phase which now has a compositiondifferent from that of the original amorphous phase Furtherthe new solid solution phase also has a composition differentfrom the original 120572-Fe phase The free energy of the mixtureof this solid solution and the amorphous phase indicatedby ldquopoint 4rdquo will have a free energy lower than that of theamorphous phase The equilibrium mixture of the 120572-Fe solidsolution and ldquointermetallicsrdquo will have the lowest free energyof all the phase mixtures as indicated by point ldquo5rdquo in thefigure The process of mechanical crystallization is uniqueto the MA process and can be achieved only on continuedmilling of the amorphous powder ormilling of an amorphousalloy obtained by other methods

5 Nanostructured Materials

Another important attribute of MA has beenmicrostructuralrefinement The most common observation is that grainrefinement takes place resulting in the formation of very finegrains often down to nanometer levels A two-phase alloyunder equilibrium conditions can transform to a single-phasealloy due to formation of a supersaturated solid solution Anamorphous alloy can crystallize under the action of severeplastic deformation Most usefully one can also introducea high volume fraction of fine second phase particles andobtain a composite with a uniform distribution of thesecond phase particles By a proper choice of the size ofthe matrix andor the reinforcement phase one can producenanocomposites a very fruitful area for investigations Infact MA has been the most popular technique to synthesizenanostructured materials and nanocomposites in a numberof alloy systems

It has been frequently demonstrated that mechanicallyalloyed materials show powder particle refinement as wellas grain refinement as a function of the milling time Thesesizes decrease very rapidly in the early stages of milling andmore gradually later Eventually in almost all the cases thegrain size of the mechanically alloyed materials is in thenanometer region unless the powder becomes amorphousThe minimum grain size achieved has been reported tobe a few nanometers ranging typically from about 5 to50 nm but depending on the material and processingconditions For example it was reported that the grain sizedecreases with increasing milling energy higher ball-to-powder weight ratio and lower temperatures As is wellknown nanostructuredmaterials exhibit high strength goodductility excellent sintering characteristics and interestingelectrical andmagnetic properties [7 8 27 43]The ease withwhich nanostructured materials can be synthesized is onereason why MA has been extensively employed to producenanocrystalline and nanocomposite materials

8 Research

From high-resolution transmission electron microscopy(TEM) observations it was noted that in the early stages ofmilling due to the high deformation rates experienced bythe powder deformation was localized within shear bands[44] These shear bands which contain a high density ofdislocations have a typical width of approximately 05 to 10120583m Small grains with a diameter of 8ndash12 nm were seenwithin the shear bands and electron diffraction patternssuggested significant preferred orientation With continuedmilling the average atomic level strain increased due toincreasing dislocation density and at a certain dislocationdensity within these heavily strained regions the crystaldisintegrated into subgrains that are separated by low-anglegrain boundaries This resulted in a decrease of the latticestrain The subgrains formed this way were of nanometerdimensions and are often between 20 and 30 nm

On further processing deformation occurred in shearbands located in previously unstrained parts of the materialThe grain size decreased steadily and the shear bands coa-lesced The small-angle boundaries are replaced by higherangle grain boundaries implying grain rotation as reflectedby the absence of texture in the electron diffraction patternsand random orientation of the grains observed from thelattice fringes in the high-resolution TEMmicrographs Con-sequently dislocation-free nanocrystalline grains are formedThis is the currently accepted mechanism of nanocrystalformation in MA processed powders

The grain size of the milled materials decreases withmilling time and reaches a saturation level when a balance isestablished between the fracturing and cold welding eventsThis minimum grain size 119889min is different depending on thematerial and milling conditionsThe value of 119889min achievableby milling is determined by the competition between theplastic deformation via dislocation motion that tends todecrease the grain size and the recovery and recrystallizationbehavior of the material that tends to increase the grain sizeThis balance gives a lower bound for the grain size of puremetals and alloys

The 119889min obtained is different for different metals and isalso found to vary with the crystal structure (Figure 4) Inmost of the metals the minimum grain size attained is inthe nanometer dimensions But metals with a body-centeredcubic (BCC) crystal structure reach much smaller valuesin comparison to metals with the other crystal structuresrelated to the difficulty of extensive plastic deformation andconsequent enhanced fracturing tendency during millingCeramics and intermetallic compounds are much harderand usually more brittle than the metals on which they arebased Therefore intuitively one expects that 119889min of thesecompounds is smaller than those of the pure metals but thisdoes not appear to be the case always

It was also reported that 119889min decreases with an increasein the melting temperature of the metal [45] As shown inFigure 4 this trend is amply clear in the case of metalswith close-packed structures (face-centered cubic (FCC) andhexagonal close-packed (HCP)) but not so clear in metalswith a BCC structure Another point of interest is that thedifference in grain size is much less among metals thathave high melting temperatures the minimum grain size is

0

5

10

15

20

25

FCCBCCHCP

Min

imum

Gra

in S

ize (

nm)

0 2000 30001000Melting Temperature (K)

Figure 4 Variation of minimum grain size with temperature inmechanically milled FCC HCP and BCC metals

virtually constant Thus for the HCP metals Co Ti Zr Hfand Ru the minimum grain size is almost the same eventhough themelting temperatures vary between 1495∘C for Coand 2310∘C for Ru

Analysis of the existing data on 119889min of mechanicallymilled pure metals shows that the normalized minimumgrain size 119889minb where b is the Burgers vector of thedislocations decreases with increasing melting temperatureactivation energy for self-diffusion hardness stacking faultenergy bulk modulus and the equilibrium distance betweentwo edge dislocations The 119889min value is also affected bymilling energy (the higher the milling energy the smaller thegrain size) milling temperature (the lower the milling tem-perature the smaller the grain size) and alloying additions(the minimum grain size is smaller for solid solutions thanfor pure metals since solid solutions are harder and strongerthan the pure metals on which they are based)

It was suggested that 119889min is determined by the minimumgrain size that can sustain a dislocation pile-up within a grainand by the rate of recovery Based on the dislocation pile-up model the critical equilibrium distance between two edgedislocations in a pile-up L119888 (which could be assumed to bethe crystallite or grain size inmilled powders) was calculated[46] using the equation

119871119888=3119866119887

(1 minus ])119867(2)

where G is the shear modulus 119887 is the Burgers vector ] isthe Poissonrsquos ratio and H is the hardness of the materialAccording to the above equation increased hardness resultsin smaller values of 119871c (grain size) and an approximate linearrelationship was observed between 119871c and the minimumgrain size obtained by milling of a number of metals Otherattempts have also beenmade to theoretically predict 119889min onthe basis of thermodynamic properties of materials [47]

Research 9

6 Nanocomposites

Achievement of a uniform distribution of the reinforcementin a matrix is essential to the achievement of good anduniform mechanical properties in composites Further themechanical properties of the composite tend to improvewith increasing volume fraction andor decreasing particlesize of the reinforcement [48] Traditionally a reasonablylarge volume fraction of the reinforcement could be addedif the size of the reinforcement is large (on a micrometerscale or larger) But if the reinforcement size is very fine(of nanometer dimensions) then the volume fraction addedis typically limited to about 2 to 4 However if a largevolume fraction of nanometer-sized reinforcement can beintroduced the mechanical properties of the composite arelikely to be vastly improved Therefore several investigationshave been undertaken to produce improved nanocompositesthrough MA [20]

Composites are traditionally produced by solidificationprocessing methods since the processing is inexpensive andlarge tonnage quantities can also be produced But it isnot easy to disperse fine reinforcement particles in a liquidmetal since agglomeration of the fine particles occurs andclusters are produced Further wetting of the particles ispoor and consequently even the most vigorous stirring isnot able to break the agglomerates Additionally the finepowders tend to float to the top of the melt during processingof the composites through solidification processing Thussolid-state processing methods such as MA are effective indispersing fine particles (including nanoparticles) An addedadvantage is that since no melting is involved even a largevolume fraction of the reinforcement can be introduced

Some of the specific goals sought in processing ofnanocomposite materials throughMA include incorporationof a high volume percentage of ultrafine reinforcements andachievement of high ductility and even superplasticity insome composites It is also possible that due to the increasedductility the fracture toughness of these composites couldbe higher than that of their coarse-grained counterparts Inline with these goals we have in recent times accomplished(a) achievement of a very uniform distribution of the rein-forcement phases in different types of matrices includingdispersion of a high volume fraction of Al2O3 in Al [49] andof Ti5Si3in 120574ndashTiAl [50 51] and (b) synthesis of MoSi

2+Si3N4

composites for high-temperature applications [52] amongothers However we will describe here only the developmentof Ti5Si3+120574ndashTiAl composites as a typical example

Lightweight intermetallic alloys based on 120574-TiAl arepromising materials for high-temperature structural appli-cations eg in aircraft engines or stationary turbines Eventhough they have many desirable properties such as highspecific strength and modulus both at room and elevatedtemperatures and good corrosion and oxidation resistancethey suffer from inadequate room temperature ductility andinsufficient creep resistance at elevated temperatures espe-cially between 800 and 850∘C an important requirement forelevated temperature applications of these materials There-fore current research programs have been addressing thedevelopment of high-temperature materials with adequate

020406080

100120140160180200

Engi

neer

ing

stres

s (M

Pa)

20 40 60 80 100 120 140 1600Engineering strain ()

-5950

∘C 4x10

-4950

∘C 4x10

-41000

∘C 4x10x10

(a)

(b)

Figure 5 (a) Scanning electronmicrograph of the 120574-TiAl + 60 volTi5Si3composite specimen showing that the twophases are very uni-

formly distributed in the microstructure Such a microstructure isconducive to achieving superplastic deformation under appropriateconditions of testing (b) Tensile stress-strain curve of 120574-TiAl + 60vol Ti

5Si3composite showing that superplastic deformation was

achieved at 950∘C with a strain rate of 4x10minus5 sminus1 and at 1000∘C witha strain rate of 4x10minus4 sminus1

room temperature ductility for easy formability and abilityto increase the high-temperature strength by a suitable heattreatment or alloying additions to obtain sufficient creepresistance at elevated temperatures

Composites of 120574-TiAl and 120585-Ti5Si3phase with the volume

fractions of the 120585-Ti5Si3phase varying from 0 to 60 vol

were produced by MA of a combination of the blendedelemental and prealloyed intermetallic powders Fully denseand porosity-free compacts were produced by hot isostaticpressing with the resulting grain size of each of the phasesbeing about 300-400 nm Figure 5(a) shows scanning elec-tron micrographs of the 120574-TiAl+60 vol 120585-Ti5Si3 compositeshowing that the two phases are very uniformly distributedthroughout the microstructure Materials exhibiting such amicrostructure (roughly equal amounts of the two phasessmall grain sizes and a uniform distribution of the twophases) are expected to be deformed superplastically Totest this hypothesis both compression and tensile testingof these composite specimens were conducted at differenttemperatures and strain rates From the tensile stress-strain

10 Research

Pure Metals Alloysand Compounds

NanostructuredMaterials

MECHANICALALLOYING

ODS Superalloys Supercorroding

Alloys

Synthesis of Novel Phases PVDTargets Solders Paints Food Heaters

Biomaterials Waste Utilization etc

Hydrogen Storage MagneticMaterials Catalysts Sensors etc

Figure 6 An overview of the applications of the MA process

plots of the 120574-TiAl+60 vol 120585-Ti5Si3composite specimens

shown in Figure 5(b) it is clear that the specimens testedat 950∘C with a strain rate of 4times10minus5 sminus1 and at 1000∘Cwith a strain rate of 4times10minus4 sminus1 exhibited large ductilityof nearly 150 and 100 respectively Considering that thiscomposite is based on a ceramic material (Ti

5Si3) this is a

very high amount of deformation suggestive of superplasticdeformation Final proof is provided by TEM investiga-tions that confirm the continued stability of the equiaxedmicrostructure after deformation

7 Applications

As has been frequently pointed out in the literature thetechnique ofMAwas developed out of an industrial necessityin 1965 to produce ODS nickel-based superalloys Sincethen a number of new applications have been exploredfor mechanically alloyed materials (Figure 6) Apart fromthe ODS alloys which continue to be used in the high-temperature applications area MA powders have also beenused for other applications and newer applications are alsobeing explored These novel applications include direct syn-thesis of pure metals and alloys from ores and scrap mate-rials super-corroding alloys for release into ocean bottomsdevelopment of PVD (physical vapor deposition) targetssolders paints etc hydrogen storage materials photovoltaicmaterials catalysts and sensors

A novel and simple application of MA powders is in thedevelopment ofmeals ready to eat (MRE) In this applicationvery finely ground Mg-based alloy powders are used Whenwater is added to these fineMg-Fe powders heat is generatedaccording to the following reaction

Mg (Fe) + 2H2O 997888rarr Mg (OH)2 +H2 +Heat (3)

This heat is utilized to warm the food in remote areas wherecooking or heating facilities may not be available eg in war-torn areas These ldquofood heatersrdquo were widely used during theDesert Stormoperations in the 1990s Let us now look at someof the other applications

71 Pure Metals and Alloys The technique of MA has beenemployed to produce pure metals and alloys from ores andother raw materials such as scrap The process is based onchemical reactions induced by mechanical activation Mostof the reactions studied have been displacement reactions ofthe type

119872119874 + 119877 997888rarr 119872 + 119877119874 (4)

where ametal oxide (MO) is reduced by amore reactivemetal(reductant R) to the pure metal M In addition to oxidesmetal chlorides and sulfides have also been reduced to puremetals this way Similar reactions have also been used toproduce alloys and nanocomposites [21 53]

All solid-state reactions involve the formation of a prod-uct phase at the interface of the component phases Thusin the above example the metal M forms at the interfacebetween the oxide MO and the reductant R and physicallyseparates the reactants Further growth of the product phaseinvolves diffusion of atoms of the reactant phases through theproduct phase which constitutes a barrier layer preventingfurther reaction from occurring In other words the reactioninterface defined as the nominal boundary surface betweenthe reactants continuously decreases during the course ofthe reaction Consequently kinetics of the reaction are slowand elevated temperatures are required to achieve reasonablereaction rates

MA can provide the means to substantially increasethe reaction kinetics of chemical reactions in general andthe reduction reactions in particular This is because therepeated cold welding and fracturing of powder particlesincrease the area of contact between the reactant powderparticles by bringing fresh surfaces to come into contactrepeatedly due to a reduction in particle size during millingThis allows the reaction to proceed without the necessityfor diffusion through the product layer As a consequencereactions that normally require high temperatures will occurat lower temperatures or even without any externally appliedheat In addition the high defect densities induced by MAaccelerate diffusion processes

Research 11

Depending on the milling conditions two entirely differ-ent reaction kinetics are possible [21 54]

(a) the reactionmay extend to a very small volumeduringeach collision resulting in a gradual transformationor

(b) if the reaction enthalpy is sufficiently high (and ifthe adiabatic temperature is above 1800 K) a self-propagating high-temperature synthesis (SHS) reaction(also known as combustion synthesis) can be initi-ated by controlling the milling conditions (eg byadding diluents) to avoid combustion reactions maybe made to proceed in a steady-state manner

The latter type of reactions requires a critical milling time forthe combustion reaction to be initiated If the temperature ofthe vial is recorded during the milling process the tempera-ture initially increases slowly with time After some time thetemperature increases abruptly suggesting that ignition hasoccurred and this is followed by a relatively slow decrease

In the above types of reactions themilled powder consistsof the pure metalM dispersed in the RO phase both usuallyof nanometer dimensions It has been shown that by anintelligent choice of the reductant the RO phase can beeasily leached out Thus by selective removal of the matrixphase by washing with appropriate solvents it is possibleto achieve well-dispersed metal nanoparticles as small as 5nm in size If the crystallinity of these particles is not highit can be improved by subsequent annealing without thefear of agglomeration due to the enclosure of nanoparticlesin a solid matrix Such nanoparticles are structurally andmorphologically uniform having a narrow size distributionThis technique has been used to produce powders of manydifferent metalsmdashAg Cd Co Cr Cu Fe Gd Nb NiTi V W Zn and Zr Several alloys intermetallics andnanocomposites have also been synthesized Figure 7 showstwo micrographs of nanocrystalline ceria particles producedby the conventional vapor synthesis and MA methods Eventhough the grain size is approximately the same in both thecases the nanoparticles produced by MA are very discreteand display a very narrow particle size distribution whilethose produced by vapor deposition are agglomerated andhave a wide particle size distribution

Novel materials have also been synthesized by MA ofblended elemental powders One of the interesting examplesin this category is the synthesis of plain carbon steels andstainless steels [55 56] Twomain advantages of this synthesisroute are that the steels can be manufactured at roomtemperature and that the steels will be nanostructured thusconferring improved properties But these powders needto be consolidated to full density for applications and itis possible that grain growth occurs during this processThe kinetics of grain growth can however be controlled byregulating the time and temperature combinations

72 Hydrogen Storage Materials Another important applica-tion of MA powders is hydrogen storage Mg is a lightweightmetal (the lightest structural material) with a density of174 gcm3 Mg and Mg-based alloys can store a significant

20 nm

50 nm

Figure 7 Transmission electron micrographs of ceria particlesproduced byMA (top) and vapor synthesis (bottom)methods Notethe severe agglomeration of the nanoparticles in the latter case

amount of hydrogen However being a reactive metal Mgalways contains a thin layer of oxide on the surface Thissurface oxide layer which inhibits asorption of hydrogenneeds to be broken down through activation A high-temperature annealing is usually done to achieve this ButMA can overcome these difficulties due to the severe plasticdeformation experienced by the powder particles duringMAA catalyst is also often required to accelerate the kinetics ofhydrogen and dehydrogenation of alloys

Mg can adsorb about 76 wt hydrogen andMg-Ni alloysmuch less However Mg is hard to activate its tempera-ture of operation is high and the sorption and desorptionkinetics are slow Therefore Mg-Ni and other alloys arebeing explored But due to the high vapor pressure of Mgand significant difference in the melting temperatures ofthe constituent elements (Mg = 650∘C and Ni = 1452∘C) itis difficult to prepare these alloys by conventional meltingand solidification processes Therefore MA has been used toconveniently synthesize these alloys since this is a completelysolid-state processing method An added advantage of MA isthat the product will have a nanocrystalline grain structureallowing the improvements described below [57 58]

Figure 8 shows the hydrogenation and dehydrogena-tion plots for conventional and nanocrystalline alloysAn additional plot for a situation where a catalyst isadded to the nanocrystalline material is also shown Fromthese two graphs the following facts become clear Firstlythe nanocrystalline alloys adsorb hydrogen much fasterthan conventional (coarse-grained) alloys even though the

12 Research

0 1 2 3 4 5 6 7 8 9 10time (min)

conventional

p = 8 bar

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

time (min)

nano + catalyst

nano + catalyst

nanocrystallinenanocrystalline

conventional

breakthrough in reaction kinetics due to nanostructure and use of catalyst

0

1

2

3

4

5

6

7

8

hydr

ogen

cont

ent (

wt

)

T = 300∘C p = 10-3 mbar

Figure 8 Comparison of the hydrogen sorption and desorption kinetics of Mg alloys with conventional and nanocrystalline grain sizesSignificant improvement in the desorption kinetics when a catalyst is added is worth noticing

amount of hydrogen adsorbed is the sameWhile the conven-tional alloys do not adsorb any significant amount of hydro-gen in about 10 min the allowable full amount of hydrogenis adsorbed in the nanocrystalline alloy Similar observationsare made by many other authors in many different alloysFurther the kinetics are slightly faster (not much different)when a catalyst is added to the nanocrystalline alloy Onthe other hand the kinetics appear to be very significantlyaffected during the desorption process For example thenanocrystalline alloys desorb hydrogen much faster thanthe conventional alloys Again while the conventional alloysdo not desorb any significant amount of hydrogen thenanocrystalline alloy desorbs over 20 of the hydrogenstored in about 10 min But the most significant effect wasobserved when a catalyst was added to the nanocrystallinealloyAll the adsorbedhydrogenwas desorbed in about 2minwhen a catalyst was added to the nanocrystalline alloy Thusnanostructure processing throughMAappears to be a fruitfulway of developing novel hydrogen storage materials [59]

Many novel complex hydrides based on light metals(Mg Li and Na) are being explored and the importantcategories of materials being investigated include the alanates(also known as aluminohydrides a family of compoundsconsisting of hydrogen and aluminum) borohydrides amideborohydrides amide-imide systems (which have a highhydrogen storage capacity and low operative temperatures)amine borane (compounds of borane groups having ammo-nia addition) and alane

73 Energy Materials The technique of MA has also beenemployed to synthesize materials for energy applicationsThese include synthesis of photovoltaic (PV) materials andultrafine particles for faster ignition and combustion pur-poses

One of the most common PV materials is CIGS acompound of Cu In Ga and Se with the compositionCu(InGa)Se2 Since PV cells have the highest efficiencywhen

the band gap for this semiconductor is 125 eV one will haveto engineer the composition for the ratio of Ga to In This isbecause the band gaps for CuInSe

2 and CuGaSe2 are 102 and170 eV respectively Accordingly a Ga(Ga+In) ratio of about03 is found most appropriate and the final ideal compositionof the PV material will be Cu097In07Ga03Se2

The traditional method of synthesizing this material isto prepare an alloy of Cu Ga and In by vapor depositionmethods and incorporate Se into this by exposure to Seatmosphere Consequently this is a complex process MAhowever can be a simple direct and efficient method ofmanufacturing Blended elemental powders of Cu In Gaand Se weremilled in a copper container for different periodsof time and the phase constitution was monitored throughX-ray diffraction studies It was noted that the tetragonalCu(InGa)Se

2phase has formed even aftermilling the powder

mix for 20 min Process variables such as mill rotation speedball-to-powder weight ratio (BPR) and milling time werevaried and it was noted that the ideal composition wasachieved at lowmilling speeds short milling times and a lowBPR (Table 2) [60] By fine-tuning these parameters it shouldbe possible to achieve the exact composition

Nanostructuredmetal particles have high specific surfaceareas and consequently they are highly reactive and can storeenergy This area has received a great impetus in recentyears since the combustion rates of nanostructured metalparticles aremuch higher than those of coarsemetal particlesThe particle size cannot be exceedingly small because thesurface may be coated with a thin oxide layer and make theparticles passive [61ndash63] MA is very well suited to preparesuch materials because the reactions are easily initiatedby impact or friction and at the same time milling isa simple scalable and controllable technology capable ofmixing reactive components on the nanoscaleThese reactivematerials could be used as additives to propellants explosivesand pyrotechnics as well asmanufacture of reactive structuralcomponents [63]

Research 13

Table 2 Chemical analysis (at) of the Cu-In-Ga-Se powder mechanically alloyed under different processing conditions

BPR Speed (rpm) Time (min) Cu In Ga Se201 300 120 3261 1523 653 4562201 300 240 3173 1563 671 4593101 150 40 2598 1698 725 4978Targeted (Cu

097In07Ga03Se2) 2443 1763 756 5038

Compared to the presently used liquid aviation fuelsmetal particles based on B Al andMg are effective HoweverB and Al have very high boiling points and high heats ofenthalpy On the other hand Mg has a low boiling point butalso low heat of enthalpy Therefore if Al and Mg could bemixed intimately on a nanometer scale one could hope tohave a high heat of enthalpy at reduced boiling temperaturesThat is Mg-Al alloy particles could be practical in achievinga high-energy density and practical ignitionboiling temper-atures MA is a very effective method to accomplish this

Al and Mg powders were blended together in differentproportions and milled Since the milled powders have arange of particle sizes they were sieved to obtain well-defined powder sizes They were then fluidized and thencombusted in a CH4-air premixed flame Both the ignitionand burning times of these metal particles decreased withdecreased particle sizes and were also shorter when the Mgcontent is higher (Figure 9) [64] Similar results have beenreported by other investigators

8 Powder Consolidation

The product of MA is in powder form Except in cases ofchemical applications such as catalysis when the productdoes not require consolidation widespread application ofMA powders requires efficient methods of consolidatingthem into bulk shapes Successful consolidation of MA pow-ders is a nontrivial problem since fully densematerials shouldbe produced while simultaneously retaining the nanometer-sized grains without coarsening or the amorphous phaseswithout crystallizing Conventional consolidation of powdersto full density through processes such as hot extrusion [65]and hot isostatic pressing (HIP) [66] requires use of highpressures and exposure to elevated temperatures for extendedperiods of time [67] Unfortunately however this resultsin significant coarsening of the nanometer-sized grains orcrystallization of amorphous phases and consequently thebenefits of nanostructure processing or amorphization arelost [68] On the other hand retention of finemicrostructuresrequires use of low consolidation temperatures and then itis difficult to achieve full interparticle bonding at these lowtemperatures Therefore novel and innovative methods ofconsolidating MA powders are required The objectives ofconsolidation of these powders are to (a) achieve full densifi-cation (without any porosity) (b) minimize microstructuralcoarsening (ie retention of fine grain size) andor (c)avoid undesirable phase transformations In fact the earlyresults of excellent mechanical properties and most specifi-cally superplasticity at room temperature [69] attributed to

20 Mg (tig)90 Mg (tig)

20 Mg (tb)90 Mg (tb)

20 40 60 80 100 1200Particle Size (m)

0

10

20

30

40

50

60

70

Igni

tion

Burn

ing

Tim

e (m

s)

Figure 9 Ignition and burning times as a function of particle sizefor the mechanically alloyed Al-20 and Al-90 at Mg powdersIn the insert tig and tb represent the ignition and burning timesrespectively

nanocrystalline ceramics have not been successfully repro-duced due to incomplete consolidation of the nanopowdersin the material tested Thus consolidation to full densityassumes even greater importance

Because of the small size of the powder particles (typicallya few micrometers even though the grain size is only afew nanometers) MA powders pose special problems Forexample such nanometric powders are very hard and strongThey also have a high level of interparticle friction Furthersince these powders have a large surface area they also exhibithigh chemical reactivity Therefore special precautions needto be taken to consolidate the metal powders to bulk shapes

Successful consolidation of MA powders has beenachieved by electrodischarge compaction plasma-activatedsintering shock (explosive) consolidation [70] HIP hydro-static extrusion strained powder rolling and sinter forging[71] By utilizing the combination of high temperature andpressure HIP can achieve a particular density at lowerpressure when compared to cold isostatic pressing or at lowertemperature when compared to sintering Because of theincreased diffusivity in nanocrystalline materials sintering(and therefore densification) takes place at temperaturesmuch lower than those in coarse-grained materials This islikely to reduce the grain growth Spark plasma sintering(SPS) has become the most popular technique to consolidatemetal powders especially those with ultrafine grain sizesdue to the fact that full density can be achieved at low

14 Research

SOLID LIQUID GAS

MILLING

USE POWDER AS-ISor Chemical Reaction

Thermal Treatment etcCONSOLIDATE TO SOLID

(Temperature Pressure)

Catalysts Pigments SolderComponents Ceramic Powder etc

Advanced materials with hightolerance to gaseous impurities

Figure 10 Alternatives to consolidation of metal powders Ifpowders are used as catalysts pigments solder components etcthen they need not be consolidated However if powders need tobe consolidated then they could be used in applications with hightolerance to gaseous impurities

temperatures and shorter times in comparison to severalothermethods [72 73] An excellent reviewbyGroza [71]maybe consulted for full details of the methods of consolidationand description of results

9 Powder Contamination

Amajor concern inmetal powders processed byMAmethodsis the nature and amount of impurities that get incorporatedinto the powder and contaminate it The small size of thepowder particles and consequent availability of large surfacearea formation of fresh surfaces during milling and wearand tear of the milling tools and the atmosphere underwhich the powder is milled all contribute to contaminationof the powder In addition to these factors one has toconsider the purity of the starting raw materials and themilling conditions employed (especially the milling atmo-sphere) Thus it appears as though powder contaminationis an inherent drawback of the method and that the milledpowder will always be contaminated with impurities unlessspecial precautions are taken to avoidminimize them Themagnitude of powder contamination appears to depend onthe time of milling intensity of milling atmosphere in whichthe powder is milled nature and size of the milling mediumand differences in the strengthhardness of the powder andthe milling medium and the container

Several attempts have been made in recent years to min-imize powder contamination during milling An importantpoint to remember is that if the starting powders are highlypure then the final product is going to be clean and purewith minimum contamination from other sources providedthat proper precautions are taken during milling It is alsoimportant to keep inmind that all applications do not requireultraclean powders and that the purity desired will depend onthe specific application (Figure 10) But it is a good practiceto minimize powder contamination at every stage and ensurethat additional impurities are not picked up subsequentlyduring handling andor consolidation

One way ofminimizing contamination from the grindingmedium and themilling container is to use the samematerial

for the container and the grinding medium as the powderbeing milled Thus one could use copper balls and coppercontainer formilling copper and copper alloy powders [60] Ifa container of the same material to be milled is not availablethen a thin adherent coating on the internal surface of thecontainer (and also on the surface of the grinding medium)with the material to be milled will minimize contaminationThe idea here is to mill the powder once allowing the powderto be coated onto the grindingmedium and the inner walls ofthe container The milled loose powder is then discarded anda fresh batch of powder is milled but with the old grindingmedia and container In general a simple rule that shouldbe followed to minimize contamination from the millingcontainer and the grinding medium is that the container andgrindingmedium should be harderstronger than the powderbeing milled

Milling atmosphere is perhaps the most important vari-able that needs to be controlled Even though nominally puregases are used to purge the glove box before loading andunloading the powders even small amounts of impurities inthe gases appear to contaminate the powders especially whenthey are reactive The best solution one could think of willbe to place the mill inside a chamber that is evacuated andfilled with high-purity argon gas Since the whole chamberis maintained under argon gas atmosphere (continuouslypurified to keep oxygen and water vapor below 1 ppm each)and the container is inside the mill which is inside thechamber contamination from the atmosphere is minimum

Contamination from PCAs is perhaps the most ubiqui-tous Since most of the PCAs used are organic compoundswhich have low melting and boiling points they decomposeduring milling due to the heat generated The decomposi-tion products consisting of carbon oxygen nitrogen andhydrogen react with the metal atoms and form undesirablecarbides oxides nitrides etc

10 Concluding Remarks and Future Prospects

We have only briefly touched upon the scientific and tech-nological aspects of MA materials along with some of theconcerns such as powder contamination and consolidationissues The historical development of MA during the last 50years or so can be divided into threemajor periods (Figure 11)The first period covering the first twenty years (from 1966up to about 1985) was mostly concerned with the devel-opment and production of oxide-dispersion-strengthened(ODS) superalloys for applications in the aerospace indus-try Several alloys with improved properties based onNi and Fe were developed and found useful applicationsThese included the MA754 MA760 MA956 MA957 andMA6000 [74] A more recent survey of application of MAto steel is also available [75] The second period of aboutfifteen years covering from about 1986 up to about 2000witnessed advances in the fundamental understanding ofthe processes that take place during MA Along with thisa large number of dedicated conferences were held andthere was a burgeoning of publication activity Comparisonshave also been frequently made between MA and othernonequilibriumprocessing techniquesmore specifically RSP

Research 15

1965 19851975 1995 2005

Production of ODS Alloys

Novel Phase SynthesisDevelopment of BasicUnderstanding of MA

Nanostructures andNew Applications

2015

Figure 11 Schematic showing the different periods of development in the field of mechanical alloying (MA) and applications ofMA productssince its inception in 1965

It was shown that themetastable effects achieved by these twononequilibrium processing techniques are similar Revival ofthe mechanochemical processing (MCP) took place and avariety of novel substances were synthesized Several mod-eling studies were also conducted to enable prediction of thephases produced or microstructures obtained although withlimited success The third period starting from about 2001saw a reemergence of the quest for new applications of theMA materials not only as structural materials but also forchemical applications such as catalysis and hydrogen storagewith the realization that contamination of themilled powdersis the limiting factor in the widespread applications of theMA materials Innovative techniques to consolidate the MApowders to full density while retaining the metastable phases(including glassy phases andor nanostructures) in themwerealso developed All these are continuing with the ongoinginvestigations to enhance the scientific understanding of theMA process

At present activities relating to both the science andtechnology of MA have been going on simultaneously Whilesome groups have been focusing on developing a betterunderstanding of the process and optimizing the processvariables to achieve a certain phase or phase combinationin the alloy system others have been concentrating onexploiting this technique for different applications It hasalso come to be realized that the MA process can be usedto recycle used and discarded materials to produce value-added materials for subsequent consumption They can bepulverized separated and used to mix with other powdersto produce composites The bottom line is that newer andimproved materials are being continuously developed fornovel applications

Conflicts of Interest

The author declares that he has no conflicts of interest

References

[1] K H J Buschow R W Cahn M C Flemings B Ilschner E JKramer and SMahajan EdsEncyclopedia ofMaterials Scienceand Technology Elsevier 2001

[2] C Suryanarayana EdNon-equilibrium Processing of MaterialsPergamon Oxford UK 1999

[3] C Suryanarayana ldquoMetallic glassesrdquo Bulletin of Materials Sci-ence vol 6 no 3 pp 579ndash594 1984

[4] C Suryanarayana and A Inoue Bulk Metallic Glasses CRCPress Boca Raton Fla USA 2nd edition 2018

[5] H R Trebin Ed Quasicrystals Structure and Physical Proper-ties Wiley-VCH Weinheim Germany 2003

[6] C Suryanarayana and H Jones ldquoFormation and characteristicsof quasicrystalline phases A reviewrdquo International Journal ofRapid Solidification vol 3 pp 253ndash293 1988

[7] H Gleiter ldquoNanocrystalline materialsrdquo Progress in MaterialsScience vol 33 no 4 pp 223ndash315 1989

[8] C Suryanarayana ldquoRecent developments in nanostructuredmaterialsrdquo Advanced Engineering Materials vol 7 no 11 pp983ndash992 2005

[9] M C Gao J-W Yeh P K Liaw and Y Zhang Eds High-Entropy Alloys Fundamentals and Applications Springer 2016

[10] J S Benjamin ldquoMechanical alloying ndash History and futurepotentialrdquo in Advances in Powder Metallurgy and ParticulateMaterials J M Capus and R M German Eds vol 7 ofNovel Powder Processing pp 155ndash168 Metal Powder IndustriesFederation Princeton NJ USA 1992

[11] M P Phaniraj D-I Kim J-H Shim and Y W CholdquoMicrostructure development in mechanically alloyed yttriadispersed austenitic steelsrdquo Acta Materialia vol 57 no 6 pp1856ndash1864 2009

[12] A Hirata T Fujita Y R Wen J H Schneibel C T Liu and MW Chen ldquoAtomic structure of nanoclusters in oxide-dispersionstrengthened steelsrdquo Nature Materials vol 10 no 12 pp 922ndash926 2011

[13] M K Miller C M Parish and Q Li ldquoAdvanced oxide disper-sion strengthened and nanostructured ferritic alloysrdquoMaterialsScience and Technology (United Kingdom) vol 29 no 10 pp1174ndash1178 2013

[14] N Oono Q X Tang and S Ukai ldquoOxide particle refinementin Ni-based ODS alloyrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 649 pp 250ndash253 2016

[15] D B Witkin and E J Lavernia ldquoSynthesis and mechanicalbehavior of nanostructured materials via cryomillingrdquo Progressin Materials Science vol 51 no 1 pp 1ndash60 2006

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

Research 3

Metal A

Metal B

Intermetallic

Dispersoid

20 m 5 m 20 m 5 m

Typical Starting Powders

Ball-Powder-Ball Collision

After Single Collision

(a)

(b)

Figure 2 (a) Deformation characteristics of representative constituents of starting powders in mechanical alloying Note that the ductilemetal powders (metals A and B) get flattened while the brittle intermetallic and dispersoid particles get fragmented into smaller particles(b) Ball-powder-ball collision of powder mixture during mechanical alloying

Themilling time decreases with an increase in the energyof the mill As a rule of thumb it can be estimated that aprocess that takes only a few minutes in the SPEX mill maytake hours in an attritor and a few days in a commercial low-energy mill even though the actual details can be differentdepending on the efficiency of the different mills and thepowder characteristics Full details of the process includingthe effect of different variables on the alloying behavior maybe found in two recent references [16 17]

21 Mechanism of Alloying The effects of a single collisionon each type of constituent powder particle are shown inFigure 2(a) The initial impact of the grinding ball causes theductile metal powders to flatten and work hardenThe severeplastic deformation increases the surface-to-volume ratio

of the particles and ruptures the surface films of adsorbedcontaminants The brittle intermetallic powder particles getfractured and are refined in size The oxide dispersoidparticles are comminuted more severely

Whenever two grinding balls collide a small amountof the powder being milled is trapped in between themTypically around 1000 particles with an aggregate weight ofabout 02 mg are trapped during each collision (Figure 2(b))During this process the powdermorphology can bemodifiedin two different ways depending on whether one is dealingwith ductilendashductile ductilendashbrittle or brittlendashbrittle powdercombinations If the starting powders are softmetal particlesthe flattened layers overlap and form cold weldsThis leads toformation of layered composite powder particles consistingof various combinations of the starting ingredients The

4 Research

more brittle constituents tend to become occluded by theductile constituents and trapped in the compositeThe work-hardened elemental or composite powder particles may frac-ture at the same timeThese competing events of coldwelding(with plastic deformation and agglomeration) and fracturing(with size reduction) continue repeatedly throughout themilling period Eventually at the steady-state condition arefined and homogenized microstructure is obtained andthe composition of the powder particles is the same as theproportion of the starting constituent powders

Along with the cold welding event described above somepowder may also coat the grinding medium andor the innerwalls of the container A thin layer of the coating is beneficialin preventing wear and tear of the grinding medium and alsoin preventing contamination of the milled powder with thedebris But a too thick layer will result in lower yield and alsopossible compositional inhomogeneity of the powder whichshould be avoided

The generally accepted explanation for alloying to occurfrom blended elemental powders and formation of differenttypes of phases is that a very fine and intimate mixture ofthe components (often lamellar if the constituent elementsare sufficiently ductile) is formed after milling if not thefinal product Alloy formation is facilitated through crys-talline defects (grain boundaries dislocations stacking faultsvacancies and others) introduced into thematerial due to theintense cold working operation which act as fast diffusionpaths and a slight rise in the powder temperature duringmilling as a result of frictional forces and impact of thegrinding balls against other balls and the surfaces of thecontainer If the final desired phase had not been formeddirectly by MA then a short annealing treatment at anelevated temperature was found to promote diffusion andconsequently alloy formation Accordingly it was shown thatby a proper choice of the process parameters during MA andby choosing an appropriate alloy composition it is possibleto produce a variety of alloys starting from metal powdersThe thermodynamic criterion that the phase with the lowestfree energy under the continuous deformation conditionswould be the most ldquostablerdquo phase has been found to be validA significant attribute of MA has been the ability to alloymetals even with positive heats of mixing that are difficult orimpossible to alloy otherwise [18 19]

The effects of MA are generally categorized under twogroups(1) Constitutional Changes Milling of the blended ele-

mental powders could lead to the formation of solid solu-tions (both equilibrium and supersaturated) intermetallicphases (equilibrium metastable (high temperature andorhigh pressure) and quasicrystalline) and amorphous phasesin alloy systems The actual type of phase formed could bedifferent depending on the alloy system its composition andthe milling conditions employed A number of metastablealloys have been produced this way(2) Microstructural Changes As a result of MA the

processed materials could develop ultrafine-grained andnanostructured phases In addition uniform dispersion ofa large volume fraction of very fine oxide or other ceramicparticles can be achieved in different matrices which is

either impossible or difficult by other processes These novelnanocomposites can exhibit properties far superior to thosethat have conventional grain sizes and sometimes evencompletely new behavior [16 17 20]

In many cases chemical reactions have also been foundto occur leading to the production of pure metals from theirores synthesis of alloys and formation of novel materialsand microstructures Such a process has been referred to asmechanochemical processing [21] All these effects have beenwell documented in the literature andTable 1 summarizes theimportant attributes of MA

Before discussing the synthesis of advanced materialsand their applications let us first describe the constitutionalchanges that can take place in MA materials

3 Constitutional Changes

The literature on MA has several examples of the differentequilibrium and nonequilibrium phases produced in differ-ent alloy systems An exhaustive list of the results (up to dateat the time of compilation) is available in [16 17] and the listis constantly growingWewill however briefly highlight heresome of the more recent and salient features in the followingparagraphs

31 Solid Solutions Solid solution alloys exhibit highstrength changes in density and suppression or eliminationof undesirable second phases Increasing the strength ofalloys through precipitation of a second phase from asupersaturated solid solution during aging (precipitationhardening) has been known to materials scientists for overa hundred years Further since the strengthening effect isrelated to the size and volume fraction of the second phaseparticles and since both of them could be controlled bythe extent of supersaturation and the aging parametersachievement of extended solid solubility limits in alloysystems is very desirable Solid solutionsmdashboth equilibriumand supersaturatedmdashare still being produced [22ndash24]

Rules similar to those developed by Hume-Rothery forsolid solution formation in binary noble-metal alloy systemsunder equilibrium conditions also seem to be generallyapplicable to supersaturated solutions obtained by MA eventhough exceptions have been noted in some cases [25 26]

As will be described later the mechanically alloyed mate-rials exhibit very fine grain sizes often reaching nanometerlevels Since these nanocrystalline materials contain a largevolume fraction of atoms in the grain boundaries thediffusivity of solute atoms is significantly enhanced [27] andthis would lead to increases in the solid solubility levelsAs a general rule of thumb solute elements which exhibitlimited solubility under equilibrium conditions exhibit highsolubility levels under the nonequilibrium conditions of MAEven though solid solubility extensions have been achievedby other nonequilibrium processing techniques as well (egrapid solidification processing vapor deposition and laserprocessing of materials) there appear to be differences in thesolid solubility extensions achieved by MA and other meth-ods [26] An important observation made in MA materials

Research 5

Table 1 Attributes of mechanical alloying

(1) Production of fine dispersion of second phase (usually oxide) particles(2) Extension of solid solubility limits(3) Refinement of grain sizes down to nanometer range(4) Synthesis of novel crystalline and quasicrystalline phases(5) Development of amorphous (glassy) phases(6) Disordering of ordered intermetallics(7) Possibility of alloying of difficult to alloy elementsmetals

(8)Inducement of chemical (displacement) reactions at low temperatures for (a) Mineral and Wasteprocessing (b) Metals refining (c) Combustion reactions and (d) Production of discrete ultrafineparticles

(9) Scaleable process

is that the solute atoms segregate to grain boundaries fromwhere they could diffuse into grains and form solid solutions

32 Intermetallic Phases Intermetallic phases (or inter-metallics for short) are typically ordered compounds existingat simple ratios of the two ormore elements involved Becauseof their ordered nature they are strong and hard stiffhave high melting points and also exhibit other interestingbehaviors such as excellent corrosionoxidation resistanceHowever they usually suffer from low ambient temperatureductility again due to the ordered nature of the compoundsA new family of intermetallics showing high ductility atroom temperature and based on rare-earth elements hasbeen recently developed [28 29] Since MA is known to (a)disorder the ordered lattices (b) refine grain sizes down tonanometer levels and (c) also alter the crystal structures allthese effects could induce some amount of plasticity into thematerial In fact the potential to ductilize the normally brittleintermetallics has been the driver for accelerated researchinto the MA of intermetallics

A number of intermetallics (both equilibrium andmetastable) and high-temperature or high-pressure phaseshave been synthesized byMAstarting fromelemental powderblends [16 17 24 30] More interestingly quasicrystallinephases with the forbidden crystal symmetries first reportedin rapidly solidified Al-Mn alloys have also been synthesizedby MA in a number of alloy systems [5 31] Applicationsfor these exotic materials have been limited Based ontheir high hardness low friction and low surface reactivityquasicrystalline materials have been used to coat nonstickfrying pans Also steels reinforcedwith small quasicrystallineparticles are very strong and have been used for acupunctureand surgery dental instruments and razor blades [32]

The most significant change that occurs in MA materialsis the formation of amorphous phases and this will bediscussed in the next section

4 Metallic Glasses

Amorphous alloys are materials that lack crystallinity andhave a random arrangement of atoms Because of thisextreme disorder these materials exhibit a very interestingcombination of high strength good corrosion resistance

and useful electrical and magnetic properties These novelmaterials have been traditionally produced starting from thevapor or liquid states Amorphous or noncrystalline alloysbased onmetals are commonly referred to as metallic glassesAt present there is worldwide activity in the area of metallicglasses and bulk metallic glasses [4 33]

Metallic glasses were first produced by rapid solidificationprocessing (RSP) in 1960 in the form of thin ribbons bysolidifying the alloy melt at rates gt106 Ks [34] Subsequentdevelopments include the synthesis of bulk metallic glasseswith a large section thickness of a few centimeters This ispossible in multicomponent alloys which could be solidifiedinto the glassy condition at low critical cooling rates of lt102Ks [4] The current record is an 80-mm diameter glassy rodproduced by solidification in a Pd

425Cu30Ni75P20

alloy (thesubscripts represent the atomic percentage of the elementsin the alloy) [35] While solidification methods produce themetallic glasses either in thin ribbon or bulk form directlyfrom the liquid the amorphous phases are in powder formbyMAmethodsThese need to be consolidated to bulk shapefor subsequent characterization and applications

Amorphous alloys were synthesized by MA first in anY-Co intermetallic compound [36] and later in a blendedelemental Ni-Nb powder mixture [37] Amorphous phaseshave now been synthesized in a very large number ofalloy systems starting from either blended elemental powdermixtures intermetallic compounds or mixtures of them[38] Comparisons have been frequently made between theamorphous alloys produced by MA and RSP techniquesand both similarities and differences have been noted [26]Further the mechanism of glass formation appears to besignificantly different depending uponwhether it is producedby MA or RSP methods

Several different criteria have been proposed to explainthe glass-forming ability of alloys produced by solidificationmethodsThese include the reduced glass transition tempera-ture119879rg =119879g119879l where119879g and119879l represent the glass transitionand liquidus temperature of the alloy respectively presenceof deep eutectics in the phase diagrams and significantdifferences in the atomic sizes of the constituent elementsleading to development of strain energy in the solid solutionWhen the strain energy exceeds a critical value determinedby the size and amount of the solute atoms it destabilizes the

6 Research

crystal lattice and results in the formation of an amorphousphase With the introduction of bulk metallic glasses nearlythirty years ago a very large number of new criteria mostlybased on the glass transformation temperatures (119879g 119879x and119879l where 119879x represents the crystallization temperature of theamorphous alloy) have been proposed In spite of a very largenumber of criteria proposed [4] it has not been possible yetto accurately predict the glass-forming ability of alloys [39]

There are some significant differences in the formation ofmetallic glasses by the MA and RSP methods For examplemetallic glasses are obtained by MA in almost every alloysystem provided that sufficient energy has been stored in thepowder But metallic glasses are obtained by RSP methodsonly in certain composition rangesmdashabout 20 at metalloidcontent in metal-metalloid systems and different composi-tions in metal-metal systems mostly at compositions cor-responding to deep eutectics Additionally glass formationby MA occurs by the accumulation of crystal defects whichraises the free energy of the crystalline phase above that of thehypothetical glassy phase under which conditions the crys-talline phase becomes destabilized On the other hand forglass formation by the RSP methods the critical cooling ratefor glass formation needs to be exceeded so that formationof the crystalline nuclei is completely suppressed Since MAis a completely solid-state powder processing technique thecriteria for amorphous phase formation should be differentfrom those of solidification methods A new criterion wasproposed that is the number of intermetallics present inthe constituent phase diagrams determines the glass-formingability of alloys processed by MAmdashthe more the number ofintermetallics in the constituent phase diagrams the easier itis to amorphize the powder blend [40]

Amorphization by MA occurs when the free energy ofthe hypothetical amorphous phase 119866A is lower than that ofthe crystalline phase G119862 ie 119866A lt 119866C A crystalline phasenormally has a lower free energy than the amorphous phaseBut its free energy can be increased by introducing a varietyof crystal defects such as dislocations grain boundaries andstacking faults If an intermetallic has formed then additionalenergy can be introduced by disordering the crystal lattice Bythis approach it is then possible to obtain a situation when

119866A lt 119866C + 119866D (1)

where 119866D is the free energy increase due to introductionof crystal defects It has been shown that the presence ofintermetallics in an alloy system can significantly increasethe energy contributing to amorphization This is due totwo important effects Firstly disordering of intermetallicscontributes an energy of about 15 kJmol to the systemSecondly a slight change in the stoichiometry of the inter-metallic increases the free energy of the system drasticallyAdditionally grain size reduction contributes about 5 kJmolFurther disordering of intermetallics has also been shownto be possible by heavy deformation Since MA reduces thegrain size to nanometer levels and also disorders the usuallyordered intermetallics the energy of the milled powders issignificantly raised In fact it is raised to a level above that ofthe hypothetical amorphous phase and thus formation of theamorphous phase is favored over the crystalline phase

41 Mechanical Crystallization It was reported that an amor-phous phase formed by milling transforms into a crystallinephase on continued milling and that this phenomenonhas been referred to as mechanical crystallization [41 42]The process of mechanical crystallization is very unusualand offers many opportunities to design novel alloys withimproved properties For example it will be possible toproduce the alloy in either a fully amorphous amorphous +nanocrystalline composite or a completely crystalline statewith different grain sizes depending upon the time ofmillingor the temperature and time of subsequent annealing of theamorphous alloy

Figure 3(a) shows the XRD patterns of the blended ele-mental powder mixture of the composition Fe42Ge28Zr10B20mechanically alloyed for 10 and 60 h [42] While the pat-tern at 10 h of milling clearly shows the presence of anamorphous phase the pattern at 60 h of milling showsthe presence of a crystalline phase superimposed over thebroad and diffuse halo representing the amorphous phaseThe crystalline phase has been identified as the 120572-Fe solidsolution containing the solute elements present in the blendThis 120572-Fe phase is the first phase to crystallize from theamorphous alloy (as in primary crystallization) and externalannealing of the milled powder showed that the amount ofthe 120572-Fe increased along with the presence of other inter-metallics Eutectic crystallization (simultaneous formation oftwo crystalline phases) has also been observed in other alloysystems

Mechanical crystallization has also been observed inseveral other alloy systems that formed the amorphous phaseon MA In fact it is now becoming apparent that thisphenomenon of mechanical crystallization is not as rare asit was once thought to be

Many reasons have been suggested for the formation of acrystalline phase after the formation of an amorphous phaseduring MA These include (a) rise in temperature to a levelabove that of the crystallization temperature of the amor-phous alloy (b) powder contamination due to which a stablecrystalline impurity phase forms (c) phenomenon of inversemelting and (d) basic thermodynamic considerations It nowappears clear that the basic thermodynamic stabilities of thedifferent phases under the different conditions of milling areresponsible for mechanical crystallization Such a situationcan be explained with reference to a free energy versuscomposition diagram

Figure 3(b) is a schematic diagram showing the variationof free energy with composition for the Fe42X28Zr10B20 (X =AlGe andNi) alloy systems investigatedThepossible (stableand metastable) constitution in this system is (a) blendedelemental (BE) powder mixture (b) 120572-Fe the solid solutionof all the alloying elements in Fe (c) an amorphous phase (d)intermetallic phases and (e) different combinations of thesephases The stability of any phase will be determined by itsrelative position in the free energy vs composition plotmdashthelower the free energy the more stable the phase is Since it ispossible to have a large number of intermetallic phases in thismulticomponent system and since it is difficult to indicateeach of them separately for simplicity all of them are groupedtogether as ldquointermetallicsrdquo

Research 7Fr

ee E

nerg

y

Solute Concentration (at)20 40 60 80Fe 100

BE powder

amorphous

-Fe

Intermetallics

1

23

45

Inte

nsity

30 40 502-Theta

(a)

(b)

10 h

60 h

60 70 80 90

1

Figure 3 (a) X-ray diffraction patterns of the Fe42Ge28Zr10B20

powder blend milled for 10 and 60 h While the powder milledfor 10 h shows the presence of an amorphous phase the powdermilled for 60 h shows the presence of a crystalline phase coexistingwith the amorphous phase suggesting that the amorphous phase hascrystallized (b) Hypothetical free energy vs composition diagramto explain the mechanism of mechanical crystallization in theFe42X28Zr10B20systemNote that point ldquo1rdquo represents the free energy

of the blended elemental powders Similarly point ldquo2rdquo representsformation of the 120572-Fe solid solution containing all the alloyingelements in Fe point ldquo3rdquo formation of the homogeneous amorphousphase point ldquo4rdquo a mixture of the amorphous phase with a differentsolute content (amorphous) from that at ldquo3rdquo and the solid solution120572-Fe with different solute content and point ldquo5rdquo the equilibriumsituation when the solid solution and intermetallic phases coexist

The free energy of the BE powder mixture is indicated bypoint ldquo1rdquo which represents the point of intersection of thecomposition vertical with the line joining the free energiesof pure Fe and the ldquosoluterdquo On milling this powder a solidsolution phase containing all the solute elements in Fe (ora mixture of intermetallics and a solid solution in somecases) is seen to form and its free energy is indicated bypoint ldquo2rdquoThe solid solution forms because it has a lower free

energy than the BE powder mixture Since MA introduces avariety of crystal defects the crystalline phases in the milledpowders will contain excess energyThis energy will continueto increase with milling time and reach a value which isabove that of the metastable amorphous phase Thus theamorphous phase gets stabilized (point ldquo3rdquo) On primarycrystallization of the amorphous phase the new constitutionwill be a mixture of the 120572-Fe solid solution (or intermetallics)and the amorphous phase which now has a compositiondifferent from that of the original amorphous phase Furtherthe new solid solution phase also has a composition differentfrom the original 120572-Fe phase The free energy of the mixtureof this solid solution and the amorphous phase indicatedby ldquopoint 4rdquo will have a free energy lower than that of theamorphous phase The equilibrium mixture of the 120572-Fe solidsolution and ldquointermetallicsrdquo will have the lowest free energyof all the phase mixtures as indicated by point ldquo5rdquo in thefigure The process of mechanical crystallization is uniqueto the MA process and can be achieved only on continuedmilling of the amorphous powder ormilling of an amorphousalloy obtained by other methods

5 Nanostructured Materials

Another important attribute of MA has beenmicrostructuralrefinement The most common observation is that grainrefinement takes place resulting in the formation of very finegrains often down to nanometer levels A two-phase alloyunder equilibrium conditions can transform to a single-phasealloy due to formation of a supersaturated solid solution Anamorphous alloy can crystallize under the action of severeplastic deformation Most usefully one can also introducea high volume fraction of fine second phase particles andobtain a composite with a uniform distribution of thesecond phase particles By a proper choice of the size ofthe matrix andor the reinforcement phase one can producenanocomposites a very fruitful area for investigations Infact MA has been the most popular technique to synthesizenanostructured materials and nanocomposites in a numberof alloy systems

It has been frequently demonstrated that mechanicallyalloyed materials show powder particle refinement as wellas grain refinement as a function of the milling time Thesesizes decrease very rapidly in the early stages of milling andmore gradually later Eventually in almost all the cases thegrain size of the mechanically alloyed materials is in thenanometer region unless the powder becomes amorphousThe minimum grain size achieved has been reported tobe a few nanometers ranging typically from about 5 to50 nm but depending on the material and processingconditions For example it was reported that the grain sizedecreases with increasing milling energy higher ball-to-powder weight ratio and lower temperatures As is wellknown nanostructuredmaterials exhibit high strength goodductility excellent sintering characteristics and interestingelectrical andmagnetic properties [7 8 27 43]The ease withwhich nanostructured materials can be synthesized is onereason why MA has been extensively employed to producenanocrystalline and nanocomposite materials

8 Research

From high-resolution transmission electron microscopy(TEM) observations it was noted that in the early stages ofmilling due to the high deformation rates experienced bythe powder deformation was localized within shear bands[44] These shear bands which contain a high density ofdislocations have a typical width of approximately 05 to 10120583m Small grains with a diameter of 8ndash12 nm were seenwithin the shear bands and electron diffraction patternssuggested significant preferred orientation With continuedmilling the average atomic level strain increased due toincreasing dislocation density and at a certain dislocationdensity within these heavily strained regions the crystaldisintegrated into subgrains that are separated by low-anglegrain boundaries This resulted in a decrease of the latticestrain The subgrains formed this way were of nanometerdimensions and are often between 20 and 30 nm

On further processing deformation occurred in shearbands located in previously unstrained parts of the materialThe grain size decreased steadily and the shear bands coa-lesced The small-angle boundaries are replaced by higherangle grain boundaries implying grain rotation as reflectedby the absence of texture in the electron diffraction patternsand random orientation of the grains observed from thelattice fringes in the high-resolution TEMmicrographs Con-sequently dislocation-free nanocrystalline grains are formedThis is the currently accepted mechanism of nanocrystalformation in MA processed powders

The grain size of the milled materials decreases withmilling time and reaches a saturation level when a balance isestablished between the fracturing and cold welding eventsThis minimum grain size 119889min is different depending on thematerial and milling conditionsThe value of 119889min achievableby milling is determined by the competition between theplastic deformation via dislocation motion that tends todecrease the grain size and the recovery and recrystallizationbehavior of the material that tends to increase the grain sizeThis balance gives a lower bound for the grain size of puremetals and alloys

The 119889min obtained is different for different metals and isalso found to vary with the crystal structure (Figure 4) Inmost of the metals the minimum grain size attained is inthe nanometer dimensions But metals with a body-centeredcubic (BCC) crystal structure reach much smaller valuesin comparison to metals with the other crystal structuresrelated to the difficulty of extensive plastic deformation andconsequent enhanced fracturing tendency during millingCeramics and intermetallic compounds are much harderand usually more brittle than the metals on which they arebased Therefore intuitively one expects that 119889min of thesecompounds is smaller than those of the pure metals but thisdoes not appear to be the case always

It was also reported that 119889min decreases with an increasein the melting temperature of the metal [45] As shown inFigure 4 this trend is amply clear in the case of metalswith close-packed structures (face-centered cubic (FCC) andhexagonal close-packed (HCP)) but not so clear in metalswith a BCC structure Another point of interest is that thedifference in grain size is much less among metals thathave high melting temperatures the minimum grain size is

0

5

10

15

20

25

FCCBCCHCP

Min

imum

Gra

in S

ize (

nm)

0 2000 30001000Melting Temperature (K)

Figure 4 Variation of minimum grain size with temperature inmechanically milled FCC HCP and BCC metals

virtually constant Thus for the HCP metals Co Ti Zr Hfand Ru the minimum grain size is almost the same eventhough themelting temperatures vary between 1495∘C for Coand 2310∘C for Ru

Analysis of the existing data on 119889min of mechanicallymilled pure metals shows that the normalized minimumgrain size 119889minb where b is the Burgers vector of thedislocations decreases with increasing melting temperatureactivation energy for self-diffusion hardness stacking faultenergy bulk modulus and the equilibrium distance betweentwo edge dislocations The 119889min value is also affected bymilling energy (the higher the milling energy the smaller thegrain size) milling temperature (the lower the milling tem-perature the smaller the grain size) and alloying additions(the minimum grain size is smaller for solid solutions thanfor pure metals since solid solutions are harder and strongerthan the pure metals on which they are based)

It was suggested that 119889min is determined by the minimumgrain size that can sustain a dislocation pile-up within a grainand by the rate of recovery Based on the dislocation pile-up model the critical equilibrium distance between two edgedislocations in a pile-up L119888 (which could be assumed to bethe crystallite or grain size inmilled powders) was calculated[46] using the equation

119871119888=3119866119887

(1 minus ])119867(2)

where G is the shear modulus 119887 is the Burgers vector ] isthe Poissonrsquos ratio and H is the hardness of the materialAccording to the above equation increased hardness resultsin smaller values of 119871c (grain size) and an approximate linearrelationship was observed between 119871c and the minimumgrain size obtained by milling of a number of metals Otherattempts have also beenmade to theoretically predict 119889min onthe basis of thermodynamic properties of materials [47]

Research 9

6 Nanocomposites

Achievement of a uniform distribution of the reinforcementin a matrix is essential to the achievement of good anduniform mechanical properties in composites Further themechanical properties of the composite tend to improvewith increasing volume fraction andor decreasing particlesize of the reinforcement [48] Traditionally a reasonablylarge volume fraction of the reinforcement could be addedif the size of the reinforcement is large (on a micrometerscale or larger) But if the reinforcement size is very fine(of nanometer dimensions) then the volume fraction addedis typically limited to about 2 to 4 However if a largevolume fraction of nanometer-sized reinforcement can beintroduced the mechanical properties of the composite arelikely to be vastly improved Therefore several investigationshave been undertaken to produce improved nanocompositesthrough MA [20]

Composites are traditionally produced by solidificationprocessing methods since the processing is inexpensive andlarge tonnage quantities can also be produced But it isnot easy to disperse fine reinforcement particles in a liquidmetal since agglomeration of the fine particles occurs andclusters are produced Further wetting of the particles ispoor and consequently even the most vigorous stirring isnot able to break the agglomerates Additionally the finepowders tend to float to the top of the melt during processingof the composites through solidification processing Thussolid-state processing methods such as MA are effective indispersing fine particles (including nanoparticles) An addedadvantage is that since no melting is involved even a largevolume fraction of the reinforcement can be introduced

Some of the specific goals sought in processing ofnanocomposite materials throughMA include incorporationof a high volume percentage of ultrafine reinforcements andachievement of high ductility and even superplasticity insome composites It is also possible that due to the increasedductility the fracture toughness of these composites couldbe higher than that of their coarse-grained counterparts Inline with these goals we have in recent times accomplished(a) achievement of a very uniform distribution of the rein-forcement phases in different types of matrices includingdispersion of a high volume fraction of Al2O3 in Al [49] andof Ti5Si3in 120574ndashTiAl [50 51] and (b) synthesis of MoSi

2+Si3N4

composites for high-temperature applications [52] amongothers However we will describe here only the developmentof Ti5Si3+120574ndashTiAl composites as a typical example

Lightweight intermetallic alloys based on 120574-TiAl arepromising materials for high-temperature structural appli-cations eg in aircraft engines or stationary turbines Eventhough they have many desirable properties such as highspecific strength and modulus both at room and elevatedtemperatures and good corrosion and oxidation resistancethey suffer from inadequate room temperature ductility andinsufficient creep resistance at elevated temperatures espe-cially between 800 and 850∘C an important requirement forelevated temperature applications of these materials There-fore current research programs have been addressing thedevelopment of high-temperature materials with adequate

020406080

100120140160180200

Engi

neer

ing

stres

s (M

Pa)

20 40 60 80 100 120 140 1600Engineering strain ()

-5950

∘C 4x10

-4950

∘C 4x10

-41000

∘C 4x10x10

(a)

(b)

Figure 5 (a) Scanning electronmicrograph of the 120574-TiAl + 60 volTi5Si3composite specimen showing that the twophases are very uni-

formly distributed in the microstructure Such a microstructure isconducive to achieving superplastic deformation under appropriateconditions of testing (b) Tensile stress-strain curve of 120574-TiAl + 60vol Ti

5Si3composite showing that superplastic deformation was

achieved at 950∘C with a strain rate of 4x10minus5 sminus1 and at 1000∘C witha strain rate of 4x10minus4 sminus1

room temperature ductility for easy formability and abilityto increase the high-temperature strength by a suitable heattreatment or alloying additions to obtain sufficient creepresistance at elevated temperatures

Composites of 120574-TiAl and 120585-Ti5Si3phase with the volume

fractions of the 120585-Ti5Si3phase varying from 0 to 60 vol

were produced by MA of a combination of the blendedelemental and prealloyed intermetallic powders Fully denseand porosity-free compacts were produced by hot isostaticpressing with the resulting grain size of each of the phasesbeing about 300-400 nm Figure 5(a) shows scanning elec-tron micrographs of the 120574-TiAl+60 vol 120585-Ti5Si3 compositeshowing that the two phases are very uniformly distributedthroughout the microstructure Materials exhibiting such amicrostructure (roughly equal amounts of the two phasessmall grain sizes and a uniform distribution of the twophases) are expected to be deformed superplastically Totest this hypothesis both compression and tensile testingof these composite specimens were conducted at differenttemperatures and strain rates From the tensile stress-strain

10 Research

Pure Metals Alloysand Compounds

NanostructuredMaterials

MECHANICALALLOYING

ODS Superalloys Supercorroding

Alloys

Synthesis of Novel Phases PVDTargets Solders Paints Food Heaters

Biomaterials Waste Utilization etc

Hydrogen Storage MagneticMaterials Catalysts Sensors etc

Figure 6 An overview of the applications of the MA process

plots of the 120574-TiAl+60 vol 120585-Ti5Si3composite specimens

shown in Figure 5(b) it is clear that the specimens testedat 950∘C with a strain rate of 4times10minus5 sminus1 and at 1000∘Cwith a strain rate of 4times10minus4 sminus1 exhibited large ductilityof nearly 150 and 100 respectively Considering that thiscomposite is based on a ceramic material (Ti

5Si3) this is a

very high amount of deformation suggestive of superplasticdeformation Final proof is provided by TEM investiga-tions that confirm the continued stability of the equiaxedmicrostructure after deformation

7 Applications

As has been frequently pointed out in the literature thetechnique ofMAwas developed out of an industrial necessityin 1965 to produce ODS nickel-based superalloys Sincethen a number of new applications have been exploredfor mechanically alloyed materials (Figure 6) Apart fromthe ODS alloys which continue to be used in the high-temperature applications area MA powders have also beenused for other applications and newer applications are alsobeing explored These novel applications include direct syn-thesis of pure metals and alloys from ores and scrap mate-rials super-corroding alloys for release into ocean bottomsdevelopment of PVD (physical vapor deposition) targetssolders paints etc hydrogen storage materials photovoltaicmaterials catalysts and sensors

A novel and simple application of MA powders is in thedevelopment ofmeals ready to eat (MRE) In this applicationvery finely ground Mg-based alloy powders are used Whenwater is added to these fineMg-Fe powders heat is generatedaccording to the following reaction

Mg (Fe) + 2H2O 997888rarr Mg (OH)2 +H2 +Heat (3)

This heat is utilized to warm the food in remote areas wherecooking or heating facilities may not be available eg in war-torn areas These ldquofood heatersrdquo were widely used during theDesert Stormoperations in the 1990s Let us now look at someof the other applications

71 Pure Metals and Alloys The technique of MA has beenemployed to produce pure metals and alloys from ores andother raw materials such as scrap The process is based onchemical reactions induced by mechanical activation Mostof the reactions studied have been displacement reactions ofthe type

119872119874 + 119877 997888rarr 119872 + 119877119874 (4)

where ametal oxide (MO) is reduced by amore reactivemetal(reductant R) to the pure metal M In addition to oxidesmetal chlorides and sulfides have also been reduced to puremetals this way Similar reactions have also been used toproduce alloys and nanocomposites [21 53]

All solid-state reactions involve the formation of a prod-uct phase at the interface of the component phases Thusin the above example the metal M forms at the interfacebetween the oxide MO and the reductant R and physicallyseparates the reactants Further growth of the product phaseinvolves diffusion of atoms of the reactant phases through theproduct phase which constitutes a barrier layer preventingfurther reaction from occurring In other words the reactioninterface defined as the nominal boundary surface betweenthe reactants continuously decreases during the course ofthe reaction Consequently kinetics of the reaction are slowand elevated temperatures are required to achieve reasonablereaction rates

MA can provide the means to substantially increasethe reaction kinetics of chemical reactions in general andthe reduction reactions in particular This is because therepeated cold welding and fracturing of powder particlesincrease the area of contact between the reactant powderparticles by bringing fresh surfaces to come into contactrepeatedly due to a reduction in particle size during millingThis allows the reaction to proceed without the necessityfor diffusion through the product layer As a consequencereactions that normally require high temperatures will occurat lower temperatures or even without any externally appliedheat In addition the high defect densities induced by MAaccelerate diffusion processes

Research 11

Depending on the milling conditions two entirely differ-ent reaction kinetics are possible [21 54]

(a) the reactionmay extend to a very small volumeduringeach collision resulting in a gradual transformationor

(b) if the reaction enthalpy is sufficiently high (and ifthe adiabatic temperature is above 1800 K) a self-propagating high-temperature synthesis (SHS) reaction(also known as combustion synthesis) can be initi-ated by controlling the milling conditions (eg byadding diluents) to avoid combustion reactions maybe made to proceed in a steady-state manner

The latter type of reactions requires a critical milling time forthe combustion reaction to be initiated If the temperature ofthe vial is recorded during the milling process the tempera-ture initially increases slowly with time After some time thetemperature increases abruptly suggesting that ignition hasoccurred and this is followed by a relatively slow decrease

In the above types of reactions themilled powder consistsof the pure metalM dispersed in the RO phase both usuallyof nanometer dimensions It has been shown that by anintelligent choice of the reductant the RO phase can beeasily leached out Thus by selective removal of the matrixphase by washing with appropriate solvents it is possibleto achieve well-dispersed metal nanoparticles as small as 5nm in size If the crystallinity of these particles is not highit can be improved by subsequent annealing without thefear of agglomeration due to the enclosure of nanoparticlesin a solid matrix Such nanoparticles are structurally andmorphologically uniform having a narrow size distributionThis technique has been used to produce powders of manydifferent metalsmdashAg Cd Co Cr Cu Fe Gd Nb NiTi V W Zn and Zr Several alloys intermetallics andnanocomposites have also been synthesized Figure 7 showstwo micrographs of nanocrystalline ceria particles producedby the conventional vapor synthesis and MA methods Eventhough the grain size is approximately the same in both thecases the nanoparticles produced by MA are very discreteand display a very narrow particle size distribution whilethose produced by vapor deposition are agglomerated andhave a wide particle size distribution

Novel materials have also been synthesized by MA ofblended elemental powders One of the interesting examplesin this category is the synthesis of plain carbon steels andstainless steels [55 56] Twomain advantages of this synthesisroute are that the steels can be manufactured at roomtemperature and that the steels will be nanostructured thusconferring improved properties But these powders needto be consolidated to full density for applications and itis possible that grain growth occurs during this processThe kinetics of grain growth can however be controlled byregulating the time and temperature combinations

72 Hydrogen Storage Materials Another important applica-tion of MA powders is hydrogen storage Mg is a lightweightmetal (the lightest structural material) with a density of174 gcm3 Mg and Mg-based alloys can store a significant

20 nm

50 nm

Figure 7 Transmission electron micrographs of ceria particlesproduced byMA (top) and vapor synthesis (bottom)methods Notethe severe agglomeration of the nanoparticles in the latter case

amount of hydrogen However being a reactive metal Mgalways contains a thin layer of oxide on the surface Thissurface oxide layer which inhibits asorption of hydrogenneeds to be broken down through activation A high-temperature annealing is usually done to achieve this ButMA can overcome these difficulties due to the severe plasticdeformation experienced by the powder particles duringMAA catalyst is also often required to accelerate the kinetics ofhydrogen and dehydrogenation of alloys

Mg can adsorb about 76 wt hydrogen andMg-Ni alloysmuch less However Mg is hard to activate its tempera-ture of operation is high and the sorption and desorptionkinetics are slow Therefore Mg-Ni and other alloys arebeing explored But due to the high vapor pressure of Mgand significant difference in the melting temperatures ofthe constituent elements (Mg = 650∘C and Ni = 1452∘C) itis difficult to prepare these alloys by conventional meltingand solidification processes Therefore MA has been used toconveniently synthesize these alloys since this is a completelysolid-state processing method An added advantage of MA isthat the product will have a nanocrystalline grain structureallowing the improvements described below [57 58]

Figure 8 shows the hydrogenation and dehydrogena-tion plots for conventional and nanocrystalline alloysAn additional plot for a situation where a catalyst isadded to the nanocrystalline material is also shown Fromthese two graphs the following facts become clear Firstlythe nanocrystalline alloys adsorb hydrogen much fasterthan conventional (coarse-grained) alloys even though the

12 Research

0 1 2 3 4 5 6 7 8 9 10time (min)

conventional

p = 8 bar

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

time (min)

nano + catalyst

nano + catalyst

nanocrystallinenanocrystalline

conventional

breakthrough in reaction kinetics due to nanostructure and use of catalyst

0

1

2

3

4

5

6

7

8

hydr

ogen

cont

ent (

wt

)

T = 300∘C p = 10-3 mbar

Figure 8 Comparison of the hydrogen sorption and desorption kinetics of Mg alloys with conventional and nanocrystalline grain sizesSignificant improvement in the desorption kinetics when a catalyst is added is worth noticing

amount of hydrogen adsorbed is the sameWhile the conven-tional alloys do not adsorb any significant amount of hydro-gen in about 10 min the allowable full amount of hydrogenis adsorbed in the nanocrystalline alloy Similar observationsare made by many other authors in many different alloysFurther the kinetics are slightly faster (not much different)when a catalyst is added to the nanocrystalline alloy Onthe other hand the kinetics appear to be very significantlyaffected during the desorption process For example thenanocrystalline alloys desorb hydrogen much faster thanthe conventional alloys Again while the conventional alloysdo not desorb any significant amount of hydrogen thenanocrystalline alloy desorbs over 20 of the hydrogenstored in about 10 min But the most significant effect wasobserved when a catalyst was added to the nanocrystallinealloyAll the adsorbedhydrogenwas desorbed in about 2minwhen a catalyst was added to the nanocrystalline alloy Thusnanostructure processing throughMAappears to be a fruitfulway of developing novel hydrogen storage materials [59]

Many novel complex hydrides based on light metals(Mg Li and Na) are being explored and the importantcategories of materials being investigated include the alanates(also known as aluminohydrides a family of compoundsconsisting of hydrogen and aluminum) borohydrides amideborohydrides amide-imide systems (which have a highhydrogen storage capacity and low operative temperatures)amine borane (compounds of borane groups having ammo-nia addition) and alane

73 Energy Materials The technique of MA has also beenemployed to synthesize materials for energy applicationsThese include synthesis of photovoltaic (PV) materials andultrafine particles for faster ignition and combustion pur-poses

One of the most common PV materials is CIGS acompound of Cu In Ga and Se with the compositionCu(InGa)Se2 Since PV cells have the highest efficiencywhen

the band gap for this semiconductor is 125 eV one will haveto engineer the composition for the ratio of Ga to In This isbecause the band gaps for CuInSe

2 and CuGaSe2 are 102 and170 eV respectively Accordingly a Ga(Ga+In) ratio of about03 is found most appropriate and the final ideal compositionof the PV material will be Cu097In07Ga03Se2

The traditional method of synthesizing this material isto prepare an alloy of Cu Ga and In by vapor depositionmethods and incorporate Se into this by exposure to Seatmosphere Consequently this is a complex process MAhowever can be a simple direct and efficient method ofmanufacturing Blended elemental powders of Cu In Gaand Se weremilled in a copper container for different periodsof time and the phase constitution was monitored throughX-ray diffraction studies It was noted that the tetragonalCu(InGa)Se

2phase has formed even aftermilling the powder

mix for 20 min Process variables such as mill rotation speedball-to-powder weight ratio (BPR) and milling time werevaried and it was noted that the ideal composition wasachieved at lowmilling speeds short milling times and a lowBPR (Table 2) [60] By fine-tuning these parameters it shouldbe possible to achieve the exact composition

Nanostructuredmetal particles have high specific surfaceareas and consequently they are highly reactive and can storeenergy This area has received a great impetus in recentyears since the combustion rates of nanostructured metalparticles aremuch higher than those of coarsemetal particlesThe particle size cannot be exceedingly small because thesurface may be coated with a thin oxide layer and make theparticles passive [61ndash63] MA is very well suited to preparesuch materials because the reactions are easily initiatedby impact or friction and at the same time milling isa simple scalable and controllable technology capable ofmixing reactive components on the nanoscaleThese reactivematerials could be used as additives to propellants explosivesand pyrotechnics as well asmanufacture of reactive structuralcomponents [63]

Research 13

Table 2 Chemical analysis (at) of the Cu-In-Ga-Se powder mechanically alloyed under different processing conditions

BPR Speed (rpm) Time (min) Cu In Ga Se201 300 120 3261 1523 653 4562201 300 240 3173 1563 671 4593101 150 40 2598 1698 725 4978Targeted (Cu

097In07Ga03Se2) 2443 1763 756 5038

Compared to the presently used liquid aviation fuelsmetal particles based on B Al andMg are effective HoweverB and Al have very high boiling points and high heats ofenthalpy On the other hand Mg has a low boiling point butalso low heat of enthalpy Therefore if Al and Mg could bemixed intimately on a nanometer scale one could hope tohave a high heat of enthalpy at reduced boiling temperaturesThat is Mg-Al alloy particles could be practical in achievinga high-energy density and practical ignitionboiling temper-atures MA is a very effective method to accomplish this

Al and Mg powders were blended together in differentproportions and milled Since the milled powders have arange of particle sizes they were sieved to obtain well-defined powder sizes They were then fluidized and thencombusted in a CH4-air premixed flame Both the ignitionand burning times of these metal particles decreased withdecreased particle sizes and were also shorter when the Mgcontent is higher (Figure 9) [64] Similar results have beenreported by other investigators

8 Powder Consolidation

The product of MA is in powder form Except in cases ofchemical applications such as catalysis when the productdoes not require consolidation widespread application ofMA powders requires efficient methods of consolidatingthem into bulk shapes Successful consolidation of MA pow-ders is a nontrivial problem since fully densematerials shouldbe produced while simultaneously retaining the nanometer-sized grains without coarsening or the amorphous phaseswithout crystallizing Conventional consolidation of powdersto full density through processes such as hot extrusion [65]and hot isostatic pressing (HIP) [66] requires use of highpressures and exposure to elevated temperatures for extendedperiods of time [67] Unfortunately however this resultsin significant coarsening of the nanometer-sized grains orcrystallization of amorphous phases and consequently thebenefits of nanostructure processing or amorphization arelost [68] On the other hand retention of finemicrostructuresrequires use of low consolidation temperatures and then itis difficult to achieve full interparticle bonding at these lowtemperatures Therefore novel and innovative methods ofconsolidating MA powders are required The objectives ofconsolidation of these powders are to (a) achieve full densifi-cation (without any porosity) (b) minimize microstructuralcoarsening (ie retention of fine grain size) andor (c)avoid undesirable phase transformations In fact the earlyresults of excellent mechanical properties and most specifi-cally superplasticity at room temperature [69] attributed to

20 Mg (tig)90 Mg (tig)

20 Mg (tb)90 Mg (tb)

20 40 60 80 100 1200Particle Size (m)

0

10

20

30

40

50

60

70

Igni

tion

Burn

ing

Tim

e (m

s)

Figure 9 Ignition and burning times as a function of particle sizefor the mechanically alloyed Al-20 and Al-90 at Mg powdersIn the insert tig and tb represent the ignition and burning timesrespectively

nanocrystalline ceramics have not been successfully repro-duced due to incomplete consolidation of the nanopowdersin the material tested Thus consolidation to full densityassumes even greater importance

Because of the small size of the powder particles (typicallya few micrometers even though the grain size is only afew nanometers) MA powders pose special problems Forexample such nanometric powders are very hard and strongThey also have a high level of interparticle friction Furthersince these powders have a large surface area they also exhibithigh chemical reactivity Therefore special precautions needto be taken to consolidate the metal powders to bulk shapes

Successful consolidation of MA powders has beenachieved by electrodischarge compaction plasma-activatedsintering shock (explosive) consolidation [70] HIP hydro-static extrusion strained powder rolling and sinter forging[71] By utilizing the combination of high temperature andpressure HIP can achieve a particular density at lowerpressure when compared to cold isostatic pressing or at lowertemperature when compared to sintering Because of theincreased diffusivity in nanocrystalline materials sintering(and therefore densification) takes place at temperaturesmuch lower than those in coarse-grained materials This islikely to reduce the grain growth Spark plasma sintering(SPS) has become the most popular technique to consolidatemetal powders especially those with ultrafine grain sizesdue to the fact that full density can be achieved at low

14 Research

SOLID LIQUID GAS

MILLING

USE POWDER AS-ISor Chemical Reaction

Thermal Treatment etcCONSOLIDATE TO SOLID

(Temperature Pressure)

Catalysts Pigments SolderComponents Ceramic Powder etc

Advanced materials with hightolerance to gaseous impurities

Figure 10 Alternatives to consolidation of metal powders Ifpowders are used as catalysts pigments solder components etcthen they need not be consolidated However if powders need tobe consolidated then they could be used in applications with hightolerance to gaseous impurities

temperatures and shorter times in comparison to severalothermethods [72 73] An excellent reviewbyGroza [71]maybe consulted for full details of the methods of consolidationand description of results

9 Powder Contamination

Amajor concern inmetal powders processed byMAmethodsis the nature and amount of impurities that get incorporatedinto the powder and contaminate it The small size of thepowder particles and consequent availability of large surfacearea formation of fresh surfaces during milling and wearand tear of the milling tools and the atmosphere underwhich the powder is milled all contribute to contaminationof the powder In addition to these factors one has toconsider the purity of the starting raw materials and themilling conditions employed (especially the milling atmo-sphere) Thus it appears as though powder contaminationis an inherent drawback of the method and that the milledpowder will always be contaminated with impurities unlessspecial precautions are taken to avoidminimize them Themagnitude of powder contamination appears to depend onthe time of milling intensity of milling atmosphere in whichthe powder is milled nature and size of the milling mediumand differences in the strengthhardness of the powder andthe milling medium and the container

Several attempts have been made in recent years to min-imize powder contamination during milling An importantpoint to remember is that if the starting powders are highlypure then the final product is going to be clean and purewith minimum contamination from other sources providedthat proper precautions are taken during milling It is alsoimportant to keep inmind that all applications do not requireultraclean powders and that the purity desired will depend onthe specific application (Figure 10) But it is a good practiceto minimize powder contamination at every stage and ensurethat additional impurities are not picked up subsequentlyduring handling andor consolidation

One way ofminimizing contamination from the grindingmedium and themilling container is to use the samematerial

for the container and the grinding medium as the powderbeing milled Thus one could use copper balls and coppercontainer formilling copper and copper alloy powders [60] Ifa container of the same material to be milled is not availablethen a thin adherent coating on the internal surface of thecontainer (and also on the surface of the grinding medium)with the material to be milled will minimize contaminationThe idea here is to mill the powder once allowing the powderto be coated onto the grindingmedium and the inner walls ofthe container The milled loose powder is then discarded anda fresh batch of powder is milled but with the old grindingmedia and container In general a simple rule that shouldbe followed to minimize contamination from the millingcontainer and the grinding medium is that the container andgrindingmedium should be harderstronger than the powderbeing milled

Milling atmosphere is perhaps the most important vari-able that needs to be controlled Even though nominally puregases are used to purge the glove box before loading andunloading the powders even small amounts of impurities inthe gases appear to contaminate the powders especially whenthey are reactive The best solution one could think of willbe to place the mill inside a chamber that is evacuated andfilled with high-purity argon gas Since the whole chamberis maintained under argon gas atmosphere (continuouslypurified to keep oxygen and water vapor below 1 ppm each)and the container is inside the mill which is inside thechamber contamination from the atmosphere is minimum

Contamination from PCAs is perhaps the most ubiqui-tous Since most of the PCAs used are organic compoundswhich have low melting and boiling points they decomposeduring milling due to the heat generated The decomposi-tion products consisting of carbon oxygen nitrogen andhydrogen react with the metal atoms and form undesirablecarbides oxides nitrides etc

10 Concluding Remarks and Future Prospects

We have only briefly touched upon the scientific and tech-nological aspects of MA materials along with some of theconcerns such as powder contamination and consolidationissues The historical development of MA during the last 50years or so can be divided into threemajor periods (Figure 11)The first period covering the first twenty years (from 1966up to about 1985) was mostly concerned with the devel-opment and production of oxide-dispersion-strengthened(ODS) superalloys for applications in the aerospace indus-try Several alloys with improved properties based onNi and Fe were developed and found useful applicationsThese included the MA754 MA760 MA956 MA957 andMA6000 [74] A more recent survey of application of MAto steel is also available [75] The second period of aboutfifteen years covering from about 1986 up to about 2000witnessed advances in the fundamental understanding ofthe processes that take place during MA Along with thisa large number of dedicated conferences were held andthere was a burgeoning of publication activity Comparisonshave also been frequently made between MA and othernonequilibriumprocessing techniquesmore specifically RSP

Research 15

1965 19851975 1995 2005

Production of ODS Alloys

Novel Phase SynthesisDevelopment of BasicUnderstanding of MA

Nanostructures andNew Applications

2015

Figure 11 Schematic showing the different periods of development in the field of mechanical alloying (MA) and applications ofMA productssince its inception in 1965

It was shown that themetastable effects achieved by these twononequilibrium processing techniques are similar Revival ofthe mechanochemical processing (MCP) took place and avariety of novel substances were synthesized Several mod-eling studies were also conducted to enable prediction of thephases produced or microstructures obtained although withlimited success The third period starting from about 2001saw a reemergence of the quest for new applications of theMA materials not only as structural materials but also forchemical applications such as catalysis and hydrogen storagewith the realization that contamination of themilled powdersis the limiting factor in the widespread applications of theMA materials Innovative techniques to consolidate the MApowders to full density while retaining the metastable phases(including glassy phases andor nanostructures) in themwerealso developed All these are continuing with the ongoinginvestigations to enhance the scientific understanding of theMA process

At present activities relating to both the science andtechnology of MA have been going on simultaneously Whilesome groups have been focusing on developing a betterunderstanding of the process and optimizing the processvariables to achieve a certain phase or phase combinationin the alloy system others have been concentrating onexploiting this technique for different applications It hasalso come to be realized that the MA process can be usedto recycle used and discarded materials to produce value-added materials for subsequent consumption They can bepulverized separated and used to mix with other powdersto produce composites The bottom line is that newer andimproved materials are being continuously developed fornovel applications

Conflicts of Interest

The author declares that he has no conflicts of interest

References

[1] K H J Buschow R W Cahn M C Flemings B Ilschner E JKramer and SMahajan EdsEncyclopedia ofMaterials Scienceand Technology Elsevier 2001

[2] C Suryanarayana EdNon-equilibrium Processing of MaterialsPergamon Oxford UK 1999

[3] C Suryanarayana ldquoMetallic glassesrdquo Bulletin of Materials Sci-ence vol 6 no 3 pp 579ndash594 1984

[4] C Suryanarayana and A Inoue Bulk Metallic Glasses CRCPress Boca Raton Fla USA 2nd edition 2018

[5] H R Trebin Ed Quasicrystals Structure and Physical Proper-ties Wiley-VCH Weinheim Germany 2003

[6] C Suryanarayana and H Jones ldquoFormation and characteristicsof quasicrystalline phases A reviewrdquo International Journal ofRapid Solidification vol 3 pp 253ndash293 1988

[7] H Gleiter ldquoNanocrystalline materialsrdquo Progress in MaterialsScience vol 33 no 4 pp 223ndash315 1989

[8] C Suryanarayana ldquoRecent developments in nanostructuredmaterialsrdquo Advanced Engineering Materials vol 7 no 11 pp983ndash992 2005

[9] M C Gao J-W Yeh P K Liaw and Y Zhang Eds High-Entropy Alloys Fundamentals and Applications Springer 2016

[10] J S Benjamin ldquoMechanical alloying ndash History and futurepotentialrdquo in Advances in Powder Metallurgy and ParticulateMaterials J M Capus and R M German Eds vol 7 ofNovel Powder Processing pp 155ndash168 Metal Powder IndustriesFederation Princeton NJ USA 1992

[11] M P Phaniraj D-I Kim J-H Shim and Y W CholdquoMicrostructure development in mechanically alloyed yttriadispersed austenitic steelsrdquo Acta Materialia vol 57 no 6 pp1856ndash1864 2009

[12] A Hirata T Fujita Y R Wen J H Schneibel C T Liu and MW Chen ldquoAtomic structure of nanoclusters in oxide-dispersionstrengthened steelsrdquo Nature Materials vol 10 no 12 pp 922ndash926 2011

[13] M K Miller C M Parish and Q Li ldquoAdvanced oxide disper-sion strengthened and nanostructured ferritic alloysrdquoMaterialsScience and Technology (United Kingdom) vol 29 no 10 pp1174ndash1178 2013

[14] N Oono Q X Tang and S Ukai ldquoOxide particle refinementin Ni-based ODS alloyrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 649 pp 250ndash253 2016

[15] D B Witkin and E J Lavernia ldquoSynthesis and mechanicalbehavior of nanostructured materials via cryomillingrdquo Progressin Materials Science vol 51 no 1 pp 1ndash60 2006

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

4 Research

more brittle constituents tend to become occluded by theductile constituents and trapped in the compositeThe work-hardened elemental or composite powder particles may frac-ture at the same timeThese competing events of coldwelding(with plastic deformation and agglomeration) and fracturing(with size reduction) continue repeatedly throughout themilling period Eventually at the steady-state condition arefined and homogenized microstructure is obtained andthe composition of the powder particles is the same as theproportion of the starting constituent powders

Along with the cold welding event described above somepowder may also coat the grinding medium andor the innerwalls of the container A thin layer of the coating is beneficialin preventing wear and tear of the grinding medium and alsoin preventing contamination of the milled powder with thedebris But a too thick layer will result in lower yield and alsopossible compositional inhomogeneity of the powder whichshould be avoided

The generally accepted explanation for alloying to occurfrom blended elemental powders and formation of differenttypes of phases is that a very fine and intimate mixture ofthe components (often lamellar if the constituent elementsare sufficiently ductile) is formed after milling if not thefinal product Alloy formation is facilitated through crys-talline defects (grain boundaries dislocations stacking faultsvacancies and others) introduced into thematerial due to theintense cold working operation which act as fast diffusionpaths and a slight rise in the powder temperature duringmilling as a result of frictional forces and impact of thegrinding balls against other balls and the surfaces of thecontainer If the final desired phase had not been formeddirectly by MA then a short annealing treatment at anelevated temperature was found to promote diffusion andconsequently alloy formation Accordingly it was shown thatby a proper choice of the process parameters during MA andby choosing an appropriate alloy composition it is possibleto produce a variety of alloys starting from metal powdersThe thermodynamic criterion that the phase with the lowestfree energy under the continuous deformation conditionswould be the most ldquostablerdquo phase has been found to be validA significant attribute of MA has been the ability to alloymetals even with positive heats of mixing that are difficult orimpossible to alloy otherwise [18 19]

The effects of MA are generally categorized under twogroups(1) Constitutional Changes Milling of the blended ele-

mental powders could lead to the formation of solid solu-tions (both equilibrium and supersaturated) intermetallicphases (equilibrium metastable (high temperature andorhigh pressure) and quasicrystalline) and amorphous phasesin alloy systems The actual type of phase formed could bedifferent depending on the alloy system its composition andthe milling conditions employed A number of metastablealloys have been produced this way(2) Microstructural Changes As a result of MA the

processed materials could develop ultrafine-grained andnanostructured phases In addition uniform dispersion ofa large volume fraction of very fine oxide or other ceramicparticles can be achieved in different matrices which is

either impossible or difficult by other processes These novelnanocomposites can exhibit properties far superior to thosethat have conventional grain sizes and sometimes evencompletely new behavior [16 17 20]

In many cases chemical reactions have also been foundto occur leading to the production of pure metals from theirores synthesis of alloys and formation of novel materialsand microstructures Such a process has been referred to asmechanochemical processing [21] All these effects have beenwell documented in the literature andTable 1 summarizes theimportant attributes of MA

Before discussing the synthesis of advanced materialsand their applications let us first describe the constitutionalchanges that can take place in MA materials

3 Constitutional Changes

The literature on MA has several examples of the differentequilibrium and nonequilibrium phases produced in differ-ent alloy systems An exhaustive list of the results (up to dateat the time of compilation) is available in [16 17] and the listis constantly growingWewill however briefly highlight heresome of the more recent and salient features in the followingparagraphs

31 Solid Solutions Solid solution alloys exhibit highstrength changes in density and suppression or eliminationof undesirable second phases Increasing the strength ofalloys through precipitation of a second phase from asupersaturated solid solution during aging (precipitationhardening) has been known to materials scientists for overa hundred years Further since the strengthening effect isrelated to the size and volume fraction of the second phaseparticles and since both of them could be controlled bythe extent of supersaturation and the aging parametersachievement of extended solid solubility limits in alloysystems is very desirable Solid solutionsmdashboth equilibriumand supersaturatedmdashare still being produced [22ndash24]

Rules similar to those developed by Hume-Rothery forsolid solution formation in binary noble-metal alloy systemsunder equilibrium conditions also seem to be generallyapplicable to supersaturated solutions obtained by MA eventhough exceptions have been noted in some cases [25 26]

As will be described later the mechanically alloyed mate-rials exhibit very fine grain sizes often reaching nanometerlevels Since these nanocrystalline materials contain a largevolume fraction of atoms in the grain boundaries thediffusivity of solute atoms is significantly enhanced [27] andthis would lead to increases in the solid solubility levelsAs a general rule of thumb solute elements which exhibitlimited solubility under equilibrium conditions exhibit highsolubility levels under the nonequilibrium conditions of MAEven though solid solubility extensions have been achievedby other nonequilibrium processing techniques as well (egrapid solidification processing vapor deposition and laserprocessing of materials) there appear to be differences in thesolid solubility extensions achieved by MA and other meth-ods [26] An important observation made in MA materials

Research 5

Table 1 Attributes of mechanical alloying

(1) Production of fine dispersion of second phase (usually oxide) particles(2) Extension of solid solubility limits(3) Refinement of grain sizes down to nanometer range(4) Synthesis of novel crystalline and quasicrystalline phases(5) Development of amorphous (glassy) phases(6) Disordering of ordered intermetallics(7) Possibility of alloying of difficult to alloy elementsmetals

(8)Inducement of chemical (displacement) reactions at low temperatures for (a) Mineral and Wasteprocessing (b) Metals refining (c) Combustion reactions and (d) Production of discrete ultrafineparticles

(9) Scaleable process

is that the solute atoms segregate to grain boundaries fromwhere they could diffuse into grains and form solid solutions

32 Intermetallic Phases Intermetallic phases (or inter-metallics for short) are typically ordered compounds existingat simple ratios of the two ormore elements involved Becauseof their ordered nature they are strong and hard stiffhave high melting points and also exhibit other interestingbehaviors such as excellent corrosionoxidation resistanceHowever they usually suffer from low ambient temperatureductility again due to the ordered nature of the compoundsA new family of intermetallics showing high ductility atroom temperature and based on rare-earth elements hasbeen recently developed [28 29] Since MA is known to (a)disorder the ordered lattices (b) refine grain sizes down tonanometer levels and (c) also alter the crystal structures allthese effects could induce some amount of plasticity into thematerial In fact the potential to ductilize the normally brittleintermetallics has been the driver for accelerated researchinto the MA of intermetallics

A number of intermetallics (both equilibrium andmetastable) and high-temperature or high-pressure phaseshave been synthesized byMAstarting fromelemental powderblends [16 17 24 30] More interestingly quasicrystallinephases with the forbidden crystal symmetries first reportedin rapidly solidified Al-Mn alloys have also been synthesizedby MA in a number of alloy systems [5 31] Applicationsfor these exotic materials have been limited Based ontheir high hardness low friction and low surface reactivityquasicrystalline materials have been used to coat nonstickfrying pans Also steels reinforcedwith small quasicrystallineparticles are very strong and have been used for acupunctureand surgery dental instruments and razor blades [32]

The most significant change that occurs in MA materialsis the formation of amorphous phases and this will bediscussed in the next section

4 Metallic Glasses

Amorphous alloys are materials that lack crystallinity andhave a random arrangement of atoms Because of thisextreme disorder these materials exhibit a very interestingcombination of high strength good corrosion resistance

and useful electrical and magnetic properties These novelmaterials have been traditionally produced starting from thevapor or liquid states Amorphous or noncrystalline alloysbased onmetals are commonly referred to as metallic glassesAt present there is worldwide activity in the area of metallicglasses and bulk metallic glasses [4 33]

Metallic glasses were first produced by rapid solidificationprocessing (RSP) in 1960 in the form of thin ribbons bysolidifying the alloy melt at rates gt106 Ks [34] Subsequentdevelopments include the synthesis of bulk metallic glasseswith a large section thickness of a few centimeters This ispossible in multicomponent alloys which could be solidifiedinto the glassy condition at low critical cooling rates of lt102Ks [4] The current record is an 80-mm diameter glassy rodproduced by solidification in a Pd

425Cu30Ni75P20

alloy (thesubscripts represent the atomic percentage of the elementsin the alloy) [35] While solidification methods produce themetallic glasses either in thin ribbon or bulk form directlyfrom the liquid the amorphous phases are in powder formbyMAmethodsThese need to be consolidated to bulk shapefor subsequent characterization and applications

Amorphous alloys were synthesized by MA first in anY-Co intermetallic compound [36] and later in a blendedelemental Ni-Nb powder mixture [37] Amorphous phaseshave now been synthesized in a very large number ofalloy systems starting from either blended elemental powdermixtures intermetallic compounds or mixtures of them[38] Comparisons have been frequently made between theamorphous alloys produced by MA and RSP techniquesand both similarities and differences have been noted [26]Further the mechanism of glass formation appears to besignificantly different depending uponwhether it is producedby MA or RSP methods

Several different criteria have been proposed to explainthe glass-forming ability of alloys produced by solidificationmethodsThese include the reduced glass transition tempera-ture119879rg =119879g119879l where119879g and119879l represent the glass transitionand liquidus temperature of the alloy respectively presenceof deep eutectics in the phase diagrams and significantdifferences in the atomic sizes of the constituent elementsleading to development of strain energy in the solid solutionWhen the strain energy exceeds a critical value determinedby the size and amount of the solute atoms it destabilizes the

6 Research

crystal lattice and results in the formation of an amorphousphase With the introduction of bulk metallic glasses nearlythirty years ago a very large number of new criteria mostlybased on the glass transformation temperatures (119879g 119879x and119879l where 119879x represents the crystallization temperature of theamorphous alloy) have been proposed In spite of a very largenumber of criteria proposed [4] it has not been possible yetto accurately predict the glass-forming ability of alloys [39]

There are some significant differences in the formation ofmetallic glasses by the MA and RSP methods For examplemetallic glasses are obtained by MA in almost every alloysystem provided that sufficient energy has been stored in thepowder But metallic glasses are obtained by RSP methodsonly in certain composition rangesmdashabout 20 at metalloidcontent in metal-metalloid systems and different composi-tions in metal-metal systems mostly at compositions cor-responding to deep eutectics Additionally glass formationby MA occurs by the accumulation of crystal defects whichraises the free energy of the crystalline phase above that of thehypothetical glassy phase under which conditions the crys-talline phase becomes destabilized On the other hand forglass formation by the RSP methods the critical cooling ratefor glass formation needs to be exceeded so that formationof the crystalline nuclei is completely suppressed Since MAis a completely solid-state powder processing technique thecriteria for amorphous phase formation should be differentfrom those of solidification methods A new criterion wasproposed that is the number of intermetallics present inthe constituent phase diagrams determines the glass-formingability of alloys processed by MAmdashthe more the number ofintermetallics in the constituent phase diagrams the easier itis to amorphize the powder blend [40]

Amorphization by MA occurs when the free energy ofthe hypothetical amorphous phase 119866A is lower than that ofthe crystalline phase G119862 ie 119866A lt 119866C A crystalline phasenormally has a lower free energy than the amorphous phaseBut its free energy can be increased by introducing a varietyof crystal defects such as dislocations grain boundaries andstacking faults If an intermetallic has formed then additionalenergy can be introduced by disordering the crystal lattice Bythis approach it is then possible to obtain a situation when

119866A lt 119866C + 119866D (1)

where 119866D is the free energy increase due to introductionof crystal defects It has been shown that the presence ofintermetallics in an alloy system can significantly increasethe energy contributing to amorphization This is due totwo important effects Firstly disordering of intermetallicscontributes an energy of about 15 kJmol to the systemSecondly a slight change in the stoichiometry of the inter-metallic increases the free energy of the system drasticallyAdditionally grain size reduction contributes about 5 kJmolFurther disordering of intermetallics has also been shownto be possible by heavy deformation Since MA reduces thegrain size to nanometer levels and also disorders the usuallyordered intermetallics the energy of the milled powders issignificantly raised In fact it is raised to a level above that ofthe hypothetical amorphous phase and thus formation of theamorphous phase is favored over the crystalline phase

41 Mechanical Crystallization It was reported that an amor-phous phase formed by milling transforms into a crystallinephase on continued milling and that this phenomenonhas been referred to as mechanical crystallization [41 42]The process of mechanical crystallization is very unusualand offers many opportunities to design novel alloys withimproved properties For example it will be possible toproduce the alloy in either a fully amorphous amorphous +nanocrystalline composite or a completely crystalline statewith different grain sizes depending upon the time ofmillingor the temperature and time of subsequent annealing of theamorphous alloy

Figure 3(a) shows the XRD patterns of the blended ele-mental powder mixture of the composition Fe42Ge28Zr10B20mechanically alloyed for 10 and 60 h [42] While the pat-tern at 10 h of milling clearly shows the presence of anamorphous phase the pattern at 60 h of milling showsthe presence of a crystalline phase superimposed over thebroad and diffuse halo representing the amorphous phaseThe crystalline phase has been identified as the 120572-Fe solidsolution containing the solute elements present in the blendThis 120572-Fe phase is the first phase to crystallize from theamorphous alloy (as in primary crystallization) and externalannealing of the milled powder showed that the amount ofthe 120572-Fe increased along with the presence of other inter-metallics Eutectic crystallization (simultaneous formation oftwo crystalline phases) has also been observed in other alloysystems

Mechanical crystallization has also been observed inseveral other alloy systems that formed the amorphous phaseon MA In fact it is now becoming apparent that thisphenomenon of mechanical crystallization is not as rare asit was once thought to be

Many reasons have been suggested for the formation of acrystalline phase after the formation of an amorphous phaseduring MA These include (a) rise in temperature to a levelabove that of the crystallization temperature of the amor-phous alloy (b) powder contamination due to which a stablecrystalline impurity phase forms (c) phenomenon of inversemelting and (d) basic thermodynamic considerations It nowappears clear that the basic thermodynamic stabilities of thedifferent phases under the different conditions of milling areresponsible for mechanical crystallization Such a situationcan be explained with reference to a free energy versuscomposition diagram

Figure 3(b) is a schematic diagram showing the variationof free energy with composition for the Fe42X28Zr10B20 (X =AlGe andNi) alloy systems investigatedThepossible (stableand metastable) constitution in this system is (a) blendedelemental (BE) powder mixture (b) 120572-Fe the solid solutionof all the alloying elements in Fe (c) an amorphous phase (d)intermetallic phases and (e) different combinations of thesephases The stability of any phase will be determined by itsrelative position in the free energy vs composition plotmdashthelower the free energy the more stable the phase is Since it ispossible to have a large number of intermetallic phases in thismulticomponent system and since it is difficult to indicateeach of them separately for simplicity all of them are groupedtogether as ldquointermetallicsrdquo

Research 7Fr

ee E

nerg

y

Solute Concentration (at)20 40 60 80Fe 100

BE powder

amorphous

-Fe

Intermetallics

1

23

45

Inte

nsity

30 40 502-Theta

(a)

(b)

10 h

60 h

60 70 80 90

1

Figure 3 (a) X-ray diffraction patterns of the Fe42Ge28Zr10B20

powder blend milled for 10 and 60 h While the powder milledfor 10 h shows the presence of an amorphous phase the powdermilled for 60 h shows the presence of a crystalline phase coexistingwith the amorphous phase suggesting that the amorphous phase hascrystallized (b) Hypothetical free energy vs composition diagramto explain the mechanism of mechanical crystallization in theFe42X28Zr10B20systemNote that point ldquo1rdquo represents the free energy

of the blended elemental powders Similarly point ldquo2rdquo representsformation of the 120572-Fe solid solution containing all the alloyingelements in Fe point ldquo3rdquo formation of the homogeneous amorphousphase point ldquo4rdquo a mixture of the amorphous phase with a differentsolute content (amorphous) from that at ldquo3rdquo and the solid solution120572-Fe with different solute content and point ldquo5rdquo the equilibriumsituation when the solid solution and intermetallic phases coexist

The free energy of the BE powder mixture is indicated bypoint ldquo1rdquo which represents the point of intersection of thecomposition vertical with the line joining the free energiesof pure Fe and the ldquosoluterdquo On milling this powder a solidsolution phase containing all the solute elements in Fe (ora mixture of intermetallics and a solid solution in somecases) is seen to form and its free energy is indicated bypoint ldquo2rdquoThe solid solution forms because it has a lower free

energy than the BE powder mixture Since MA introduces avariety of crystal defects the crystalline phases in the milledpowders will contain excess energyThis energy will continueto increase with milling time and reach a value which isabove that of the metastable amorphous phase Thus theamorphous phase gets stabilized (point ldquo3rdquo) On primarycrystallization of the amorphous phase the new constitutionwill be a mixture of the 120572-Fe solid solution (or intermetallics)and the amorphous phase which now has a compositiondifferent from that of the original amorphous phase Furtherthe new solid solution phase also has a composition differentfrom the original 120572-Fe phase The free energy of the mixtureof this solid solution and the amorphous phase indicatedby ldquopoint 4rdquo will have a free energy lower than that of theamorphous phase The equilibrium mixture of the 120572-Fe solidsolution and ldquointermetallicsrdquo will have the lowest free energyof all the phase mixtures as indicated by point ldquo5rdquo in thefigure The process of mechanical crystallization is uniqueto the MA process and can be achieved only on continuedmilling of the amorphous powder ormilling of an amorphousalloy obtained by other methods

5 Nanostructured Materials

Another important attribute of MA has beenmicrostructuralrefinement The most common observation is that grainrefinement takes place resulting in the formation of very finegrains often down to nanometer levels A two-phase alloyunder equilibrium conditions can transform to a single-phasealloy due to formation of a supersaturated solid solution Anamorphous alloy can crystallize under the action of severeplastic deformation Most usefully one can also introducea high volume fraction of fine second phase particles andobtain a composite with a uniform distribution of thesecond phase particles By a proper choice of the size ofthe matrix andor the reinforcement phase one can producenanocomposites a very fruitful area for investigations Infact MA has been the most popular technique to synthesizenanostructured materials and nanocomposites in a numberof alloy systems

It has been frequently demonstrated that mechanicallyalloyed materials show powder particle refinement as wellas grain refinement as a function of the milling time Thesesizes decrease very rapidly in the early stages of milling andmore gradually later Eventually in almost all the cases thegrain size of the mechanically alloyed materials is in thenanometer region unless the powder becomes amorphousThe minimum grain size achieved has been reported tobe a few nanometers ranging typically from about 5 to50 nm but depending on the material and processingconditions For example it was reported that the grain sizedecreases with increasing milling energy higher ball-to-powder weight ratio and lower temperatures As is wellknown nanostructuredmaterials exhibit high strength goodductility excellent sintering characteristics and interestingelectrical andmagnetic properties [7 8 27 43]The ease withwhich nanostructured materials can be synthesized is onereason why MA has been extensively employed to producenanocrystalline and nanocomposite materials

8 Research

From high-resolution transmission electron microscopy(TEM) observations it was noted that in the early stages ofmilling due to the high deformation rates experienced bythe powder deformation was localized within shear bands[44] These shear bands which contain a high density ofdislocations have a typical width of approximately 05 to 10120583m Small grains with a diameter of 8ndash12 nm were seenwithin the shear bands and electron diffraction patternssuggested significant preferred orientation With continuedmilling the average atomic level strain increased due toincreasing dislocation density and at a certain dislocationdensity within these heavily strained regions the crystaldisintegrated into subgrains that are separated by low-anglegrain boundaries This resulted in a decrease of the latticestrain The subgrains formed this way were of nanometerdimensions and are often between 20 and 30 nm

On further processing deformation occurred in shearbands located in previously unstrained parts of the materialThe grain size decreased steadily and the shear bands coa-lesced The small-angle boundaries are replaced by higherangle grain boundaries implying grain rotation as reflectedby the absence of texture in the electron diffraction patternsand random orientation of the grains observed from thelattice fringes in the high-resolution TEMmicrographs Con-sequently dislocation-free nanocrystalline grains are formedThis is the currently accepted mechanism of nanocrystalformation in MA processed powders

The grain size of the milled materials decreases withmilling time and reaches a saturation level when a balance isestablished between the fracturing and cold welding eventsThis minimum grain size 119889min is different depending on thematerial and milling conditionsThe value of 119889min achievableby milling is determined by the competition between theplastic deformation via dislocation motion that tends todecrease the grain size and the recovery and recrystallizationbehavior of the material that tends to increase the grain sizeThis balance gives a lower bound for the grain size of puremetals and alloys

The 119889min obtained is different for different metals and isalso found to vary with the crystal structure (Figure 4) Inmost of the metals the minimum grain size attained is inthe nanometer dimensions But metals with a body-centeredcubic (BCC) crystal structure reach much smaller valuesin comparison to metals with the other crystal structuresrelated to the difficulty of extensive plastic deformation andconsequent enhanced fracturing tendency during millingCeramics and intermetallic compounds are much harderand usually more brittle than the metals on which they arebased Therefore intuitively one expects that 119889min of thesecompounds is smaller than those of the pure metals but thisdoes not appear to be the case always

It was also reported that 119889min decreases with an increasein the melting temperature of the metal [45] As shown inFigure 4 this trend is amply clear in the case of metalswith close-packed structures (face-centered cubic (FCC) andhexagonal close-packed (HCP)) but not so clear in metalswith a BCC structure Another point of interest is that thedifference in grain size is much less among metals thathave high melting temperatures the minimum grain size is

0

5

10

15

20

25

FCCBCCHCP

Min

imum

Gra

in S

ize (

nm)

0 2000 30001000Melting Temperature (K)

Figure 4 Variation of minimum grain size with temperature inmechanically milled FCC HCP and BCC metals

virtually constant Thus for the HCP metals Co Ti Zr Hfand Ru the minimum grain size is almost the same eventhough themelting temperatures vary between 1495∘C for Coand 2310∘C for Ru

Analysis of the existing data on 119889min of mechanicallymilled pure metals shows that the normalized minimumgrain size 119889minb where b is the Burgers vector of thedislocations decreases with increasing melting temperatureactivation energy for self-diffusion hardness stacking faultenergy bulk modulus and the equilibrium distance betweentwo edge dislocations The 119889min value is also affected bymilling energy (the higher the milling energy the smaller thegrain size) milling temperature (the lower the milling tem-perature the smaller the grain size) and alloying additions(the minimum grain size is smaller for solid solutions thanfor pure metals since solid solutions are harder and strongerthan the pure metals on which they are based)

It was suggested that 119889min is determined by the minimumgrain size that can sustain a dislocation pile-up within a grainand by the rate of recovery Based on the dislocation pile-up model the critical equilibrium distance between two edgedislocations in a pile-up L119888 (which could be assumed to bethe crystallite or grain size inmilled powders) was calculated[46] using the equation

119871119888=3119866119887

(1 minus ])119867(2)

where G is the shear modulus 119887 is the Burgers vector ] isthe Poissonrsquos ratio and H is the hardness of the materialAccording to the above equation increased hardness resultsin smaller values of 119871c (grain size) and an approximate linearrelationship was observed between 119871c and the minimumgrain size obtained by milling of a number of metals Otherattempts have also beenmade to theoretically predict 119889min onthe basis of thermodynamic properties of materials [47]

Research 9

6 Nanocomposites

Achievement of a uniform distribution of the reinforcementin a matrix is essential to the achievement of good anduniform mechanical properties in composites Further themechanical properties of the composite tend to improvewith increasing volume fraction andor decreasing particlesize of the reinforcement [48] Traditionally a reasonablylarge volume fraction of the reinforcement could be addedif the size of the reinforcement is large (on a micrometerscale or larger) But if the reinforcement size is very fine(of nanometer dimensions) then the volume fraction addedis typically limited to about 2 to 4 However if a largevolume fraction of nanometer-sized reinforcement can beintroduced the mechanical properties of the composite arelikely to be vastly improved Therefore several investigationshave been undertaken to produce improved nanocompositesthrough MA [20]

Composites are traditionally produced by solidificationprocessing methods since the processing is inexpensive andlarge tonnage quantities can also be produced But it isnot easy to disperse fine reinforcement particles in a liquidmetal since agglomeration of the fine particles occurs andclusters are produced Further wetting of the particles ispoor and consequently even the most vigorous stirring isnot able to break the agglomerates Additionally the finepowders tend to float to the top of the melt during processingof the composites through solidification processing Thussolid-state processing methods such as MA are effective indispersing fine particles (including nanoparticles) An addedadvantage is that since no melting is involved even a largevolume fraction of the reinforcement can be introduced

Some of the specific goals sought in processing ofnanocomposite materials throughMA include incorporationof a high volume percentage of ultrafine reinforcements andachievement of high ductility and even superplasticity insome composites It is also possible that due to the increasedductility the fracture toughness of these composites couldbe higher than that of their coarse-grained counterparts Inline with these goals we have in recent times accomplished(a) achievement of a very uniform distribution of the rein-forcement phases in different types of matrices includingdispersion of a high volume fraction of Al2O3 in Al [49] andof Ti5Si3in 120574ndashTiAl [50 51] and (b) synthesis of MoSi

2+Si3N4

composites for high-temperature applications [52] amongothers However we will describe here only the developmentof Ti5Si3+120574ndashTiAl composites as a typical example

Lightweight intermetallic alloys based on 120574-TiAl arepromising materials for high-temperature structural appli-cations eg in aircraft engines or stationary turbines Eventhough they have many desirable properties such as highspecific strength and modulus both at room and elevatedtemperatures and good corrosion and oxidation resistancethey suffer from inadequate room temperature ductility andinsufficient creep resistance at elevated temperatures espe-cially between 800 and 850∘C an important requirement forelevated temperature applications of these materials There-fore current research programs have been addressing thedevelopment of high-temperature materials with adequate

020406080

100120140160180200

Engi

neer

ing

stres

s (M

Pa)

20 40 60 80 100 120 140 1600Engineering strain ()

-5950

∘C 4x10

-4950

∘C 4x10

-41000

∘C 4x10x10

(a)

(b)

Figure 5 (a) Scanning electronmicrograph of the 120574-TiAl + 60 volTi5Si3composite specimen showing that the twophases are very uni-

formly distributed in the microstructure Such a microstructure isconducive to achieving superplastic deformation under appropriateconditions of testing (b) Tensile stress-strain curve of 120574-TiAl + 60vol Ti

5Si3composite showing that superplastic deformation was

achieved at 950∘C with a strain rate of 4x10minus5 sminus1 and at 1000∘C witha strain rate of 4x10minus4 sminus1

room temperature ductility for easy formability and abilityto increase the high-temperature strength by a suitable heattreatment or alloying additions to obtain sufficient creepresistance at elevated temperatures

Composites of 120574-TiAl and 120585-Ti5Si3phase with the volume

fractions of the 120585-Ti5Si3phase varying from 0 to 60 vol

were produced by MA of a combination of the blendedelemental and prealloyed intermetallic powders Fully denseand porosity-free compacts were produced by hot isostaticpressing with the resulting grain size of each of the phasesbeing about 300-400 nm Figure 5(a) shows scanning elec-tron micrographs of the 120574-TiAl+60 vol 120585-Ti5Si3 compositeshowing that the two phases are very uniformly distributedthroughout the microstructure Materials exhibiting such amicrostructure (roughly equal amounts of the two phasessmall grain sizes and a uniform distribution of the twophases) are expected to be deformed superplastically Totest this hypothesis both compression and tensile testingof these composite specimens were conducted at differenttemperatures and strain rates From the tensile stress-strain

10 Research

Pure Metals Alloysand Compounds

NanostructuredMaterials

MECHANICALALLOYING

ODS Superalloys Supercorroding

Alloys

Synthesis of Novel Phases PVDTargets Solders Paints Food Heaters

Biomaterials Waste Utilization etc

Hydrogen Storage MagneticMaterials Catalysts Sensors etc

Figure 6 An overview of the applications of the MA process

plots of the 120574-TiAl+60 vol 120585-Ti5Si3composite specimens

shown in Figure 5(b) it is clear that the specimens testedat 950∘C with a strain rate of 4times10minus5 sminus1 and at 1000∘Cwith a strain rate of 4times10minus4 sminus1 exhibited large ductilityof nearly 150 and 100 respectively Considering that thiscomposite is based on a ceramic material (Ti

5Si3) this is a

very high amount of deformation suggestive of superplasticdeformation Final proof is provided by TEM investiga-tions that confirm the continued stability of the equiaxedmicrostructure after deformation

7 Applications

As has been frequently pointed out in the literature thetechnique ofMAwas developed out of an industrial necessityin 1965 to produce ODS nickel-based superalloys Sincethen a number of new applications have been exploredfor mechanically alloyed materials (Figure 6) Apart fromthe ODS alloys which continue to be used in the high-temperature applications area MA powders have also beenused for other applications and newer applications are alsobeing explored These novel applications include direct syn-thesis of pure metals and alloys from ores and scrap mate-rials super-corroding alloys for release into ocean bottomsdevelopment of PVD (physical vapor deposition) targetssolders paints etc hydrogen storage materials photovoltaicmaterials catalysts and sensors

A novel and simple application of MA powders is in thedevelopment ofmeals ready to eat (MRE) In this applicationvery finely ground Mg-based alloy powders are used Whenwater is added to these fineMg-Fe powders heat is generatedaccording to the following reaction

Mg (Fe) + 2H2O 997888rarr Mg (OH)2 +H2 +Heat (3)

This heat is utilized to warm the food in remote areas wherecooking or heating facilities may not be available eg in war-torn areas These ldquofood heatersrdquo were widely used during theDesert Stormoperations in the 1990s Let us now look at someof the other applications

71 Pure Metals and Alloys The technique of MA has beenemployed to produce pure metals and alloys from ores andother raw materials such as scrap The process is based onchemical reactions induced by mechanical activation Mostof the reactions studied have been displacement reactions ofthe type

119872119874 + 119877 997888rarr 119872 + 119877119874 (4)

where ametal oxide (MO) is reduced by amore reactivemetal(reductant R) to the pure metal M In addition to oxidesmetal chlorides and sulfides have also been reduced to puremetals this way Similar reactions have also been used toproduce alloys and nanocomposites [21 53]

All solid-state reactions involve the formation of a prod-uct phase at the interface of the component phases Thusin the above example the metal M forms at the interfacebetween the oxide MO and the reductant R and physicallyseparates the reactants Further growth of the product phaseinvolves diffusion of atoms of the reactant phases through theproduct phase which constitutes a barrier layer preventingfurther reaction from occurring In other words the reactioninterface defined as the nominal boundary surface betweenthe reactants continuously decreases during the course ofthe reaction Consequently kinetics of the reaction are slowand elevated temperatures are required to achieve reasonablereaction rates

MA can provide the means to substantially increasethe reaction kinetics of chemical reactions in general andthe reduction reactions in particular This is because therepeated cold welding and fracturing of powder particlesincrease the area of contact between the reactant powderparticles by bringing fresh surfaces to come into contactrepeatedly due to a reduction in particle size during millingThis allows the reaction to proceed without the necessityfor diffusion through the product layer As a consequencereactions that normally require high temperatures will occurat lower temperatures or even without any externally appliedheat In addition the high defect densities induced by MAaccelerate diffusion processes

Research 11

Depending on the milling conditions two entirely differ-ent reaction kinetics are possible [21 54]

(a) the reactionmay extend to a very small volumeduringeach collision resulting in a gradual transformationor

(b) if the reaction enthalpy is sufficiently high (and ifthe adiabatic temperature is above 1800 K) a self-propagating high-temperature synthesis (SHS) reaction(also known as combustion synthesis) can be initi-ated by controlling the milling conditions (eg byadding diluents) to avoid combustion reactions maybe made to proceed in a steady-state manner

The latter type of reactions requires a critical milling time forthe combustion reaction to be initiated If the temperature ofthe vial is recorded during the milling process the tempera-ture initially increases slowly with time After some time thetemperature increases abruptly suggesting that ignition hasoccurred and this is followed by a relatively slow decrease

In the above types of reactions themilled powder consistsof the pure metalM dispersed in the RO phase both usuallyof nanometer dimensions It has been shown that by anintelligent choice of the reductant the RO phase can beeasily leached out Thus by selective removal of the matrixphase by washing with appropriate solvents it is possibleto achieve well-dispersed metal nanoparticles as small as 5nm in size If the crystallinity of these particles is not highit can be improved by subsequent annealing without thefear of agglomeration due to the enclosure of nanoparticlesin a solid matrix Such nanoparticles are structurally andmorphologically uniform having a narrow size distributionThis technique has been used to produce powders of manydifferent metalsmdashAg Cd Co Cr Cu Fe Gd Nb NiTi V W Zn and Zr Several alloys intermetallics andnanocomposites have also been synthesized Figure 7 showstwo micrographs of nanocrystalline ceria particles producedby the conventional vapor synthesis and MA methods Eventhough the grain size is approximately the same in both thecases the nanoparticles produced by MA are very discreteand display a very narrow particle size distribution whilethose produced by vapor deposition are agglomerated andhave a wide particle size distribution

Novel materials have also been synthesized by MA ofblended elemental powders One of the interesting examplesin this category is the synthesis of plain carbon steels andstainless steels [55 56] Twomain advantages of this synthesisroute are that the steels can be manufactured at roomtemperature and that the steels will be nanostructured thusconferring improved properties But these powders needto be consolidated to full density for applications and itis possible that grain growth occurs during this processThe kinetics of grain growth can however be controlled byregulating the time and temperature combinations

72 Hydrogen Storage Materials Another important applica-tion of MA powders is hydrogen storage Mg is a lightweightmetal (the lightest structural material) with a density of174 gcm3 Mg and Mg-based alloys can store a significant

20 nm

50 nm

Figure 7 Transmission electron micrographs of ceria particlesproduced byMA (top) and vapor synthesis (bottom)methods Notethe severe agglomeration of the nanoparticles in the latter case

amount of hydrogen However being a reactive metal Mgalways contains a thin layer of oxide on the surface Thissurface oxide layer which inhibits asorption of hydrogenneeds to be broken down through activation A high-temperature annealing is usually done to achieve this ButMA can overcome these difficulties due to the severe plasticdeformation experienced by the powder particles duringMAA catalyst is also often required to accelerate the kinetics ofhydrogen and dehydrogenation of alloys

Mg can adsorb about 76 wt hydrogen andMg-Ni alloysmuch less However Mg is hard to activate its tempera-ture of operation is high and the sorption and desorptionkinetics are slow Therefore Mg-Ni and other alloys arebeing explored But due to the high vapor pressure of Mgand significant difference in the melting temperatures ofthe constituent elements (Mg = 650∘C and Ni = 1452∘C) itis difficult to prepare these alloys by conventional meltingand solidification processes Therefore MA has been used toconveniently synthesize these alloys since this is a completelysolid-state processing method An added advantage of MA isthat the product will have a nanocrystalline grain structureallowing the improvements described below [57 58]

Figure 8 shows the hydrogenation and dehydrogena-tion plots for conventional and nanocrystalline alloysAn additional plot for a situation where a catalyst isadded to the nanocrystalline material is also shown Fromthese two graphs the following facts become clear Firstlythe nanocrystalline alloys adsorb hydrogen much fasterthan conventional (coarse-grained) alloys even though the

12 Research

0 1 2 3 4 5 6 7 8 9 10time (min)

conventional

p = 8 bar

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

time (min)

nano + catalyst

nano + catalyst

nanocrystallinenanocrystalline

conventional

breakthrough in reaction kinetics due to nanostructure and use of catalyst

0

1

2

3

4

5

6

7

8

hydr

ogen

cont

ent (

wt

)

T = 300∘C p = 10-3 mbar

Figure 8 Comparison of the hydrogen sorption and desorption kinetics of Mg alloys with conventional and nanocrystalline grain sizesSignificant improvement in the desorption kinetics when a catalyst is added is worth noticing

amount of hydrogen adsorbed is the sameWhile the conven-tional alloys do not adsorb any significant amount of hydro-gen in about 10 min the allowable full amount of hydrogenis adsorbed in the nanocrystalline alloy Similar observationsare made by many other authors in many different alloysFurther the kinetics are slightly faster (not much different)when a catalyst is added to the nanocrystalline alloy Onthe other hand the kinetics appear to be very significantlyaffected during the desorption process For example thenanocrystalline alloys desorb hydrogen much faster thanthe conventional alloys Again while the conventional alloysdo not desorb any significant amount of hydrogen thenanocrystalline alloy desorbs over 20 of the hydrogenstored in about 10 min But the most significant effect wasobserved when a catalyst was added to the nanocrystallinealloyAll the adsorbedhydrogenwas desorbed in about 2minwhen a catalyst was added to the nanocrystalline alloy Thusnanostructure processing throughMAappears to be a fruitfulway of developing novel hydrogen storage materials [59]

Many novel complex hydrides based on light metals(Mg Li and Na) are being explored and the importantcategories of materials being investigated include the alanates(also known as aluminohydrides a family of compoundsconsisting of hydrogen and aluminum) borohydrides amideborohydrides amide-imide systems (which have a highhydrogen storage capacity and low operative temperatures)amine borane (compounds of borane groups having ammo-nia addition) and alane

73 Energy Materials The technique of MA has also beenemployed to synthesize materials for energy applicationsThese include synthesis of photovoltaic (PV) materials andultrafine particles for faster ignition and combustion pur-poses

One of the most common PV materials is CIGS acompound of Cu In Ga and Se with the compositionCu(InGa)Se2 Since PV cells have the highest efficiencywhen

the band gap for this semiconductor is 125 eV one will haveto engineer the composition for the ratio of Ga to In This isbecause the band gaps for CuInSe

2 and CuGaSe2 are 102 and170 eV respectively Accordingly a Ga(Ga+In) ratio of about03 is found most appropriate and the final ideal compositionof the PV material will be Cu097In07Ga03Se2

The traditional method of synthesizing this material isto prepare an alloy of Cu Ga and In by vapor depositionmethods and incorporate Se into this by exposure to Seatmosphere Consequently this is a complex process MAhowever can be a simple direct and efficient method ofmanufacturing Blended elemental powders of Cu In Gaand Se weremilled in a copper container for different periodsof time and the phase constitution was monitored throughX-ray diffraction studies It was noted that the tetragonalCu(InGa)Se

2phase has formed even aftermilling the powder

mix for 20 min Process variables such as mill rotation speedball-to-powder weight ratio (BPR) and milling time werevaried and it was noted that the ideal composition wasachieved at lowmilling speeds short milling times and a lowBPR (Table 2) [60] By fine-tuning these parameters it shouldbe possible to achieve the exact composition

Nanostructuredmetal particles have high specific surfaceareas and consequently they are highly reactive and can storeenergy This area has received a great impetus in recentyears since the combustion rates of nanostructured metalparticles aremuch higher than those of coarsemetal particlesThe particle size cannot be exceedingly small because thesurface may be coated with a thin oxide layer and make theparticles passive [61ndash63] MA is very well suited to preparesuch materials because the reactions are easily initiatedby impact or friction and at the same time milling isa simple scalable and controllable technology capable ofmixing reactive components on the nanoscaleThese reactivematerials could be used as additives to propellants explosivesand pyrotechnics as well asmanufacture of reactive structuralcomponents [63]

Research 13

Table 2 Chemical analysis (at) of the Cu-In-Ga-Se powder mechanically alloyed under different processing conditions

BPR Speed (rpm) Time (min) Cu In Ga Se201 300 120 3261 1523 653 4562201 300 240 3173 1563 671 4593101 150 40 2598 1698 725 4978Targeted (Cu

097In07Ga03Se2) 2443 1763 756 5038

Compared to the presently used liquid aviation fuelsmetal particles based on B Al andMg are effective HoweverB and Al have very high boiling points and high heats ofenthalpy On the other hand Mg has a low boiling point butalso low heat of enthalpy Therefore if Al and Mg could bemixed intimately on a nanometer scale one could hope tohave a high heat of enthalpy at reduced boiling temperaturesThat is Mg-Al alloy particles could be practical in achievinga high-energy density and practical ignitionboiling temper-atures MA is a very effective method to accomplish this

Al and Mg powders were blended together in differentproportions and milled Since the milled powders have arange of particle sizes they were sieved to obtain well-defined powder sizes They were then fluidized and thencombusted in a CH4-air premixed flame Both the ignitionand burning times of these metal particles decreased withdecreased particle sizes and were also shorter when the Mgcontent is higher (Figure 9) [64] Similar results have beenreported by other investigators

8 Powder Consolidation

The product of MA is in powder form Except in cases ofchemical applications such as catalysis when the productdoes not require consolidation widespread application ofMA powders requires efficient methods of consolidatingthem into bulk shapes Successful consolidation of MA pow-ders is a nontrivial problem since fully densematerials shouldbe produced while simultaneously retaining the nanometer-sized grains without coarsening or the amorphous phaseswithout crystallizing Conventional consolidation of powdersto full density through processes such as hot extrusion [65]and hot isostatic pressing (HIP) [66] requires use of highpressures and exposure to elevated temperatures for extendedperiods of time [67] Unfortunately however this resultsin significant coarsening of the nanometer-sized grains orcrystallization of amorphous phases and consequently thebenefits of nanostructure processing or amorphization arelost [68] On the other hand retention of finemicrostructuresrequires use of low consolidation temperatures and then itis difficult to achieve full interparticle bonding at these lowtemperatures Therefore novel and innovative methods ofconsolidating MA powders are required The objectives ofconsolidation of these powders are to (a) achieve full densifi-cation (without any porosity) (b) minimize microstructuralcoarsening (ie retention of fine grain size) andor (c)avoid undesirable phase transformations In fact the earlyresults of excellent mechanical properties and most specifi-cally superplasticity at room temperature [69] attributed to

20 Mg (tig)90 Mg (tig)

20 Mg (tb)90 Mg (tb)

20 40 60 80 100 1200Particle Size (m)

0

10

20

30

40

50

60

70

Igni

tion

Burn

ing

Tim

e (m

s)

Figure 9 Ignition and burning times as a function of particle sizefor the mechanically alloyed Al-20 and Al-90 at Mg powdersIn the insert tig and tb represent the ignition and burning timesrespectively

nanocrystalline ceramics have not been successfully repro-duced due to incomplete consolidation of the nanopowdersin the material tested Thus consolidation to full densityassumes even greater importance

Because of the small size of the powder particles (typicallya few micrometers even though the grain size is only afew nanometers) MA powders pose special problems Forexample such nanometric powders are very hard and strongThey also have a high level of interparticle friction Furthersince these powders have a large surface area they also exhibithigh chemical reactivity Therefore special precautions needto be taken to consolidate the metal powders to bulk shapes

Successful consolidation of MA powders has beenachieved by electrodischarge compaction plasma-activatedsintering shock (explosive) consolidation [70] HIP hydro-static extrusion strained powder rolling and sinter forging[71] By utilizing the combination of high temperature andpressure HIP can achieve a particular density at lowerpressure when compared to cold isostatic pressing or at lowertemperature when compared to sintering Because of theincreased diffusivity in nanocrystalline materials sintering(and therefore densification) takes place at temperaturesmuch lower than those in coarse-grained materials This islikely to reduce the grain growth Spark plasma sintering(SPS) has become the most popular technique to consolidatemetal powders especially those with ultrafine grain sizesdue to the fact that full density can be achieved at low

14 Research

SOLID LIQUID GAS

MILLING

USE POWDER AS-ISor Chemical Reaction

Thermal Treatment etcCONSOLIDATE TO SOLID

(Temperature Pressure)

Catalysts Pigments SolderComponents Ceramic Powder etc

Advanced materials with hightolerance to gaseous impurities

Figure 10 Alternatives to consolidation of metal powders Ifpowders are used as catalysts pigments solder components etcthen they need not be consolidated However if powders need tobe consolidated then they could be used in applications with hightolerance to gaseous impurities

temperatures and shorter times in comparison to severalothermethods [72 73] An excellent reviewbyGroza [71]maybe consulted for full details of the methods of consolidationand description of results

9 Powder Contamination

Amajor concern inmetal powders processed byMAmethodsis the nature and amount of impurities that get incorporatedinto the powder and contaminate it The small size of thepowder particles and consequent availability of large surfacearea formation of fresh surfaces during milling and wearand tear of the milling tools and the atmosphere underwhich the powder is milled all contribute to contaminationof the powder In addition to these factors one has toconsider the purity of the starting raw materials and themilling conditions employed (especially the milling atmo-sphere) Thus it appears as though powder contaminationis an inherent drawback of the method and that the milledpowder will always be contaminated with impurities unlessspecial precautions are taken to avoidminimize them Themagnitude of powder contamination appears to depend onthe time of milling intensity of milling atmosphere in whichthe powder is milled nature and size of the milling mediumand differences in the strengthhardness of the powder andthe milling medium and the container

Several attempts have been made in recent years to min-imize powder contamination during milling An importantpoint to remember is that if the starting powders are highlypure then the final product is going to be clean and purewith minimum contamination from other sources providedthat proper precautions are taken during milling It is alsoimportant to keep inmind that all applications do not requireultraclean powders and that the purity desired will depend onthe specific application (Figure 10) But it is a good practiceto minimize powder contamination at every stage and ensurethat additional impurities are not picked up subsequentlyduring handling andor consolidation

One way ofminimizing contamination from the grindingmedium and themilling container is to use the samematerial

for the container and the grinding medium as the powderbeing milled Thus one could use copper balls and coppercontainer formilling copper and copper alloy powders [60] Ifa container of the same material to be milled is not availablethen a thin adherent coating on the internal surface of thecontainer (and also on the surface of the grinding medium)with the material to be milled will minimize contaminationThe idea here is to mill the powder once allowing the powderto be coated onto the grindingmedium and the inner walls ofthe container The milled loose powder is then discarded anda fresh batch of powder is milled but with the old grindingmedia and container In general a simple rule that shouldbe followed to minimize contamination from the millingcontainer and the grinding medium is that the container andgrindingmedium should be harderstronger than the powderbeing milled

Milling atmosphere is perhaps the most important vari-able that needs to be controlled Even though nominally puregases are used to purge the glove box before loading andunloading the powders even small amounts of impurities inthe gases appear to contaminate the powders especially whenthey are reactive The best solution one could think of willbe to place the mill inside a chamber that is evacuated andfilled with high-purity argon gas Since the whole chamberis maintained under argon gas atmosphere (continuouslypurified to keep oxygen and water vapor below 1 ppm each)and the container is inside the mill which is inside thechamber contamination from the atmosphere is minimum

Contamination from PCAs is perhaps the most ubiqui-tous Since most of the PCAs used are organic compoundswhich have low melting and boiling points they decomposeduring milling due to the heat generated The decomposi-tion products consisting of carbon oxygen nitrogen andhydrogen react with the metal atoms and form undesirablecarbides oxides nitrides etc

10 Concluding Remarks and Future Prospects

We have only briefly touched upon the scientific and tech-nological aspects of MA materials along with some of theconcerns such as powder contamination and consolidationissues The historical development of MA during the last 50years or so can be divided into threemajor periods (Figure 11)The first period covering the first twenty years (from 1966up to about 1985) was mostly concerned with the devel-opment and production of oxide-dispersion-strengthened(ODS) superalloys for applications in the aerospace indus-try Several alloys with improved properties based onNi and Fe were developed and found useful applicationsThese included the MA754 MA760 MA956 MA957 andMA6000 [74] A more recent survey of application of MAto steel is also available [75] The second period of aboutfifteen years covering from about 1986 up to about 2000witnessed advances in the fundamental understanding ofthe processes that take place during MA Along with thisa large number of dedicated conferences were held andthere was a burgeoning of publication activity Comparisonshave also been frequently made between MA and othernonequilibriumprocessing techniquesmore specifically RSP

Research 15

1965 19851975 1995 2005

Production of ODS Alloys

Novel Phase SynthesisDevelopment of BasicUnderstanding of MA

Nanostructures andNew Applications

2015

Figure 11 Schematic showing the different periods of development in the field of mechanical alloying (MA) and applications ofMA productssince its inception in 1965

It was shown that themetastable effects achieved by these twononequilibrium processing techniques are similar Revival ofthe mechanochemical processing (MCP) took place and avariety of novel substances were synthesized Several mod-eling studies were also conducted to enable prediction of thephases produced or microstructures obtained although withlimited success The third period starting from about 2001saw a reemergence of the quest for new applications of theMA materials not only as structural materials but also forchemical applications such as catalysis and hydrogen storagewith the realization that contamination of themilled powdersis the limiting factor in the widespread applications of theMA materials Innovative techniques to consolidate the MApowders to full density while retaining the metastable phases(including glassy phases andor nanostructures) in themwerealso developed All these are continuing with the ongoinginvestigations to enhance the scientific understanding of theMA process

At present activities relating to both the science andtechnology of MA have been going on simultaneously Whilesome groups have been focusing on developing a betterunderstanding of the process and optimizing the processvariables to achieve a certain phase or phase combinationin the alloy system others have been concentrating onexploiting this technique for different applications It hasalso come to be realized that the MA process can be usedto recycle used and discarded materials to produce value-added materials for subsequent consumption They can bepulverized separated and used to mix with other powdersto produce composites The bottom line is that newer andimproved materials are being continuously developed fornovel applications

Conflicts of Interest

The author declares that he has no conflicts of interest

References

[1] K H J Buschow R W Cahn M C Flemings B Ilschner E JKramer and SMahajan EdsEncyclopedia ofMaterials Scienceand Technology Elsevier 2001

[2] C Suryanarayana EdNon-equilibrium Processing of MaterialsPergamon Oxford UK 1999

[3] C Suryanarayana ldquoMetallic glassesrdquo Bulletin of Materials Sci-ence vol 6 no 3 pp 579ndash594 1984

[4] C Suryanarayana and A Inoue Bulk Metallic Glasses CRCPress Boca Raton Fla USA 2nd edition 2018

[5] H R Trebin Ed Quasicrystals Structure and Physical Proper-ties Wiley-VCH Weinheim Germany 2003

[6] C Suryanarayana and H Jones ldquoFormation and characteristicsof quasicrystalline phases A reviewrdquo International Journal ofRapid Solidification vol 3 pp 253ndash293 1988

[7] H Gleiter ldquoNanocrystalline materialsrdquo Progress in MaterialsScience vol 33 no 4 pp 223ndash315 1989

[8] C Suryanarayana ldquoRecent developments in nanostructuredmaterialsrdquo Advanced Engineering Materials vol 7 no 11 pp983ndash992 2005

[9] M C Gao J-W Yeh P K Liaw and Y Zhang Eds High-Entropy Alloys Fundamentals and Applications Springer 2016

[10] J S Benjamin ldquoMechanical alloying ndash History and futurepotentialrdquo in Advances in Powder Metallurgy and ParticulateMaterials J M Capus and R M German Eds vol 7 ofNovel Powder Processing pp 155ndash168 Metal Powder IndustriesFederation Princeton NJ USA 1992

[11] M P Phaniraj D-I Kim J-H Shim and Y W CholdquoMicrostructure development in mechanically alloyed yttriadispersed austenitic steelsrdquo Acta Materialia vol 57 no 6 pp1856ndash1864 2009

[12] A Hirata T Fujita Y R Wen J H Schneibel C T Liu and MW Chen ldquoAtomic structure of nanoclusters in oxide-dispersionstrengthened steelsrdquo Nature Materials vol 10 no 12 pp 922ndash926 2011

[13] M K Miller C M Parish and Q Li ldquoAdvanced oxide disper-sion strengthened and nanostructured ferritic alloysrdquoMaterialsScience and Technology (United Kingdom) vol 29 no 10 pp1174ndash1178 2013

[14] N Oono Q X Tang and S Ukai ldquoOxide particle refinementin Ni-based ODS alloyrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 649 pp 250ndash253 2016

[15] D B Witkin and E J Lavernia ldquoSynthesis and mechanicalbehavior of nanostructured materials via cryomillingrdquo Progressin Materials Science vol 51 no 1 pp 1ndash60 2006

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

Research 5

Table 1 Attributes of mechanical alloying

(1) Production of fine dispersion of second phase (usually oxide) particles(2) Extension of solid solubility limits(3) Refinement of grain sizes down to nanometer range(4) Synthesis of novel crystalline and quasicrystalline phases(5) Development of amorphous (glassy) phases(6) Disordering of ordered intermetallics(7) Possibility of alloying of difficult to alloy elementsmetals

(8)Inducement of chemical (displacement) reactions at low temperatures for (a) Mineral and Wasteprocessing (b) Metals refining (c) Combustion reactions and (d) Production of discrete ultrafineparticles

(9) Scaleable process

is that the solute atoms segregate to grain boundaries fromwhere they could diffuse into grains and form solid solutions

32 Intermetallic Phases Intermetallic phases (or inter-metallics for short) are typically ordered compounds existingat simple ratios of the two ormore elements involved Becauseof their ordered nature they are strong and hard stiffhave high melting points and also exhibit other interestingbehaviors such as excellent corrosionoxidation resistanceHowever they usually suffer from low ambient temperatureductility again due to the ordered nature of the compoundsA new family of intermetallics showing high ductility atroom temperature and based on rare-earth elements hasbeen recently developed [28 29] Since MA is known to (a)disorder the ordered lattices (b) refine grain sizes down tonanometer levels and (c) also alter the crystal structures allthese effects could induce some amount of plasticity into thematerial In fact the potential to ductilize the normally brittleintermetallics has been the driver for accelerated researchinto the MA of intermetallics

A number of intermetallics (both equilibrium andmetastable) and high-temperature or high-pressure phaseshave been synthesized byMAstarting fromelemental powderblends [16 17 24 30] More interestingly quasicrystallinephases with the forbidden crystal symmetries first reportedin rapidly solidified Al-Mn alloys have also been synthesizedby MA in a number of alloy systems [5 31] Applicationsfor these exotic materials have been limited Based ontheir high hardness low friction and low surface reactivityquasicrystalline materials have been used to coat nonstickfrying pans Also steels reinforcedwith small quasicrystallineparticles are very strong and have been used for acupunctureand surgery dental instruments and razor blades [32]

The most significant change that occurs in MA materialsis the formation of amorphous phases and this will bediscussed in the next section

4 Metallic Glasses

Amorphous alloys are materials that lack crystallinity andhave a random arrangement of atoms Because of thisextreme disorder these materials exhibit a very interestingcombination of high strength good corrosion resistance

and useful electrical and magnetic properties These novelmaterials have been traditionally produced starting from thevapor or liquid states Amorphous or noncrystalline alloysbased onmetals are commonly referred to as metallic glassesAt present there is worldwide activity in the area of metallicglasses and bulk metallic glasses [4 33]

Metallic glasses were first produced by rapid solidificationprocessing (RSP) in 1960 in the form of thin ribbons bysolidifying the alloy melt at rates gt106 Ks [34] Subsequentdevelopments include the synthesis of bulk metallic glasseswith a large section thickness of a few centimeters This ispossible in multicomponent alloys which could be solidifiedinto the glassy condition at low critical cooling rates of lt102Ks [4] The current record is an 80-mm diameter glassy rodproduced by solidification in a Pd

425Cu30Ni75P20

alloy (thesubscripts represent the atomic percentage of the elementsin the alloy) [35] While solidification methods produce themetallic glasses either in thin ribbon or bulk form directlyfrom the liquid the amorphous phases are in powder formbyMAmethodsThese need to be consolidated to bulk shapefor subsequent characterization and applications

Amorphous alloys were synthesized by MA first in anY-Co intermetallic compound [36] and later in a blendedelemental Ni-Nb powder mixture [37] Amorphous phaseshave now been synthesized in a very large number ofalloy systems starting from either blended elemental powdermixtures intermetallic compounds or mixtures of them[38] Comparisons have been frequently made between theamorphous alloys produced by MA and RSP techniquesand both similarities and differences have been noted [26]Further the mechanism of glass formation appears to besignificantly different depending uponwhether it is producedby MA or RSP methods

Several different criteria have been proposed to explainthe glass-forming ability of alloys produced by solidificationmethodsThese include the reduced glass transition tempera-ture119879rg =119879g119879l where119879g and119879l represent the glass transitionand liquidus temperature of the alloy respectively presenceof deep eutectics in the phase diagrams and significantdifferences in the atomic sizes of the constituent elementsleading to development of strain energy in the solid solutionWhen the strain energy exceeds a critical value determinedby the size and amount of the solute atoms it destabilizes the

6 Research

crystal lattice and results in the formation of an amorphousphase With the introduction of bulk metallic glasses nearlythirty years ago a very large number of new criteria mostlybased on the glass transformation temperatures (119879g 119879x and119879l where 119879x represents the crystallization temperature of theamorphous alloy) have been proposed In spite of a very largenumber of criteria proposed [4] it has not been possible yetto accurately predict the glass-forming ability of alloys [39]

There are some significant differences in the formation ofmetallic glasses by the MA and RSP methods For examplemetallic glasses are obtained by MA in almost every alloysystem provided that sufficient energy has been stored in thepowder But metallic glasses are obtained by RSP methodsonly in certain composition rangesmdashabout 20 at metalloidcontent in metal-metalloid systems and different composi-tions in metal-metal systems mostly at compositions cor-responding to deep eutectics Additionally glass formationby MA occurs by the accumulation of crystal defects whichraises the free energy of the crystalline phase above that of thehypothetical glassy phase under which conditions the crys-talline phase becomes destabilized On the other hand forglass formation by the RSP methods the critical cooling ratefor glass formation needs to be exceeded so that formationof the crystalline nuclei is completely suppressed Since MAis a completely solid-state powder processing technique thecriteria for amorphous phase formation should be differentfrom those of solidification methods A new criterion wasproposed that is the number of intermetallics present inthe constituent phase diagrams determines the glass-formingability of alloys processed by MAmdashthe more the number ofintermetallics in the constituent phase diagrams the easier itis to amorphize the powder blend [40]

Amorphization by MA occurs when the free energy ofthe hypothetical amorphous phase 119866A is lower than that ofthe crystalline phase G119862 ie 119866A lt 119866C A crystalline phasenormally has a lower free energy than the amorphous phaseBut its free energy can be increased by introducing a varietyof crystal defects such as dislocations grain boundaries andstacking faults If an intermetallic has formed then additionalenergy can be introduced by disordering the crystal lattice Bythis approach it is then possible to obtain a situation when

119866A lt 119866C + 119866D (1)

where 119866D is the free energy increase due to introductionof crystal defects It has been shown that the presence ofintermetallics in an alloy system can significantly increasethe energy contributing to amorphization This is due totwo important effects Firstly disordering of intermetallicscontributes an energy of about 15 kJmol to the systemSecondly a slight change in the stoichiometry of the inter-metallic increases the free energy of the system drasticallyAdditionally grain size reduction contributes about 5 kJmolFurther disordering of intermetallics has also been shownto be possible by heavy deformation Since MA reduces thegrain size to nanometer levels and also disorders the usuallyordered intermetallics the energy of the milled powders issignificantly raised In fact it is raised to a level above that ofthe hypothetical amorphous phase and thus formation of theamorphous phase is favored over the crystalline phase

41 Mechanical Crystallization It was reported that an amor-phous phase formed by milling transforms into a crystallinephase on continued milling and that this phenomenonhas been referred to as mechanical crystallization [41 42]The process of mechanical crystallization is very unusualand offers many opportunities to design novel alloys withimproved properties For example it will be possible toproduce the alloy in either a fully amorphous amorphous +nanocrystalline composite or a completely crystalline statewith different grain sizes depending upon the time ofmillingor the temperature and time of subsequent annealing of theamorphous alloy

Figure 3(a) shows the XRD patterns of the blended ele-mental powder mixture of the composition Fe42Ge28Zr10B20mechanically alloyed for 10 and 60 h [42] While the pat-tern at 10 h of milling clearly shows the presence of anamorphous phase the pattern at 60 h of milling showsthe presence of a crystalline phase superimposed over thebroad and diffuse halo representing the amorphous phaseThe crystalline phase has been identified as the 120572-Fe solidsolution containing the solute elements present in the blendThis 120572-Fe phase is the first phase to crystallize from theamorphous alloy (as in primary crystallization) and externalannealing of the milled powder showed that the amount ofthe 120572-Fe increased along with the presence of other inter-metallics Eutectic crystallization (simultaneous formation oftwo crystalline phases) has also been observed in other alloysystems

Mechanical crystallization has also been observed inseveral other alloy systems that formed the amorphous phaseon MA In fact it is now becoming apparent that thisphenomenon of mechanical crystallization is not as rare asit was once thought to be

Many reasons have been suggested for the formation of acrystalline phase after the formation of an amorphous phaseduring MA These include (a) rise in temperature to a levelabove that of the crystallization temperature of the amor-phous alloy (b) powder contamination due to which a stablecrystalline impurity phase forms (c) phenomenon of inversemelting and (d) basic thermodynamic considerations It nowappears clear that the basic thermodynamic stabilities of thedifferent phases under the different conditions of milling areresponsible for mechanical crystallization Such a situationcan be explained with reference to a free energy versuscomposition diagram

Figure 3(b) is a schematic diagram showing the variationof free energy with composition for the Fe42X28Zr10B20 (X =AlGe andNi) alloy systems investigatedThepossible (stableand metastable) constitution in this system is (a) blendedelemental (BE) powder mixture (b) 120572-Fe the solid solutionof all the alloying elements in Fe (c) an amorphous phase (d)intermetallic phases and (e) different combinations of thesephases The stability of any phase will be determined by itsrelative position in the free energy vs composition plotmdashthelower the free energy the more stable the phase is Since it ispossible to have a large number of intermetallic phases in thismulticomponent system and since it is difficult to indicateeach of them separately for simplicity all of them are groupedtogether as ldquointermetallicsrdquo

Research 7Fr

ee E

nerg

y

Solute Concentration (at)20 40 60 80Fe 100

BE powder

amorphous

-Fe

Intermetallics

1

23

45

Inte

nsity

30 40 502-Theta

(a)

(b)

10 h

60 h

60 70 80 90

1

Figure 3 (a) X-ray diffraction patterns of the Fe42Ge28Zr10B20

powder blend milled for 10 and 60 h While the powder milledfor 10 h shows the presence of an amorphous phase the powdermilled for 60 h shows the presence of a crystalline phase coexistingwith the amorphous phase suggesting that the amorphous phase hascrystallized (b) Hypothetical free energy vs composition diagramto explain the mechanism of mechanical crystallization in theFe42X28Zr10B20systemNote that point ldquo1rdquo represents the free energy

of the blended elemental powders Similarly point ldquo2rdquo representsformation of the 120572-Fe solid solution containing all the alloyingelements in Fe point ldquo3rdquo formation of the homogeneous amorphousphase point ldquo4rdquo a mixture of the amorphous phase with a differentsolute content (amorphous) from that at ldquo3rdquo and the solid solution120572-Fe with different solute content and point ldquo5rdquo the equilibriumsituation when the solid solution and intermetallic phases coexist

The free energy of the BE powder mixture is indicated bypoint ldquo1rdquo which represents the point of intersection of thecomposition vertical with the line joining the free energiesof pure Fe and the ldquosoluterdquo On milling this powder a solidsolution phase containing all the solute elements in Fe (ora mixture of intermetallics and a solid solution in somecases) is seen to form and its free energy is indicated bypoint ldquo2rdquoThe solid solution forms because it has a lower free

energy than the BE powder mixture Since MA introduces avariety of crystal defects the crystalline phases in the milledpowders will contain excess energyThis energy will continueto increase with milling time and reach a value which isabove that of the metastable amorphous phase Thus theamorphous phase gets stabilized (point ldquo3rdquo) On primarycrystallization of the amorphous phase the new constitutionwill be a mixture of the 120572-Fe solid solution (or intermetallics)and the amorphous phase which now has a compositiondifferent from that of the original amorphous phase Furtherthe new solid solution phase also has a composition differentfrom the original 120572-Fe phase The free energy of the mixtureof this solid solution and the amorphous phase indicatedby ldquopoint 4rdquo will have a free energy lower than that of theamorphous phase The equilibrium mixture of the 120572-Fe solidsolution and ldquointermetallicsrdquo will have the lowest free energyof all the phase mixtures as indicated by point ldquo5rdquo in thefigure The process of mechanical crystallization is uniqueto the MA process and can be achieved only on continuedmilling of the amorphous powder ormilling of an amorphousalloy obtained by other methods

5 Nanostructured Materials

Another important attribute of MA has beenmicrostructuralrefinement The most common observation is that grainrefinement takes place resulting in the formation of very finegrains often down to nanometer levels A two-phase alloyunder equilibrium conditions can transform to a single-phasealloy due to formation of a supersaturated solid solution Anamorphous alloy can crystallize under the action of severeplastic deformation Most usefully one can also introducea high volume fraction of fine second phase particles andobtain a composite with a uniform distribution of thesecond phase particles By a proper choice of the size ofthe matrix andor the reinforcement phase one can producenanocomposites a very fruitful area for investigations Infact MA has been the most popular technique to synthesizenanostructured materials and nanocomposites in a numberof alloy systems

It has been frequently demonstrated that mechanicallyalloyed materials show powder particle refinement as wellas grain refinement as a function of the milling time Thesesizes decrease very rapidly in the early stages of milling andmore gradually later Eventually in almost all the cases thegrain size of the mechanically alloyed materials is in thenanometer region unless the powder becomes amorphousThe minimum grain size achieved has been reported tobe a few nanometers ranging typically from about 5 to50 nm but depending on the material and processingconditions For example it was reported that the grain sizedecreases with increasing milling energy higher ball-to-powder weight ratio and lower temperatures As is wellknown nanostructuredmaterials exhibit high strength goodductility excellent sintering characteristics and interestingelectrical andmagnetic properties [7 8 27 43]The ease withwhich nanostructured materials can be synthesized is onereason why MA has been extensively employed to producenanocrystalline and nanocomposite materials

8 Research

From high-resolution transmission electron microscopy(TEM) observations it was noted that in the early stages ofmilling due to the high deformation rates experienced bythe powder deformation was localized within shear bands[44] These shear bands which contain a high density ofdislocations have a typical width of approximately 05 to 10120583m Small grains with a diameter of 8ndash12 nm were seenwithin the shear bands and electron diffraction patternssuggested significant preferred orientation With continuedmilling the average atomic level strain increased due toincreasing dislocation density and at a certain dislocationdensity within these heavily strained regions the crystaldisintegrated into subgrains that are separated by low-anglegrain boundaries This resulted in a decrease of the latticestrain The subgrains formed this way were of nanometerdimensions and are often between 20 and 30 nm

On further processing deformation occurred in shearbands located in previously unstrained parts of the materialThe grain size decreased steadily and the shear bands coa-lesced The small-angle boundaries are replaced by higherangle grain boundaries implying grain rotation as reflectedby the absence of texture in the electron diffraction patternsand random orientation of the grains observed from thelattice fringes in the high-resolution TEMmicrographs Con-sequently dislocation-free nanocrystalline grains are formedThis is the currently accepted mechanism of nanocrystalformation in MA processed powders

The grain size of the milled materials decreases withmilling time and reaches a saturation level when a balance isestablished between the fracturing and cold welding eventsThis minimum grain size 119889min is different depending on thematerial and milling conditionsThe value of 119889min achievableby milling is determined by the competition between theplastic deformation via dislocation motion that tends todecrease the grain size and the recovery and recrystallizationbehavior of the material that tends to increase the grain sizeThis balance gives a lower bound for the grain size of puremetals and alloys

The 119889min obtained is different for different metals and isalso found to vary with the crystal structure (Figure 4) Inmost of the metals the minimum grain size attained is inthe nanometer dimensions But metals with a body-centeredcubic (BCC) crystal structure reach much smaller valuesin comparison to metals with the other crystal structuresrelated to the difficulty of extensive plastic deformation andconsequent enhanced fracturing tendency during millingCeramics and intermetallic compounds are much harderand usually more brittle than the metals on which they arebased Therefore intuitively one expects that 119889min of thesecompounds is smaller than those of the pure metals but thisdoes not appear to be the case always

It was also reported that 119889min decreases with an increasein the melting temperature of the metal [45] As shown inFigure 4 this trend is amply clear in the case of metalswith close-packed structures (face-centered cubic (FCC) andhexagonal close-packed (HCP)) but not so clear in metalswith a BCC structure Another point of interest is that thedifference in grain size is much less among metals thathave high melting temperatures the minimum grain size is

0

5

10

15

20

25

FCCBCCHCP

Min

imum

Gra

in S

ize (

nm)

0 2000 30001000Melting Temperature (K)

Figure 4 Variation of minimum grain size with temperature inmechanically milled FCC HCP and BCC metals

virtually constant Thus for the HCP metals Co Ti Zr Hfand Ru the minimum grain size is almost the same eventhough themelting temperatures vary between 1495∘C for Coand 2310∘C for Ru

Analysis of the existing data on 119889min of mechanicallymilled pure metals shows that the normalized minimumgrain size 119889minb where b is the Burgers vector of thedislocations decreases with increasing melting temperatureactivation energy for self-diffusion hardness stacking faultenergy bulk modulus and the equilibrium distance betweentwo edge dislocations The 119889min value is also affected bymilling energy (the higher the milling energy the smaller thegrain size) milling temperature (the lower the milling tem-perature the smaller the grain size) and alloying additions(the minimum grain size is smaller for solid solutions thanfor pure metals since solid solutions are harder and strongerthan the pure metals on which they are based)

It was suggested that 119889min is determined by the minimumgrain size that can sustain a dislocation pile-up within a grainand by the rate of recovery Based on the dislocation pile-up model the critical equilibrium distance between two edgedislocations in a pile-up L119888 (which could be assumed to bethe crystallite or grain size inmilled powders) was calculated[46] using the equation

119871119888=3119866119887

(1 minus ])119867(2)

where G is the shear modulus 119887 is the Burgers vector ] isthe Poissonrsquos ratio and H is the hardness of the materialAccording to the above equation increased hardness resultsin smaller values of 119871c (grain size) and an approximate linearrelationship was observed between 119871c and the minimumgrain size obtained by milling of a number of metals Otherattempts have also beenmade to theoretically predict 119889min onthe basis of thermodynamic properties of materials [47]

Research 9

6 Nanocomposites

Achievement of a uniform distribution of the reinforcementin a matrix is essential to the achievement of good anduniform mechanical properties in composites Further themechanical properties of the composite tend to improvewith increasing volume fraction andor decreasing particlesize of the reinforcement [48] Traditionally a reasonablylarge volume fraction of the reinforcement could be addedif the size of the reinforcement is large (on a micrometerscale or larger) But if the reinforcement size is very fine(of nanometer dimensions) then the volume fraction addedis typically limited to about 2 to 4 However if a largevolume fraction of nanometer-sized reinforcement can beintroduced the mechanical properties of the composite arelikely to be vastly improved Therefore several investigationshave been undertaken to produce improved nanocompositesthrough MA [20]

Composites are traditionally produced by solidificationprocessing methods since the processing is inexpensive andlarge tonnage quantities can also be produced But it isnot easy to disperse fine reinforcement particles in a liquidmetal since agglomeration of the fine particles occurs andclusters are produced Further wetting of the particles ispoor and consequently even the most vigorous stirring isnot able to break the agglomerates Additionally the finepowders tend to float to the top of the melt during processingof the composites through solidification processing Thussolid-state processing methods such as MA are effective indispersing fine particles (including nanoparticles) An addedadvantage is that since no melting is involved even a largevolume fraction of the reinforcement can be introduced

Some of the specific goals sought in processing ofnanocomposite materials throughMA include incorporationof a high volume percentage of ultrafine reinforcements andachievement of high ductility and even superplasticity insome composites It is also possible that due to the increasedductility the fracture toughness of these composites couldbe higher than that of their coarse-grained counterparts Inline with these goals we have in recent times accomplished(a) achievement of a very uniform distribution of the rein-forcement phases in different types of matrices includingdispersion of a high volume fraction of Al2O3 in Al [49] andof Ti5Si3in 120574ndashTiAl [50 51] and (b) synthesis of MoSi

2+Si3N4

composites for high-temperature applications [52] amongothers However we will describe here only the developmentof Ti5Si3+120574ndashTiAl composites as a typical example

Lightweight intermetallic alloys based on 120574-TiAl arepromising materials for high-temperature structural appli-cations eg in aircraft engines or stationary turbines Eventhough they have many desirable properties such as highspecific strength and modulus both at room and elevatedtemperatures and good corrosion and oxidation resistancethey suffer from inadequate room temperature ductility andinsufficient creep resistance at elevated temperatures espe-cially between 800 and 850∘C an important requirement forelevated temperature applications of these materials There-fore current research programs have been addressing thedevelopment of high-temperature materials with adequate

020406080

100120140160180200

Engi

neer

ing

stres

s (M

Pa)

20 40 60 80 100 120 140 1600Engineering strain ()

-5950

∘C 4x10

-4950

∘C 4x10

-41000

∘C 4x10x10

(a)

(b)

Figure 5 (a) Scanning electronmicrograph of the 120574-TiAl + 60 volTi5Si3composite specimen showing that the twophases are very uni-

formly distributed in the microstructure Such a microstructure isconducive to achieving superplastic deformation under appropriateconditions of testing (b) Tensile stress-strain curve of 120574-TiAl + 60vol Ti

5Si3composite showing that superplastic deformation was

achieved at 950∘C with a strain rate of 4x10minus5 sminus1 and at 1000∘C witha strain rate of 4x10minus4 sminus1

room temperature ductility for easy formability and abilityto increase the high-temperature strength by a suitable heattreatment or alloying additions to obtain sufficient creepresistance at elevated temperatures

Composites of 120574-TiAl and 120585-Ti5Si3phase with the volume

fractions of the 120585-Ti5Si3phase varying from 0 to 60 vol

were produced by MA of a combination of the blendedelemental and prealloyed intermetallic powders Fully denseand porosity-free compacts were produced by hot isostaticpressing with the resulting grain size of each of the phasesbeing about 300-400 nm Figure 5(a) shows scanning elec-tron micrographs of the 120574-TiAl+60 vol 120585-Ti5Si3 compositeshowing that the two phases are very uniformly distributedthroughout the microstructure Materials exhibiting such amicrostructure (roughly equal amounts of the two phasessmall grain sizes and a uniform distribution of the twophases) are expected to be deformed superplastically Totest this hypothesis both compression and tensile testingof these composite specimens were conducted at differenttemperatures and strain rates From the tensile stress-strain

10 Research

Pure Metals Alloysand Compounds

NanostructuredMaterials

MECHANICALALLOYING

ODS Superalloys Supercorroding

Alloys

Synthesis of Novel Phases PVDTargets Solders Paints Food Heaters

Biomaterials Waste Utilization etc

Hydrogen Storage MagneticMaterials Catalysts Sensors etc

Figure 6 An overview of the applications of the MA process

plots of the 120574-TiAl+60 vol 120585-Ti5Si3composite specimens

shown in Figure 5(b) it is clear that the specimens testedat 950∘C with a strain rate of 4times10minus5 sminus1 and at 1000∘Cwith a strain rate of 4times10minus4 sminus1 exhibited large ductilityof nearly 150 and 100 respectively Considering that thiscomposite is based on a ceramic material (Ti

5Si3) this is a

very high amount of deformation suggestive of superplasticdeformation Final proof is provided by TEM investiga-tions that confirm the continued stability of the equiaxedmicrostructure after deformation

7 Applications

As has been frequently pointed out in the literature thetechnique ofMAwas developed out of an industrial necessityin 1965 to produce ODS nickel-based superalloys Sincethen a number of new applications have been exploredfor mechanically alloyed materials (Figure 6) Apart fromthe ODS alloys which continue to be used in the high-temperature applications area MA powders have also beenused for other applications and newer applications are alsobeing explored These novel applications include direct syn-thesis of pure metals and alloys from ores and scrap mate-rials super-corroding alloys for release into ocean bottomsdevelopment of PVD (physical vapor deposition) targetssolders paints etc hydrogen storage materials photovoltaicmaterials catalysts and sensors

A novel and simple application of MA powders is in thedevelopment ofmeals ready to eat (MRE) In this applicationvery finely ground Mg-based alloy powders are used Whenwater is added to these fineMg-Fe powders heat is generatedaccording to the following reaction

Mg (Fe) + 2H2O 997888rarr Mg (OH)2 +H2 +Heat (3)

This heat is utilized to warm the food in remote areas wherecooking or heating facilities may not be available eg in war-torn areas These ldquofood heatersrdquo were widely used during theDesert Stormoperations in the 1990s Let us now look at someof the other applications

71 Pure Metals and Alloys The technique of MA has beenemployed to produce pure metals and alloys from ores andother raw materials such as scrap The process is based onchemical reactions induced by mechanical activation Mostof the reactions studied have been displacement reactions ofthe type

119872119874 + 119877 997888rarr 119872 + 119877119874 (4)

where ametal oxide (MO) is reduced by amore reactivemetal(reductant R) to the pure metal M In addition to oxidesmetal chlorides and sulfides have also been reduced to puremetals this way Similar reactions have also been used toproduce alloys and nanocomposites [21 53]

All solid-state reactions involve the formation of a prod-uct phase at the interface of the component phases Thusin the above example the metal M forms at the interfacebetween the oxide MO and the reductant R and physicallyseparates the reactants Further growth of the product phaseinvolves diffusion of atoms of the reactant phases through theproduct phase which constitutes a barrier layer preventingfurther reaction from occurring In other words the reactioninterface defined as the nominal boundary surface betweenthe reactants continuously decreases during the course ofthe reaction Consequently kinetics of the reaction are slowand elevated temperatures are required to achieve reasonablereaction rates

MA can provide the means to substantially increasethe reaction kinetics of chemical reactions in general andthe reduction reactions in particular This is because therepeated cold welding and fracturing of powder particlesincrease the area of contact between the reactant powderparticles by bringing fresh surfaces to come into contactrepeatedly due to a reduction in particle size during millingThis allows the reaction to proceed without the necessityfor diffusion through the product layer As a consequencereactions that normally require high temperatures will occurat lower temperatures or even without any externally appliedheat In addition the high defect densities induced by MAaccelerate diffusion processes

Research 11

Depending on the milling conditions two entirely differ-ent reaction kinetics are possible [21 54]

(a) the reactionmay extend to a very small volumeduringeach collision resulting in a gradual transformationor

(b) if the reaction enthalpy is sufficiently high (and ifthe adiabatic temperature is above 1800 K) a self-propagating high-temperature synthesis (SHS) reaction(also known as combustion synthesis) can be initi-ated by controlling the milling conditions (eg byadding diluents) to avoid combustion reactions maybe made to proceed in a steady-state manner

The latter type of reactions requires a critical milling time forthe combustion reaction to be initiated If the temperature ofthe vial is recorded during the milling process the tempera-ture initially increases slowly with time After some time thetemperature increases abruptly suggesting that ignition hasoccurred and this is followed by a relatively slow decrease

In the above types of reactions themilled powder consistsof the pure metalM dispersed in the RO phase both usuallyof nanometer dimensions It has been shown that by anintelligent choice of the reductant the RO phase can beeasily leached out Thus by selective removal of the matrixphase by washing with appropriate solvents it is possibleto achieve well-dispersed metal nanoparticles as small as 5nm in size If the crystallinity of these particles is not highit can be improved by subsequent annealing without thefear of agglomeration due to the enclosure of nanoparticlesin a solid matrix Such nanoparticles are structurally andmorphologically uniform having a narrow size distributionThis technique has been used to produce powders of manydifferent metalsmdashAg Cd Co Cr Cu Fe Gd Nb NiTi V W Zn and Zr Several alloys intermetallics andnanocomposites have also been synthesized Figure 7 showstwo micrographs of nanocrystalline ceria particles producedby the conventional vapor synthesis and MA methods Eventhough the grain size is approximately the same in both thecases the nanoparticles produced by MA are very discreteand display a very narrow particle size distribution whilethose produced by vapor deposition are agglomerated andhave a wide particle size distribution

Novel materials have also been synthesized by MA ofblended elemental powders One of the interesting examplesin this category is the synthesis of plain carbon steels andstainless steels [55 56] Twomain advantages of this synthesisroute are that the steels can be manufactured at roomtemperature and that the steels will be nanostructured thusconferring improved properties But these powders needto be consolidated to full density for applications and itis possible that grain growth occurs during this processThe kinetics of grain growth can however be controlled byregulating the time and temperature combinations

72 Hydrogen Storage Materials Another important applica-tion of MA powders is hydrogen storage Mg is a lightweightmetal (the lightest structural material) with a density of174 gcm3 Mg and Mg-based alloys can store a significant

20 nm

50 nm

Figure 7 Transmission electron micrographs of ceria particlesproduced byMA (top) and vapor synthesis (bottom)methods Notethe severe agglomeration of the nanoparticles in the latter case

amount of hydrogen However being a reactive metal Mgalways contains a thin layer of oxide on the surface Thissurface oxide layer which inhibits asorption of hydrogenneeds to be broken down through activation A high-temperature annealing is usually done to achieve this ButMA can overcome these difficulties due to the severe plasticdeformation experienced by the powder particles duringMAA catalyst is also often required to accelerate the kinetics ofhydrogen and dehydrogenation of alloys

Mg can adsorb about 76 wt hydrogen andMg-Ni alloysmuch less However Mg is hard to activate its tempera-ture of operation is high and the sorption and desorptionkinetics are slow Therefore Mg-Ni and other alloys arebeing explored But due to the high vapor pressure of Mgand significant difference in the melting temperatures ofthe constituent elements (Mg = 650∘C and Ni = 1452∘C) itis difficult to prepare these alloys by conventional meltingand solidification processes Therefore MA has been used toconveniently synthesize these alloys since this is a completelysolid-state processing method An added advantage of MA isthat the product will have a nanocrystalline grain structureallowing the improvements described below [57 58]

Figure 8 shows the hydrogenation and dehydrogena-tion plots for conventional and nanocrystalline alloysAn additional plot for a situation where a catalyst isadded to the nanocrystalline material is also shown Fromthese two graphs the following facts become clear Firstlythe nanocrystalline alloys adsorb hydrogen much fasterthan conventional (coarse-grained) alloys even though the

12 Research

0 1 2 3 4 5 6 7 8 9 10time (min)

conventional

p = 8 bar

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

time (min)

nano + catalyst

nano + catalyst

nanocrystallinenanocrystalline

conventional

breakthrough in reaction kinetics due to nanostructure and use of catalyst

0

1

2

3

4

5

6

7

8

hydr

ogen

cont

ent (

wt

)

T = 300∘C p = 10-3 mbar

Figure 8 Comparison of the hydrogen sorption and desorption kinetics of Mg alloys with conventional and nanocrystalline grain sizesSignificant improvement in the desorption kinetics when a catalyst is added is worth noticing

amount of hydrogen adsorbed is the sameWhile the conven-tional alloys do not adsorb any significant amount of hydro-gen in about 10 min the allowable full amount of hydrogenis adsorbed in the nanocrystalline alloy Similar observationsare made by many other authors in many different alloysFurther the kinetics are slightly faster (not much different)when a catalyst is added to the nanocrystalline alloy Onthe other hand the kinetics appear to be very significantlyaffected during the desorption process For example thenanocrystalline alloys desorb hydrogen much faster thanthe conventional alloys Again while the conventional alloysdo not desorb any significant amount of hydrogen thenanocrystalline alloy desorbs over 20 of the hydrogenstored in about 10 min But the most significant effect wasobserved when a catalyst was added to the nanocrystallinealloyAll the adsorbedhydrogenwas desorbed in about 2minwhen a catalyst was added to the nanocrystalline alloy Thusnanostructure processing throughMAappears to be a fruitfulway of developing novel hydrogen storage materials [59]

Many novel complex hydrides based on light metals(Mg Li and Na) are being explored and the importantcategories of materials being investigated include the alanates(also known as aluminohydrides a family of compoundsconsisting of hydrogen and aluminum) borohydrides amideborohydrides amide-imide systems (which have a highhydrogen storage capacity and low operative temperatures)amine borane (compounds of borane groups having ammo-nia addition) and alane

73 Energy Materials The technique of MA has also beenemployed to synthesize materials for energy applicationsThese include synthesis of photovoltaic (PV) materials andultrafine particles for faster ignition and combustion pur-poses

One of the most common PV materials is CIGS acompound of Cu In Ga and Se with the compositionCu(InGa)Se2 Since PV cells have the highest efficiencywhen

the band gap for this semiconductor is 125 eV one will haveto engineer the composition for the ratio of Ga to In This isbecause the band gaps for CuInSe

2 and CuGaSe2 are 102 and170 eV respectively Accordingly a Ga(Ga+In) ratio of about03 is found most appropriate and the final ideal compositionof the PV material will be Cu097In07Ga03Se2

The traditional method of synthesizing this material isto prepare an alloy of Cu Ga and In by vapor depositionmethods and incorporate Se into this by exposure to Seatmosphere Consequently this is a complex process MAhowever can be a simple direct and efficient method ofmanufacturing Blended elemental powders of Cu In Gaand Se weremilled in a copper container for different periodsof time and the phase constitution was monitored throughX-ray diffraction studies It was noted that the tetragonalCu(InGa)Se

2phase has formed even aftermilling the powder

mix for 20 min Process variables such as mill rotation speedball-to-powder weight ratio (BPR) and milling time werevaried and it was noted that the ideal composition wasachieved at lowmilling speeds short milling times and a lowBPR (Table 2) [60] By fine-tuning these parameters it shouldbe possible to achieve the exact composition

Nanostructuredmetal particles have high specific surfaceareas and consequently they are highly reactive and can storeenergy This area has received a great impetus in recentyears since the combustion rates of nanostructured metalparticles aremuch higher than those of coarsemetal particlesThe particle size cannot be exceedingly small because thesurface may be coated with a thin oxide layer and make theparticles passive [61ndash63] MA is very well suited to preparesuch materials because the reactions are easily initiatedby impact or friction and at the same time milling isa simple scalable and controllable technology capable ofmixing reactive components on the nanoscaleThese reactivematerials could be used as additives to propellants explosivesand pyrotechnics as well asmanufacture of reactive structuralcomponents [63]

Research 13

Table 2 Chemical analysis (at) of the Cu-In-Ga-Se powder mechanically alloyed under different processing conditions

BPR Speed (rpm) Time (min) Cu In Ga Se201 300 120 3261 1523 653 4562201 300 240 3173 1563 671 4593101 150 40 2598 1698 725 4978Targeted (Cu

097In07Ga03Se2) 2443 1763 756 5038

Compared to the presently used liquid aviation fuelsmetal particles based on B Al andMg are effective HoweverB and Al have very high boiling points and high heats ofenthalpy On the other hand Mg has a low boiling point butalso low heat of enthalpy Therefore if Al and Mg could bemixed intimately on a nanometer scale one could hope tohave a high heat of enthalpy at reduced boiling temperaturesThat is Mg-Al alloy particles could be practical in achievinga high-energy density and practical ignitionboiling temper-atures MA is a very effective method to accomplish this

Al and Mg powders were blended together in differentproportions and milled Since the milled powders have arange of particle sizes they were sieved to obtain well-defined powder sizes They were then fluidized and thencombusted in a CH4-air premixed flame Both the ignitionand burning times of these metal particles decreased withdecreased particle sizes and were also shorter when the Mgcontent is higher (Figure 9) [64] Similar results have beenreported by other investigators

8 Powder Consolidation

The product of MA is in powder form Except in cases ofchemical applications such as catalysis when the productdoes not require consolidation widespread application ofMA powders requires efficient methods of consolidatingthem into bulk shapes Successful consolidation of MA pow-ders is a nontrivial problem since fully densematerials shouldbe produced while simultaneously retaining the nanometer-sized grains without coarsening or the amorphous phaseswithout crystallizing Conventional consolidation of powdersto full density through processes such as hot extrusion [65]and hot isostatic pressing (HIP) [66] requires use of highpressures and exposure to elevated temperatures for extendedperiods of time [67] Unfortunately however this resultsin significant coarsening of the nanometer-sized grains orcrystallization of amorphous phases and consequently thebenefits of nanostructure processing or amorphization arelost [68] On the other hand retention of finemicrostructuresrequires use of low consolidation temperatures and then itis difficult to achieve full interparticle bonding at these lowtemperatures Therefore novel and innovative methods ofconsolidating MA powders are required The objectives ofconsolidation of these powders are to (a) achieve full densifi-cation (without any porosity) (b) minimize microstructuralcoarsening (ie retention of fine grain size) andor (c)avoid undesirable phase transformations In fact the earlyresults of excellent mechanical properties and most specifi-cally superplasticity at room temperature [69] attributed to

20 Mg (tig)90 Mg (tig)

20 Mg (tb)90 Mg (tb)

20 40 60 80 100 1200Particle Size (m)

0

10

20

30

40

50

60

70

Igni

tion

Burn

ing

Tim

e (m

s)

Figure 9 Ignition and burning times as a function of particle sizefor the mechanically alloyed Al-20 and Al-90 at Mg powdersIn the insert tig and tb represent the ignition and burning timesrespectively

nanocrystalline ceramics have not been successfully repro-duced due to incomplete consolidation of the nanopowdersin the material tested Thus consolidation to full densityassumes even greater importance

Because of the small size of the powder particles (typicallya few micrometers even though the grain size is only afew nanometers) MA powders pose special problems Forexample such nanometric powders are very hard and strongThey also have a high level of interparticle friction Furthersince these powders have a large surface area they also exhibithigh chemical reactivity Therefore special precautions needto be taken to consolidate the metal powders to bulk shapes

Successful consolidation of MA powders has beenachieved by electrodischarge compaction plasma-activatedsintering shock (explosive) consolidation [70] HIP hydro-static extrusion strained powder rolling and sinter forging[71] By utilizing the combination of high temperature andpressure HIP can achieve a particular density at lowerpressure when compared to cold isostatic pressing or at lowertemperature when compared to sintering Because of theincreased diffusivity in nanocrystalline materials sintering(and therefore densification) takes place at temperaturesmuch lower than those in coarse-grained materials This islikely to reduce the grain growth Spark plasma sintering(SPS) has become the most popular technique to consolidatemetal powders especially those with ultrafine grain sizesdue to the fact that full density can be achieved at low

14 Research

SOLID LIQUID GAS

MILLING

USE POWDER AS-ISor Chemical Reaction

Thermal Treatment etcCONSOLIDATE TO SOLID

(Temperature Pressure)

Catalysts Pigments SolderComponents Ceramic Powder etc

Advanced materials with hightolerance to gaseous impurities

Figure 10 Alternatives to consolidation of metal powders Ifpowders are used as catalysts pigments solder components etcthen they need not be consolidated However if powders need tobe consolidated then they could be used in applications with hightolerance to gaseous impurities

temperatures and shorter times in comparison to severalothermethods [72 73] An excellent reviewbyGroza [71]maybe consulted for full details of the methods of consolidationand description of results

9 Powder Contamination

Amajor concern inmetal powders processed byMAmethodsis the nature and amount of impurities that get incorporatedinto the powder and contaminate it The small size of thepowder particles and consequent availability of large surfacearea formation of fresh surfaces during milling and wearand tear of the milling tools and the atmosphere underwhich the powder is milled all contribute to contaminationof the powder In addition to these factors one has toconsider the purity of the starting raw materials and themilling conditions employed (especially the milling atmo-sphere) Thus it appears as though powder contaminationis an inherent drawback of the method and that the milledpowder will always be contaminated with impurities unlessspecial precautions are taken to avoidminimize them Themagnitude of powder contamination appears to depend onthe time of milling intensity of milling atmosphere in whichthe powder is milled nature and size of the milling mediumand differences in the strengthhardness of the powder andthe milling medium and the container

Several attempts have been made in recent years to min-imize powder contamination during milling An importantpoint to remember is that if the starting powders are highlypure then the final product is going to be clean and purewith minimum contamination from other sources providedthat proper precautions are taken during milling It is alsoimportant to keep inmind that all applications do not requireultraclean powders and that the purity desired will depend onthe specific application (Figure 10) But it is a good practiceto minimize powder contamination at every stage and ensurethat additional impurities are not picked up subsequentlyduring handling andor consolidation

One way ofminimizing contamination from the grindingmedium and themilling container is to use the samematerial

for the container and the grinding medium as the powderbeing milled Thus one could use copper balls and coppercontainer formilling copper and copper alloy powders [60] Ifa container of the same material to be milled is not availablethen a thin adherent coating on the internal surface of thecontainer (and also on the surface of the grinding medium)with the material to be milled will minimize contaminationThe idea here is to mill the powder once allowing the powderto be coated onto the grindingmedium and the inner walls ofthe container The milled loose powder is then discarded anda fresh batch of powder is milled but with the old grindingmedia and container In general a simple rule that shouldbe followed to minimize contamination from the millingcontainer and the grinding medium is that the container andgrindingmedium should be harderstronger than the powderbeing milled

Milling atmosphere is perhaps the most important vari-able that needs to be controlled Even though nominally puregases are used to purge the glove box before loading andunloading the powders even small amounts of impurities inthe gases appear to contaminate the powders especially whenthey are reactive The best solution one could think of willbe to place the mill inside a chamber that is evacuated andfilled with high-purity argon gas Since the whole chamberis maintained under argon gas atmosphere (continuouslypurified to keep oxygen and water vapor below 1 ppm each)and the container is inside the mill which is inside thechamber contamination from the atmosphere is minimum

Contamination from PCAs is perhaps the most ubiqui-tous Since most of the PCAs used are organic compoundswhich have low melting and boiling points they decomposeduring milling due to the heat generated The decomposi-tion products consisting of carbon oxygen nitrogen andhydrogen react with the metal atoms and form undesirablecarbides oxides nitrides etc

10 Concluding Remarks and Future Prospects

We have only briefly touched upon the scientific and tech-nological aspects of MA materials along with some of theconcerns such as powder contamination and consolidationissues The historical development of MA during the last 50years or so can be divided into threemajor periods (Figure 11)The first period covering the first twenty years (from 1966up to about 1985) was mostly concerned with the devel-opment and production of oxide-dispersion-strengthened(ODS) superalloys for applications in the aerospace indus-try Several alloys with improved properties based onNi and Fe were developed and found useful applicationsThese included the MA754 MA760 MA956 MA957 andMA6000 [74] A more recent survey of application of MAto steel is also available [75] The second period of aboutfifteen years covering from about 1986 up to about 2000witnessed advances in the fundamental understanding ofthe processes that take place during MA Along with thisa large number of dedicated conferences were held andthere was a burgeoning of publication activity Comparisonshave also been frequently made between MA and othernonequilibriumprocessing techniquesmore specifically RSP

Research 15

1965 19851975 1995 2005

Production of ODS Alloys

Novel Phase SynthesisDevelopment of BasicUnderstanding of MA

Nanostructures andNew Applications

2015

Figure 11 Schematic showing the different periods of development in the field of mechanical alloying (MA) and applications ofMA productssince its inception in 1965

It was shown that themetastable effects achieved by these twononequilibrium processing techniques are similar Revival ofthe mechanochemical processing (MCP) took place and avariety of novel substances were synthesized Several mod-eling studies were also conducted to enable prediction of thephases produced or microstructures obtained although withlimited success The third period starting from about 2001saw a reemergence of the quest for new applications of theMA materials not only as structural materials but also forchemical applications such as catalysis and hydrogen storagewith the realization that contamination of themilled powdersis the limiting factor in the widespread applications of theMA materials Innovative techniques to consolidate the MApowders to full density while retaining the metastable phases(including glassy phases andor nanostructures) in themwerealso developed All these are continuing with the ongoinginvestigations to enhance the scientific understanding of theMA process

At present activities relating to both the science andtechnology of MA have been going on simultaneously Whilesome groups have been focusing on developing a betterunderstanding of the process and optimizing the processvariables to achieve a certain phase or phase combinationin the alloy system others have been concentrating onexploiting this technique for different applications It hasalso come to be realized that the MA process can be usedto recycle used and discarded materials to produce value-added materials for subsequent consumption They can bepulverized separated and used to mix with other powdersto produce composites The bottom line is that newer andimproved materials are being continuously developed fornovel applications

Conflicts of Interest

The author declares that he has no conflicts of interest

References

[1] K H J Buschow R W Cahn M C Flemings B Ilschner E JKramer and SMahajan EdsEncyclopedia ofMaterials Scienceand Technology Elsevier 2001

[2] C Suryanarayana EdNon-equilibrium Processing of MaterialsPergamon Oxford UK 1999

[3] C Suryanarayana ldquoMetallic glassesrdquo Bulletin of Materials Sci-ence vol 6 no 3 pp 579ndash594 1984

[4] C Suryanarayana and A Inoue Bulk Metallic Glasses CRCPress Boca Raton Fla USA 2nd edition 2018

[5] H R Trebin Ed Quasicrystals Structure and Physical Proper-ties Wiley-VCH Weinheim Germany 2003

[6] C Suryanarayana and H Jones ldquoFormation and characteristicsof quasicrystalline phases A reviewrdquo International Journal ofRapid Solidification vol 3 pp 253ndash293 1988

[7] H Gleiter ldquoNanocrystalline materialsrdquo Progress in MaterialsScience vol 33 no 4 pp 223ndash315 1989

[8] C Suryanarayana ldquoRecent developments in nanostructuredmaterialsrdquo Advanced Engineering Materials vol 7 no 11 pp983ndash992 2005

[9] M C Gao J-W Yeh P K Liaw and Y Zhang Eds High-Entropy Alloys Fundamentals and Applications Springer 2016

[10] J S Benjamin ldquoMechanical alloying ndash History and futurepotentialrdquo in Advances in Powder Metallurgy and ParticulateMaterials J M Capus and R M German Eds vol 7 ofNovel Powder Processing pp 155ndash168 Metal Powder IndustriesFederation Princeton NJ USA 1992

[11] M P Phaniraj D-I Kim J-H Shim and Y W CholdquoMicrostructure development in mechanically alloyed yttriadispersed austenitic steelsrdquo Acta Materialia vol 57 no 6 pp1856ndash1864 2009

[12] A Hirata T Fujita Y R Wen J H Schneibel C T Liu and MW Chen ldquoAtomic structure of nanoclusters in oxide-dispersionstrengthened steelsrdquo Nature Materials vol 10 no 12 pp 922ndash926 2011

[13] M K Miller C M Parish and Q Li ldquoAdvanced oxide disper-sion strengthened and nanostructured ferritic alloysrdquoMaterialsScience and Technology (United Kingdom) vol 29 no 10 pp1174ndash1178 2013

[14] N Oono Q X Tang and S Ukai ldquoOxide particle refinementin Ni-based ODS alloyrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 649 pp 250ndash253 2016

[15] D B Witkin and E J Lavernia ldquoSynthesis and mechanicalbehavior of nanostructured materials via cryomillingrdquo Progressin Materials Science vol 51 no 1 pp 1ndash60 2006

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

6 Research

crystal lattice and results in the formation of an amorphousphase With the introduction of bulk metallic glasses nearlythirty years ago a very large number of new criteria mostlybased on the glass transformation temperatures (119879g 119879x and119879l where 119879x represents the crystallization temperature of theamorphous alloy) have been proposed In spite of a very largenumber of criteria proposed [4] it has not been possible yetto accurately predict the glass-forming ability of alloys [39]

There are some significant differences in the formation ofmetallic glasses by the MA and RSP methods For examplemetallic glasses are obtained by MA in almost every alloysystem provided that sufficient energy has been stored in thepowder But metallic glasses are obtained by RSP methodsonly in certain composition rangesmdashabout 20 at metalloidcontent in metal-metalloid systems and different composi-tions in metal-metal systems mostly at compositions cor-responding to deep eutectics Additionally glass formationby MA occurs by the accumulation of crystal defects whichraises the free energy of the crystalline phase above that of thehypothetical glassy phase under which conditions the crys-talline phase becomes destabilized On the other hand forglass formation by the RSP methods the critical cooling ratefor glass formation needs to be exceeded so that formationof the crystalline nuclei is completely suppressed Since MAis a completely solid-state powder processing technique thecriteria for amorphous phase formation should be differentfrom those of solidification methods A new criterion wasproposed that is the number of intermetallics present inthe constituent phase diagrams determines the glass-formingability of alloys processed by MAmdashthe more the number ofintermetallics in the constituent phase diagrams the easier itis to amorphize the powder blend [40]

Amorphization by MA occurs when the free energy ofthe hypothetical amorphous phase 119866A is lower than that ofthe crystalline phase G119862 ie 119866A lt 119866C A crystalline phasenormally has a lower free energy than the amorphous phaseBut its free energy can be increased by introducing a varietyof crystal defects such as dislocations grain boundaries andstacking faults If an intermetallic has formed then additionalenergy can be introduced by disordering the crystal lattice Bythis approach it is then possible to obtain a situation when

119866A lt 119866C + 119866D (1)

where 119866D is the free energy increase due to introductionof crystal defects It has been shown that the presence ofintermetallics in an alloy system can significantly increasethe energy contributing to amorphization This is due totwo important effects Firstly disordering of intermetallicscontributes an energy of about 15 kJmol to the systemSecondly a slight change in the stoichiometry of the inter-metallic increases the free energy of the system drasticallyAdditionally grain size reduction contributes about 5 kJmolFurther disordering of intermetallics has also been shownto be possible by heavy deformation Since MA reduces thegrain size to nanometer levels and also disorders the usuallyordered intermetallics the energy of the milled powders issignificantly raised In fact it is raised to a level above that ofthe hypothetical amorphous phase and thus formation of theamorphous phase is favored over the crystalline phase

41 Mechanical Crystallization It was reported that an amor-phous phase formed by milling transforms into a crystallinephase on continued milling and that this phenomenonhas been referred to as mechanical crystallization [41 42]The process of mechanical crystallization is very unusualand offers many opportunities to design novel alloys withimproved properties For example it will be possible toproduce the alloy in either a fully amorphous amorphous +nanocrystalline composite or a completely crystalline statewith different grain sizes depending upon the time ofmillingor the temperature and time of subsequent annealing of theamorphous alloy

Figure 3(a) shows the XRD patterns of the blended ele-mental powder mixture of the composition Fe42Ge28Zr10B20mechanically alloyed for 10 and 60 h [42] While the pat-tern at 10 h of milling clearly shows the presence of anamorphous phase the pattern at 60 h of milling showsthe presence of a crystalline phase superimposed over thebroad and diffuse halo representing the amorphous phaseThe crystalline phase has been identified as the 120572-Fe solidsolution containing the solute elements present in the blendThis 120572-Fe phase is the first phase to crystallize from theamorphous alloy (as in primary crystallization) and externalannealing of the milled powder showed that the amount ofthe 120572-Fe increased along with the presence of other inter-metallics Eutectic crystallization (simultaneous formation oftwo crystalline phases) has also been observed in other alloysystems

Mechanical crystallization has also been observed inseveral other alloy systems that formed the amorphous phaseon MA In fact it is now becoming apparent that thisphenomenon of mechanical crystallization is not as rare asit was once thought to be

Many reasons have been suggested for the formation of acrystalline phase after the formation of an amorphous phaseduring MA These include (a) rise in temperature to a levelabove that of the crystallization temperature of the amor-phous alloy (b) powder contamination due to which a stablecrystalline impurity phase forms (c) phenomenon of inversemelting and (d) basic thermodynamic considerations It nowappears clear that the basic thermodynamic stabilities of thedifferent phases under the different conditions of milling areresponsible for mechanical crystallization Such a situationcan be explained with reference to a free energy versuscomposition diagram

Figure 3(b) is a schematic diagram showing the variationof free energy with composition for the Fe42X28Zr10B20 (X =AlGe andNi) alloy systems investigatedThepossible (stableand metastable) constitution in this system is (a) blendedelemental (BE) powder mixture (b) 120572-Fe the solid solutionof all the alloying elements in Fe (c) an amorphous phase (d)intermetallic phases and (e) different combinations of thesephases The stability of any phase will be determined by itsrelative position in the free energy vs composition plotmdashthelower the free energy the more stable the phase is Since it ispossible to have a large number of intermetallic phases in thismulticomponent system and since it is difficult to indicateeach of them separately for simplicity all of them are groupedtogether as ldquointermetallicsrdquo

Research 7Fr

ee E

nerg

y

Solute Concentration (at)20 40 60 80Fe 100

BE powder

amorphous

-Fe

Intermetallics

1

23

45

Inte

nsity

30 40 502-Theta

(a)

(b)

10 h

60 h

60 70 80 90

1

Figure 3 (a) X-ray diffraction patterns of the Fe42Ge28Zr10B20

powder blend milled for 10 and 60 h While the powder milledfor 10 h shows the presence of an amorphous phase the powdermilled for 60 h shows the presence of a crystalline phase coexistingwith the amorphous phase suggesting that the amorphous phase hascrystallized (b) Hypothetical free energy vs composition diagramto explain the mechanism of mechanical crystallization in theFe42X28Zr10B20systemNote that point ldquo1rdquo represents the free energy

of the blended elemental powders Similarly point ldquo2rdquo representsformation of the 120572-Fe solid solution containing all the alloyingelements in Fe point ldquo3rdquo formation of the homogeneous amorphousphase point ldquo4rdquo a mixture of the amorphous phase with a differentsolute content (amorphous) from that at ldquo3rdquo and the solid solution120572-Fe with different solute content and point ldquo5rdquo the equilibriumsituation when the solid solution and intermetallic phases coexist

The free energy of the BE powder mixture is indicated bypoint ldquo1rdquo which represents the point of intersection of thecomposition vertical with the line joining the free energiesof pure Fe and the ldquosoluterdquo On milling this powder a solidsolution phase containing all the solute elements in Fe (ora mixture of intermetallics and a solid solution in somecases) is seen to form and its free energy is indicated bypoint ldquo2rdquoThe solid solution forms because it has a lower free

energy than the BE powder mixture Since MA introduces avariety of crystal defects the crystalline phases in the milledpowders will contain excess energyThis energy will continueto increase with milling time and reach a value which isabove that of the metastable amorphous phase Thus theamorphous phase gets stabilized (point ldquo3rdquo) On primarycrystallization of the amorphous phase the new constitutionwill be a mixture of the 120572-Fe solid solution (or intermetallics)and the amorphous phase which now has a compositiondifferent from that of the original amorphous phase Furtherthe new solid solution phase also has a composition differentfrom the original 120572-Fe phase The free energy of the mixtureof this solid solution and the amorphous phase indicatedby ldquopoint 4rdquo will have a free energy lower than that of theamorphous phase The equilibrium mixture of the 120572-Fe solidsolution and ldquointermetallicsrdquo will have the lowest free energyof all the phase mixtures as indicated by point ldquo5rdquo in thefigure The process of mechanical crystallization is uniqueto the MA process and can be achieved only on continuedmilling of the amorphous powder ormilling of an amorphousalloy obtained by other methods

5 Nanostructured Materials

Another important attribute of MA has beenmicrostructuralrefinement The most common observation is that grainrefinement takes place resulting in the formation of very finegrains often down to nanometer levels A two-phase alloyunder equilibrium conditions can transform to a single-phasealloy due to formation of a supersaturated solid solution Anamorphous alloy can crystallize under the action of severeplastic deformation Most usefully one can also introducea high volume fraction of fine second phase particles andobtain a composite with a uniform distribution of thesecond phase particles By a proper choice of the size ofthe matrix andor the reinforcement phase one can producenanocomposites a very fruitful area for investigations Infact MA has been the most popular technique to synthesizenanostructured materials and nanocomposites in a numberof alloy systems

It has been frequently demonstrated that mechanicallyalloyed materials show powder particle refinement as wellas grain refinement as a function of the milling time Thesesizes decrease very rapidly in the early stages of milling andmore gradually later Eventually in almost all the cases thegrain size of the mechanically alloyed materials is in thenanometer region unless the powder becomes amorphousThe minimum grain size achieved has been reported tobe a few nanometers ranging typically from about 5 to50 nm but depending on the material and processingconditions For example it was reported that the grain sizedecreases with increasing milling energy higher ball-to-powder weight ratio and lower temperatures As is wellknown nanostructuredmaterials exhibit high strength goodductility excellent sintering characteristics and interestingelectrical andmagnetic properties [7 8 27 43]The ease withwhich nanostructured materials can be synthesized is onereason why MA has been extensively employed to producenanocrystalline and nanocomposite materials

8 Research

From high-resolution transmission electron microscopy(TEM) observations it was noted that in the early stages ofmilling due to the high deformation rates experienced bythe powder deformation was localized within shear bands[44] These shear bands which contain a high density ofdislocations have a typical width of approximately 05 to 10120583m Small grains with a diameter of 8ndash12 nm were seenwithin the shear bands and electron diffraction patternssuggested significant preferred orientation With continuedmilling the average atomic level strain increased due toincreasing dislocation density and at a certain dislocationdensity within these heavily strained regions the crystaldisintegrated into subgrains that are separated by low-anglegrain boundaries This resulted in a decrease of the latticestrain The subgrains formed this way were of nanometerdimensions and are often between 20 and 30 nm

On further processing deformation occurred in shearbands located in previously unstrained parts of the materialThe grain size decreased steadily and the shear bands coa-lesced The small-angle boundaries are replaced by higherangle grain boundaries implying grain rotation as reflectedby the absence of texture in the electron diffraction patternsand random orientation of the grains observed from thelattice fringes in the high-resolution TEMmicrographs Con-sequently dislocation-free nanocrystalline grains are formedThis is the currently accepted mechanism of nanocrystalformation in MA processed powders

The grain size of the milled materials decreases withmilling time and reaches a saturation level when a balance isestablished between the fracturing and cold welding eventsThis minimum grain size 119889min is different depending on thematerial and milling conditionsThe value of 119889min achievableby milling is determined by the competition between theplastic deformation via dislocation motion that tends todecrease the grain size and the recovery and recrystallizationbehavior of the material that tends to increase the grain sizeThis balance gives a lower bound for the grain size of puremetals and alloys

The 119889min obtained is different for different metals and isalso found to vary with the crystal structure (Figure 4) Inmost of the metals the minimum grain size attained is inthe nanometer dimensions But metals with a body-centeredcubic (BCC) crystal structure reach much smaller valuesin comparison to metals with the other crystal structuresrelated to the difficulty of extensive plastic deformation andconsequent enhanced fracturing tendency during millingCeramics and intermetallic compounds are much harderand usually more brittle than the metals on which they arebased Therefore intuitively one expects that 119889min of thesecompounds is smaller than those of the pure metals but thisdoes not appear to be the case always

It was also reported that 119889min decreases with an increasein the melting temperature of the metal [45] As shown inFigure 4 this trend is amply clear in the case of metalswith close-packed structures (face-centered cubic (FCC) andhexagonal close-packed (HCP)) but not so clear in metalswith a BCC structure Another point of interest is that thedifference in grain size is much less among metals thathave high melting temperatures the minimum grain size is

0

5

10

15

20

25

FCCBCCHCP

Min

imum

Gra

in S

ize (

nm)

0 2000 30001000Melting Temperature (K)

Figure 4 Variation of minimum grain size with temperature inmechanically milled FCC HCP and BCC metals

virtually constant Thus for the HCP metals Co Ti Zr Hfand Ru the minimum grain size is almost the same eventhough themelting temperatures vary between 1495∘C for Coand 2310∘C for Ru

Analysis of the existing data on 119889min of mechanicallymilled pure metals shows that the normalized minimumgrain size 119889minb where b is the Burgers vector of thedislocations decreases with increasing melting temperatureactivation energy for self-diffusion hardness stacking faultenergy bulk modulus and the equilibrium distance betweentwo edge dislocations The 119889min value is also affected bymilling energy (the higher the milling energy the smaller thegrain size) milling temperature (the lower the milling tem-perature the smaller the grain size) and alloying additions(the minimum grain size is smaller for solid solutions thanfor pure metals since solid solutions are harder and strongerthan the pure metals on which they are based)

It was suggested that 119889min is determined by the minimumgrain size that can sustain a dislocation pile-up within a grainand by the rate of recovery Based on the dislocation pile-up model the critical equilibrium distance between two edgedislocations in a pile-up L119888 (which could be assumed to bethe crystallite or grain size inmilled powders) was calculated[46] using the equation

119871119888=3119866119887

(1 minus ])119867(2)

where G is the shear modulus 119887 is the Burgers vector ] isthe Poissonrsquos ratio and H is the hardness of the materialAccording to the above equation increased hardness resultsin smaller values of 119871c (grain size) and an approximate linearrelationship was observed between 119871c and the minimumgrain size obtained by milling of a number of metals Otherattempts have also beenmade to theoretically predict 119889min onthe basis of thermodynamic properties of materials [47]

Research 9

6 Nanocomposites

Achievement of a uniform distribution of the reinforcementin a matrix is essential to the achievement of good anduniform mechanical properties in composites Further themechanical properties of the composite tend to improvewith increasing volume fraction andor decreasing particlesize of the reinforcement [48] Traditionally a reasonablylarge volume fraction of the reinforcement could be addedif the size of the reinforcement is large (on a micrometerscale or larger) But if the reinforcement size is very fine(of nanometer dimensions) then the volume fraction addedis typically limited to about 2 to 4 However if a largevolume fraction of nanometer-sized reinforcement can beintroduced the mechanical properties of the composite arelikely to be vastly improved Therefore several investigationshave been undertaken to produce improved nanocompositesthrough MA [20]

Composites are traditionally produced by solidificationprocessing methods since the processing is inexpensive andlarge tonnage quantities can also be produced But it isnot easy to disperse fine reinforcement particles in a liquidmetal since agglomeration of the fine particles occurs andclusters are produced Further wetting of the particles ispoor and consequently even the most vigorous stirring isnot able to break the agglomerates Additionally the finepowders tend to float to the top of the melt during processingof the composites through solidification processing Thussolid-state processing methods such as MA are effective indispersing fine particles (including nanoparticles) An addedadvantage is that since no melting is involved even a largevolume fraction of the reinforcement can be introduced

Some of the specific goals sought in processing ofnanocomposite materials throughMA include incorporationof a high volume percentage of ultrafine reinforcements andachievement of high ductility and even superplasticity insome composites It is also possible that due to the increasedductility the fracture toughness of these composites couldbe higher than that of their coarse-grained counterparts Inline with these goals we have in recent times accomplished(a) achievement of a very uniform distribution of the rein-forcement phases in different types of matrices includingdispersion of a high volume fraction of Al2O3 in Al [49] andof Ti5Si3in 120574ndashTiAl [50 51] and (b) synthesis of MoSi

2+Si3N4

composites for high-temperature applications [52] amongothers However we will describe here only the developmentof Ti5Si3+120574ndashTiAl composites as a typical example

Lightweight intermetallic alloys based on 120574-TiAl arepromising materials for high-temperature structural appli-cations eg in aircraft engines or stationary turbines Eventhough they have many desirable properties such as highspecific strength and modulus both at room and elevatedtemperatures and good corrosion and oxidation resistancethey suffer from inadequate room temperature ductility andinsufficient creep resistance at elevated temperatures espe-cially between 800 and 850∘C an important requirement forelevated temperature applications of these materials There-fore current research programs have been addressing thedevelopment of high-temperature materials with adequate

020406080

100120140160180200

Engi

neer

ing

stres

s (M

Pa)

20 40 60 80 100 120 140 1600Engineering strain ()

-5950

∘C 4x10

-4950

∘C 4x10

-41000

∘C 4x10x10

(a)

(b)

Figure 5 (a) Scanning electronmicrograph of the 120574-TiAl + 60 volTi5Si3composite specimen showing that the twophases are very uni-

formly distributed in the microstructure Such a microstructure isconducive to achieving superplastic deformation under appropriateconditions of testing (b) Tensile stress-strain curve of 120574-TiAl + 60vol Ti

5Si3composite showing that superplastic deformation was

achieved at 950∘C with a strain rate of 4x10minus5 sminus1 and at 1000∘C witha strain rate of 4x10minus4 sminus1

room temperature ductility for easy formability and abilityto increase the high-temperature strength by a suitable heattreatment or alloying additions to obtain sufficient creepresistance at elevated temperatures

Composites of 120574-TiAl and 120585-Ti5Si3phase with the volume

fractions of the 120585-Ti5Si3phase varying from 0 to 60 vol

were produced by MA of a combination of the blendedelemental and prealloyed intermetallic powders Fully denseand porosity-free compacts were produced by hot isostaticpressing with the resulting grain size of each of the phasesbeing about 300-400 nm Figure 5(a) shows scanning elec-tron micrographs of the 120574-TiAl+60 vol 120585-Ti5Si3 compositeshowing that the two phases are very uniformly distributedthroughout the microstructure Materials exhibiting such amicrostructure (roughly equal amounts of the two phasessmall grain sizes and a uniform distribution of the twophases) are expected to be deformed superplastically Totest this hypothesis both compression and tensile testingof these composite specimens were conducted at differenttemperatures and strain rates From the tensile stress-strain

10 Research

Pure Metals Alloysand Compounds

NanostructuredMaterials

MECHANICALALLOYING

ODS Superalloys Supercorroding

Alloys

Synthesis of Novel Phases PVDTargets Solders Paints Food Heaters

Biomaterials Waste Utilization etc

Hydrogen Storage MagneticMaterials Catalysts Sensors etc

Figure 6 An overview of the applications of the MA process

plots of the 120574-TiAl+60 vol 120585-Ti5Si3composite specimens

shown in Figure 5(b) it is clear that the specimens testedat 950∘C with a strain rate of 4times10minus5 sminus1 and at 1000∘Cwith a strain rate of 4times10minus4 sminus1 exhibited large ductilityof nearly 150 and 100 respectively Considering that thiscomposite is based on a ceramic material (Ti

5Si3) this is a

very high amount of deformation suggestive of superplasticdeformation Final proof is provided by TEM investiga-tions that confirm the continued stability of the equiaxedmicrostructure after deformation

7 Applications

As has been frequently pointed out in the literature thetechnique ofMAwas developed out of an industrial necessityin 1965 to produce ODS nickel-based superalloys Sincethen a number of new applications have been exploredfor mechanically alloyed materials (Figure 6) Apart fromthe ODS alloys which continue to be used in the high-temperature applications area MA powders have also beenused for other applications and newer applications are alsobeing explored These novel applications include direct syn-thesis of pure metals and alloys from ores and scrap mate-rials super-corroding alloys for release into ocean bottomsdevelopment of PVD (physical vapor deposition) targetssolders paints etc hydrogen storage materials photovoltaicmaterials catalysts and sensors

A novel and simple application of MA powders is in thedevelopment ofmeals ready to eat (MRE) In this applicationvery finely ground Mg-based alloy powders are used Whenwater is added to these fineMg-Fe powders heat is generatedaccording to the following reaction

Mg (Fe) + 2H2O 997888rarr Mg (OH)2 +H2 +Heat (3)

This heat is utilized to warm the food in remote areas wherecooking or heating facilities may not be available eg in war-torn areas These ldquofood heatersrdquo were widely used during theDesert Stormoperations in the 1990s Let us now look at someof the other applications

71 Pure Metals and Alloys The technique of MA has beenemployed to produce pure metals and alloys from ores andother raw materials such as scrap The process is based onchemical reactions induced by mechanical activation Mostof the reactions studied have been displacement reactions ofthe type

119872119874 + 119877 997888rarr 119872 + 119877119874 (4)

where ametal oxide (MO) is reduced by amore reactivemetal(reductant R) to the pure metal M In addition to oxidesmetal chlorides and sulfides have also been reduced to puremetals this way Similar reactions have also been used toproduce alloys and nanocomposites [21 53]

All solid-state reactions involve the formation of a prod-uct phase at the interface of the component phases Thusin the above example the metal M forms at the interfacebetween the oxide MO and the reductant R and physicallyseparates the reactants Further growth of the product phaseinvolves diffusion of atoms of the reactant phases through theproduct phase which constitutes a barrier layer preventingfurther reaction from occurring In other words the reactioninterface defined as the nominal boundary surface betweenthe reactants continuously decreases during the course ofthe reaction Consequently kinetics of the reaction are slowand elevated temperatures are required to achieve reasonablereaction rates

MA can provide the means to substantially increasethe reaction kinetics of chemical reactions in general andthe reduction reactions in particular This is because therepeated cold welding and fracturing of powder particlesincrease the area of contact between the reactant powderparticles by bringing fresh surfaces to come into contactrepeatedly due to a reduction in particle size during millingThis allows the reaction to proceed without the necessityfor diffusion through the product layer As a consequencereactions that normally require high temperatures will occurat lower temperatures or even without any externally appliedheat In addition the high defect densities induced by MAaccelerate diffusion processes

Research 11

Depending on the milling conditions two entirely differ-ent reaction kinetics are possible [21 54]

(a) the reactionmay extend to a very small volumeduringeach collision resulting in a gradual transformationor

(b) if the reaction enthalpy is sufficiently high (and ifthe adiabatic temperature is above 1800 K) a self-propagating high-temperature synthesis (SHS) reaction(also known as combustion synthesis) can be initi-ated by controlling the milling conditions (eg byadding diluents) to avoid combustion reactions maybe made to proceed in a steady-state manner

The latter type of reactions requires a critical milling time forthe combustion reaction to be initiated If the temperature ofthe vial is recorded during the milling process the tempera-ture initially increases slowly with time After some time thetemperature increases abruptly suggesting that ignition hasoccurred and this is followed by a relatively slow decrease

In the above types of reactions themilled powder consistsof the pure metalM dispersed in the RO phase both usuallyof nanometer dimensions It has been shown that by anintelligent choice of the reductant the RO phase can beeasily leached out Thus by selective removal of the matrixphase by washing with appropriate solvents it is possibleto achieve well-dispersed metal nanoparticles as small as 5nm in size If the crystallinity of these particles is not highit can be improved by subsequent annealing without thefear of agglomeration due to the enclosure of nanoparticlesin a solid matrix Such nanoparticles are structurally andmorphologically uniform having a narrow size distributionThis technique has been used to produce powders of manydifferent metalsmdashAg Cd Co Cr Cu Fe Gd Nb NiTi V W Zn and Zr Several alloys intermetallics andnanocomposites have also been synthesized Figure 7 showstwo micrographs of nanocrystalline ceria particles producedby the conventional vapor synthesis and MA methods Eventhough the grain size is approximately the same in both thecases the nanoparticles produced by MA are very discreteand display a very narrow particle size distribution whilethose produced by vapor deposition are agglomerated andhave a wide particle size distribution

Novel materials have also been synthesized by MA ofblended elemental powders One of the interesting examplesin this category is the synthesis of plain carbon steels andstainless steels [55 56] Twomain advantages of this synthesisroute are that the steels can be manufactured at roomtemperature and that the steels will be nanostructured thusconferring improved properties But these powders needto be consolidated to full density for applications and itis possible that grain growth occurs during this processThe kinetics of grain growth can however be controlled byregulating the time and temperature combinations

72 Hydrogen Storage Materials Another important applica-tion of MA powders is hydrogen storage Mg is a lightweightmetal (the lightest structural material) with a density of174 gcm3 Mg and Mg-based alloys can store a significant

20 nm

50 nm

Figure 7 Transmission electron micrographs of ceria particlesproduced byMA (top) and vapor synthesis (bottom)methods Notethe severe agglomeration of the nanoparticles in the latter case

amount of hydrogen However being a reactive metal Mgalways contains a thin layer of oxide on the surface Thissurface oxide layer which inhibits asorption of hydrogenneeds to be broken down through activation A high-temperature annealing is usually done to achieve this ButMA can overcome these difficulties due to the severe plasticdeformation experienced by the powder particles duringMAA catalyst is also often required to accelerate the kinetics ofhydrogen and dehydrogenation of alloys

Mg can adsorb about 76 wt hydrogen andMg-Ni alloysmuch less However Mg is hard to activate its tempera-ture of operation is high and the sorption and desorptionkinetics are slow Therefore Mg-Ni and other alloys arebeing explored But due to the high vapor pressure of Mgand significant difference in the melting temperatures ofthe constituent elements (Mg = 650∘C and Ni = 1452∘C) itis difficult to prepare these alloys by conventional meltingand solidification processes Therefore MA has been used toconveniently synthesize these alloys since this is a completelysolid-state processing method An added advantage of MA isthat the product will have a nanocrystalline grain structureallowing the improvements described below [57 58]

Figure 8 shows the hydrogenation and dehydrogena-tion plots for conventional and nanocrystalline alloysAn additional plot for a situation where a catalyst isadded to the nanocrystalline material is also shown Fromthese two graphs the following facts become clear Firstlythe nanocrystalline alloys adsorb hydrogen much fasterthan conventional (coarse-grained) alloys even though the

12 Research

0 1 2 3 4 5 6 7 8 9 10time (min)

conventional

p = 8 bar

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

time (min)

nano + catalyst

nano + catalyst

nanocrystallinenanocrystalline

conventional

breakthrough in reaction kinetics due to nanostructure and use of catalyst

0

1

2

3

4

5

6

7

8

hydr

ogen

cont

ent (

wt

)

T = 300∘C p = 10-3 mbar

Figure 8 Comparison of the hydrogen sorption and desorption kinetics of Mg alloys with conventional and nanocrystalline grain sizesSignificant improvement in the desorption kinetics when a catalyst is added is worth noticing

amount of hydrogen adsorbed is the sameWhile the conven-tional alloys do not adsorb any significant amount of hydro-gen in about 10 min the allowable full amount of hydrogenis adsorbed in the nanocrystalline alloy Similar observationsare made by many other authors in many different alloysFurther the kinetics are slightly faster (not much different)when a catalyst is added to the nanocrystalline alloy Onthe other hand the kinetics appear to be very significantlyaffected during the desorption process For example thenanocrystalline alloys desorb hydrogen much faster thanthe conventional alloys Again while the conventional alloysdo not desorb any significant amount of hydrogen thenanocrystalline alloy desorbs over 20 of the hydrogenstored in about 10 min But the most significant effect wasobserved when a catalyst was added to the nanocrystallinealloyAll the adsorbedhydrogenwas desorbed in about 2minwhen a catalyst was added to the nanocrystalline alloy Thusnanostructure processing throughMAappears to be a fruitfulway of developing novel hydrogen storage materials [59]

Many novel complex hydrides based on light metals(Mg Li and Na) are being explored and the importantcategories of materials being investigated include the alanates(also known as aluminohydrides a family of compoundsconsisting of hydrogen and aluminum) borohydrides amideborohydrides amide-imide systems (which have a highhydrogen storage capacity and low operative temperatures)amine borane (compounds of borane groups having ammo-nia addition) and alane

73 Energy Materials The technique of MA has also beenemployed to synthesize materials for energy applicationsThese include synthesis of photovoltaic (PV) materials andultrafine particles for faster ignition and combustion pur-poses

One of the most common PV materials is CIGS acompound of Cu In Ga and Se with the compositionCu(InGa)Se2 Since PV cells have the highest efficiencywhen

the band gap for this semiconductor is 125 eV one will haveto engineer the composition for the ratio of Ga to In This isbecause the band gaps for CuInSe

2 and CuGaSe2 are 102 and170 eV respectively Accordingly a Ga(Ga+In) ratio of about03 is found most appropriate and the final ideal compositionof the PV material will be Cu097In07Ga03Se2

The traditional method of synthesizing this material isto prepare an alloy of Cu Ga and In by vapor depositionmethods and incorporate Se into this by exposure to Seatmosphere Consequently this is a complex process MAhowever can be a simple direct and efficient method ofmanufacturing Blended elemental powders of Cu In Gaand Se weremilled in a copper container for different periodsof time and the phase constitution was monitored throughX-ray diffraction studies It was noted that the tetragonalCu(InGa)Se

2phase has formed even aftermilling the powder

mix for 20 min Process variables such as mill rotation speedball-to-powder weight ratio (BPR) and milling time werevaried and it was noted that the ideal composition wasachieved at lowmilling speeds short milling times and a lowBPR (Table 2) [60] By fine-tuning these parameters it shouldbe possible to achieve the exact composition

Nanostructuredmetal particles have high specific surfaceareas and consequently they are highly reactive and can storeenergy This area has received a great impetus in recentyears since the combustion rates of nanostructured metalparticles aremuch higher than those of coarsemetal particlesThe particle size cannot be exceedingly small because thesurface may be coated with a thin oxide layer and make theparticles passive [61ndash63] MA is very well suited to preparesuch materials because the reactions are easily initiatedby impact or friction and at the same time milling isa simple scalable and controllable technology capable ofmixing reactive components on the nanoscaleThese reactivematerials could be used as additives to propellants explosivesand pyrotechnics as well asmanufacture of reactive structuralcomponents [63]

Research 13

Table 2 Chemical analysis (at) of the Cu-In-Ga-Se powder mechanically alloyed under different processing conditions

BPR Speed (rpm) Time (min) Cu In Ga Se201 300 120 3261 1523 653 4562201 300 240 3173 1563 671 4593101 150 40 2598 1698 725 4978Targeted (Cu

097In07Ga03Se2) 2443 1763 756 5038

Compared to the presently used liquid aviation fuelsmetal particles based on B Al andMg are effective HoweverB and Al have very high boiling points and high heats ofenthalpy On the other hand Mg has a low boiling point butalso low heat of enthalpy Therefore if Al and Mg could bemixed intimately on a nanometer scale one could hope tohave a high heat of enthalpy at reduced boiling temperaturesThat is Mg-Al alloy particles could be practical in achievinga high-energy density and practical ignitionboiling temper-atures MA is a very effective method to accomplish this

Al and Mg powders were blended together in differentproportions and milled Since the milled powders have arange of particle sizes they were sieved to obtain well-defined powder sizes They were then fluidized and thencombusted in a CH4-air premixed flame Both the ignitionand burning times of these metal particles decreased withdecreased particle sizes and were also shorter when the Mgcontent is higher (Figure 9) [64] Similar results have beenreported by other investigators

8 Powder Consolidation

The product of MA is in powder form Except in cases ofchemical applications such as catalysis when the productdoes not require consolidation widespread application ofMA powders requires efficient methods of consolidatingthem into bulk shapes Successful consolidation of MA pow-ders is a nontrivial problem since fully densematerials shouldbe produced while simultaneously retaining the nanometer-sized grains without coarsening or the amorphous phaseswithout crystallizing Conventional consolidation of powdersto full density through processes such as hot extrusion [65]and hot isostatic pressing (HIP) [66] requires use of highpressures and exposure to elevated temperatures for extendedperiods of time [67] Unfortunately however this resultsin significant coarsening of the nanometer-sized grains orcrystallization of amorphous phases and consequently thebenefits of nanostructure processing or amorphization arelost [68] On the other hand retention of finemicrostructuresrequires use of low consolidation temperatures and then itis difficult to achieve full interparticle bonding at these lowtemperatures Therefore novel and innovative methods ofconsolidating MA powders are required The objectives ofconsolidation of these powders are to (a) achieve full densifi-cation (without any porosity) (b) minimize microstructuralcoarsening (ie retention of fine grain size) andor (c)avoid undesirable phase transformations In fact the earlyresults of excellent mechanical properties and most specifi-cally superplasticity at room temperature [69] attributed to

20 Mg (tig)90 Mg (tig)

20 Mg (tb)90 Mg (tb)

20 40 60 80 100 1200Particle Size (m)

0

10

20

30

40

50

60

70

Igni

tion

Burn

ing

Tim

e (m

s)

Figure 9 Ignition and burning times as a function of particle sizefor the mechanically alloyed Al-20 and Al-90 at Mg powdersIn the insert tig and tb represent the ignition and burning timesrespectively

nanocrystalline ceramics have not been successfully repro-duced due to incomplete consolidation of the nanopowdersin the material tested Thus consolidation to full densityassumes even greater importance

Because of the small size of the powder particles (typicallya few micrometers even though the grain size is only afew nanometers) MA powders pose special problems Forexample such nanometric powders are very hard and strongThey also have a high level of interparticle friction Furthersince these powders have a large surface area they also exhibithigh chemical reactivity Therefore special precautions needto be taken to consolidate the metal powders to bulk shapes

Successful consolidation of MA powders has beenachieved by electrodischarge compaction plasma-activatedsintering shock (explosive) consolidation [70] HIP hydro-static extrusion strained powder rolling and sinter forging[71] By utilizing the combination of high temperature andpressure HIP can achieve a particular density at lowerpressure when compared to cold isostatic pressing or at lowertemperature when compared to sintering Because of theincreased diffusivity in nanocrystalline materials sintering(and therefore densification) takes place at temperaturesmuch lower than those in coarse-grained materials This islikely to reduce the grain growth Spark plasma sintering(SPS) has become the most popular technique to consolidatemetal powders especially those with ultrafine grain sizesdue to the fact that full density can be achieved at low

14 Research

SOLID LIQUID GAS

MILLING

USE POWDER AS-ISor Chemical Reaction

Thermal Treatment etcCONSOLIDATE TO SOLID

(Temperature Pressure)

Catalysts Pigments SolderComponents Ceramic Powder etc

Advanced materials with hightolerance to gaseous impurities

Figure 10 Alternatives to consolidation of metal powders Ifpowders are used as catalysts pigments solder components etcthen they need not be consolidated However if powders need tobe consolidated then they could be used in applications with hightolerance to gaseous impurities

temperatures and shorter times in comparison to severalothermethods [72 73] An excellent reviewbyGroza [71]maybe consulted for full details of the methods of consolidationand description of results

9 Powder Contamination

Amajor concern inmetal powders processed byMAmethodsis the nature and amount of impurities that get incorporatedinto the powder and contaminate it The small size of thepowder particles and consequent availability of large surfacearea formation of fresh surfaces during milling and wearand tear of the milling tools and the atmosphere underwhich the powder is milled all contribute to contaminationof the powder In addition to these factors one has toconsider the purity of the starting raw materials and themilling conditions employed (especially the milling atmo-sphere) Thus it appears as though powder contaminationis an inherent drawback of the method and that the milledpowder will always be contaminated with impurities unlessspecial precautions are taken to avoidminimize them Themagnitude of powder contamination appears to depend onthe time of milling intensity of milling atmosphere in whichthe powder is milled nature and size of the milling mediumand differences in the strengthhardness of the powder andthe milling medium and the container

Several attempts have been made in recent years to min-imize powder contamination during milling An importantpoint to remember is that if the starting powders are highlypure then the final product is going to be clean and purewith minimum contamination from other sources providedthat proper precautions are taken during milling It is alsoimportant to keep inmind that all applications do not requireultraclean powders and that the purity desired will depend onthe specific application (Figure 10) But it is a good practiceto minimize powder contamination at every stage and ensurethat additional impurities are not picked up subsequentlyduring handling andor consolidation

One way ofminimizing contamination from the grindingmedium and themilling container is to use the samematerial

for the container and the grinding medium as the powderbeing milled Thus one could use copper balls and coppercontainer formilling copper and copper alloy powders [60] Ifa container of the same material to be milled is not availablethen a thin adherent coating on the internal surface of thecontainer (and also on the surface of the grinding medium)with the material to be milled will minimize contaminationThe idea here is to mill the powder once allowing the powderto be coated onto the grindingmedium and the inner walls ofthe container The milled loose powder is then discarded anda fresh batch of powder is milled but with the old grindingmedia and container In general a simple rule that shouldbe followed to minimize contamination from the millingcontainer and the grinding medium is that the container andgrindingmedium should be harderstronger than the powderbeing milled

Milling atmosphere is perhaps the most important vari-able that needs to be controlled Even though nominally puregases are used to purge the glove box before loading andunloading the powders even small amounts of impurities inthe gases appear to contaminate the powders especially whenthey are reactive The best solution one could think of willbe to place the mill inside a chamber that is evacuated andfilled with high-purity argon gas Since the whole chamberis maintained under argon gas atmosphere (continuouslypurified to keep oxygen and water vapor below 1 ppm each)and the container is inside the mill which is inside thechamber contamination from the atmosphere is minimum

Contamination from PCAs is perhaps the most ubiqui-tous Since most of the PCAs used are organic compoundswhich have low melting and boiling points they decomposeduring milling due to the heat generated The decomposi-tion products consisting of carbon oxygen nitrogen andhydrogen react with the metal atoms and form undesirablecarbides oxides nitrides etc

10 Concluding Remarks and Future Prospects

We have only briefly touched upon the scientific and tech-nological aspects of MA materials along with some of theconcerns such as powder contamination and consolidationissues The historical development of MA during the last 50years or so can be divided into threemajor periods (Figure 11)The first period covering the first twenty years (from 1966up to about 1985) was mostly concerned with the devel-opment and production of oxide-dispersion-strengthened(ODS) superalloys for applications in the aerospace indus-try Several alloys with improved properties based onNi and Fe were developed and found useful applicationsThese included the MA754 MA760 MA956 MA957 andMA6000 [74] A more recent survey of application of MAto steel is also available [75] The second period of aboutfifteen years covering from about 1986 up to about 2000witnessed advances in the fundamental understanding ofthe processes that take place during MA Along with thisa large number of dedicated conferences were held andthere was a burgeoning of publication activity Comparisonshave also been frequently made between MA and othernonequilibriumprocessing techniquesmore specifically RSP

Research 15

1965 19851975 1995 2005

Production of ODS Alloys

Novel Phase SynthesisDevelopment of BasicUnderstanding of MA

Nanostructures andNew Applications

2015

Figure 11 Schematic showing the different periods of development in the field of mechanical alloying (MA) and applications ofMA productssince its inception in 1965

It was shown that themetastable effects achieved by these twononequilibrium processing techniques are similar Revival ofthe mechanochemical processing (MCP) took place and avariety of novel substances were synthesized Several mod-eling studies were also conducted to enable prediction of thephases produced or microstructures obtained although withlimited success The third period starting from about 2001saw a reemergence of the quest for new applications of theMA materials not only as structural materials but also forchemical applications such as catalysis and hydrogen storagewith the realization that contamination of themilled powdersis the limiting factor in the widespread applications of theMA materials Innovative techniques to consolidate the MApowders to full density while retaining the metastable phases(including glassy phases andor nanostructures) in themwerealso developed All these are continuing with the ongoinginvestigations to enhance the scientific understanding of theMA process

At present activities relating to both the science andtechnology of MA have been going on simultaneously Whilesome groups have been focusing on developing a betterunderstanding of the process and optimizing the processvariables to achieve a certain phase or phase combinationin the alloy system others have been concentrating onexploiting this technique for different applications It hasalso come to be realized that the MA process can be usedto recycle used and discarded materials to produce value-added materials for subsequent consumption They can bepulverized separated and used to mix with other powdersto produce composites The bottom line is that newer andimproved materials are being continuously developed fornovel applications

Conflicts of Interest

The author declares that he has no conflicts of interest

References

[1] K H J Buschow R W Cahn M C Flemings B Ilschner E JKramer and SMahajan EdsEncyclopedia ofMaterials Scienceand Technology Elsevier 2001

[2] C Suryanarayana EdNon-equilibrium Processing of MaterialsPergamon Oxford UK 1999

[3] C Suryanarayana ldquoMetallic glassesrdquo Bulletin of Materials Sci-ence vol 6 no 3 pp 579ndash594 1984

[4] C Suryanarayana and A Inoue Bulk Metallic Glasses CRCPress Boca Raton Fla USA 2nd edition 2018

[5] H R Trebin Ed Quasicrystals Structure and Physical Proper-ties Wiley-VCH Weinheim Germany 2003

[6] C Suryanarayana and H Jones ldquoFormation and characteristicsof quasicrystalline phases A reviewrdquo International Journal ofRapid Solidification vol 3 pp 253ndash293 1988

[7] H Gleiter ldquoNanocrystalline materialsrdquo Progress in MaterialsScience vol 33 no 4 pp 223ndash315 1989

[8] C Suryanarayana ldquoRecent developments in nanostructuredmaterialsrdquo Advanced Engineering Materials vol 7 no 11 pp983ndash992 2005

[9] M C Gao J-W Yeh P K Liaw and Y Zhang Eds High-Entropy Alloys Fundamentals and Applications Springer 2016

[10] J S Benjamin ldquoMechanical alloying ndash History and futurepotentialrdquo in Advances in Powder Metallurgy and ParticulateMaterials J M Capus and R M German Eds vol 7 ofNovel Powder Processing pp 155ndash168 Metal Powder IndustriesFederation Princeton NJ USA 1992

[11] M P Phaniraj D-I Kim J-H Shim and Y W CholdquoMicrostructure development in mechanically alloyed yttriadispersed austenitic steelsrdquo Acta Materialia vol 57 no 6 pp1856ndash1864 2009

[12] A Hirata T Fujita Y R Wen J H Schneibel C T Liu and MW Chen ldquoAtomic structure of nanoclusters in oxide-dispersionstrengthened steelsrdquo Nature Materials vol 10 no 12 pp 922ndash926 2011

[13] M K Miller C M Parish and Q Li ldquoAdvanced oxide disper-sion strengthened and nanostructured ferritic alloysrdquoMaterialsScience and Technology (United Kingdom) vol 29 no 10 pp1174ndash1178 2013

[14] N Oono Q X Tang and S Ukai ldquoOxide particle refinementin Ni-based ODS alloyrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 649 pp 250ndash253 2016

[15] D B Witkin and E J Lavernia ldquoSynthesis and mechanicalbehavior of nanostructured materials via cryomillingrdquo Progressin Materials Science vol 51 no 1 pp 1ndash60 2006

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

Research 7Fr

ee E

nerg

y

Solute Concentration (at)20 40 60 80Fe 100

BE powder

amorphous

-Fe

Intermetallics

1

23

45

Inte

nsity

30 40 502-Theta

(a)

(b)

10 h

60 h

60 70 80 90

1

Figure 3 (a) X-ray diffraction patterns of the Fe42Ge28Zr10B20

powder blend milled for 10 and 60 h While the powder milledfor 10 h shows the presence of an amorphous phase the powdermilled for 60 h shows the presence of a crystalline phase coexistingwith the amorphous phase suggesting that the amorphous phase hascrystallized (b) Hypothetical free energy vs composition diagramto explain the mechanism of mechanical crystallization in theFe42X28Zr10B20systemNote that point ldquo1rdquo represents the free energy

of the blended elemental powders Similarly point ldquo2rdquo representsformation of the 120572-Fe solid solution containing all the alloyingelements in Fe point ldquo3rdquo formation of the homogeneous amorphousphase point ldquo4rdquo a mixture of the amorphous phase with a differentsolute content (amorphous) from that at ldquo3rdquo and the solid solution120572-Fe with different solute content and point ldquo5rdquo the equilibriumsituation when the solid solution and intermetallic phases coexist

The free energy of the BE powder mixture is indicated bypoint ldquo1rdquo which represents the point of intersection of thecomposition vertical with the line joining the free energiesof pure Fe and the ldquosoluterdquo On milling this powder a solidsolution phase containing all the solute elements in Fe (ora mixture of intermetallics and a solid solution in somecases) is seen to form and its free energy is indicated bypoint ldquo2rdquoThe solid solution forms because it has a lower free

energy than the BE powder mixture Since MA introduces avariety of crystal defects the crystalline phases in the milledpowders will contain excess energyThis energy will continueto increase with milling time and reach a value which isabove that of the metastable amorphous phase Thus theamorphous phase gets stabilized (point ldquo3rdquo) On primarycrystallization of the amorphous phase the new constitutionwill be a mixture of the 120572-Fe solid solution (or intermetallics)and the amorphous phase which now has a compositiondifferent from that of the original amorphous phase Furtherthe new solid solution phase also has a composition differentfrom the original 120572-Fe phase The free energy of the mixtureof this solid solution and the amorphous phase indicatedby ldquopoint 4rdquo will have a free energy lower than that of theamorphous phase The equilibrium mixture of the 120572-Fe solidsolution and ldquointermetallicsrdquo will have the lowest free energyof all the phase mixtures as indicated by point ldquo5rdquo in thefigure The process of mechanical crystallization is uniqueto the MA process and can be achieved only on continuedmilling of the amorphous powder ormilling of an amorphousalloy obtained by other methods

5 Nanostructured Materials

Another important attribute of MA has beenmicrostructuralrefinement The most common observation is that grainrefinement takes place resulting in the formation of very finegrains often down to nanometer levels A two-phase alloyunder equilibrium conditions can transform to a single-phasealloy due to formation of a supersaturated solid solution Anamorphous alloy can crystallize under the action of severeplastic deformation Most usefully one can also introducea high volume fraction of fine second phase particles andobtain a composite with a uniform distribution of thesecond phase particles By a proper choice of the size ofthe matrix andor the reinforcement phase one can producenanocomposites a very fruitful area for investigations Infact MA has been the most popular technique to synthesizenanostructured materials and nanocomposites in a numberof alloy systems

It has been frequently demonstrated that mechanicallyalloyed materials show powder particle refinement as wellas grain refinement as a function of the milling time Thesesizes decrease very rapidly in the early stages of milling andmore gradually later Eventually in almost all the cases thegrain size of the mechanically alloyed materials is in thenanometer region unless the powder becomes amorphousThe minimum grain size achieved has been reported tobe a few nanometers ranging typically from about 5 to50 nm but depending on the material and processingconditions For example it was reported that the grain sizedecreases with increasing milling energy higher ball-to-powder weight ratio and lower temperatures As is wellknown nanostructuredmaterials exhibit high strength goodductility excellent sintering characteristics and interestingelectrical andmagnetic properties [7 8 27 43]The ease withwhich nanostructured materials can be synthesized is onereason why MA has been extensively employed to producenanocrystalline and nanocomposite materials

8 Research

From high-resolution transmission electron microscopy(TEM) observations it was noted that in the early stages ofmilling due to the high deformation rates experienced bythe powder deformation was localized within shear bands[44] These shear bands which contain a high density ofdislocations have a typical width of approximately 05 to 10120583m Small grains with a diameter of 8ndash12 nm were seenwithin the shear bands and electron diffraction patternssuggested significant preferred orientation With continuedmilling the average atomic level strain increased due toincreasing dislocation density and at a certain dislocationdensity within these heavily strained regions the crystaldisintegrated into subgrains that are separated by low-anglegrain boundaries This resulted in a decrease of the latticestrain The subgrains formed this way were of nanometerdimensions and are often between 20 and 30 nm

On further processing deformation occurred in shearbands located in previously unstrained parts of the materialThe grain size decreased steadily and the shear bands coa-lesced The small-angle boundaries are replaced by higherangle grain boundaries implying grain rotation as reflectedby the absence of texture in the electron diffraction patternsand random orientation of the grains observed from thelattice fringes in the high-resolution TEMmicrographs Con-sequently dislocation-free nanocrystalline grains are formedThis is the currently accepted mechanism of nanocrystalformation in MA processed powders

The grain size of the milled materials decreases withmilling time and reaches a saturation level when a balance isestablished between the fracturing and cold welding eventsThis minimum grain size 119889min is different depending on thematerial and milling conditionsThe value of 119889min achievableby milling is determined by the competition between theplastic deformation via dislocation motion that tends todecrease the grain size and the recovery and recrystallizationbehavior of the material that tends to increase the grain sizeThis balance gives a lower bound for the grain size of puremetals and alloys

The 119889min obtained is different for different metals and isalso found to vary with the crystal structure (Figure 4) Inmost of the metals the minimum grain size attained is inthe nanometer dimensions But metals with a body-centeredcubic (BCC) crystal structure reach much smaller valuesin comparison to metals with the other crystal structuresrelated to the difficulty of extensive plastic deformation andconsequent enhanced fracturing tendency during millingCeramics and intermetallic compounds are much harderand usually more brittle than the metals on which they arebased Therefore intuitively one expects that 119889min of thesecompounds is smaller than those of the pure metals but thisdoes not appear to be the case always

It was also reported that 119889min decreases with an increasein the melting temperature of the metal [45] As shown inFigure 4 this trend is amply clear in the case of metalswith close-packed structures (face-centered cubic (FCC) andhexagonal close-packed (HCP)) but not so clear in metalswith a BCC structure Another point of interest is that thedifference in grain size is much less among metals thathave high melting temperatures the minimum grain size is

0

5

10

15

20

25

FCCBCCHCP

Min

imum

Gra

in S

ize (

nm)

0 2000 30001000Melting Temperature (K)

Figure 4 Variation of minimum grain size with temperature inmechanically milled FCC HCP and BCC metals

virtually constant Thus for the HCP metals Co Ti Zr Hfand Ru the minimum grain size is almost the same eventhough themelting temperatures vary between 1495∘C for Coand 2310∘C for Ru

Analysis of the existing data on 119889min of mechanicallymilled pure metals shows that the normalized minimumgrain size 119889minb where b is the Burgers vector of thedislocations decreases with increasing melting temperatureactivation energy for self-diffusion hardness stacking faultenergy bulk modulus and the equilibrium distance betweentwo edge dislocations The 119889min value is also affected bymilling energy (the higher the milling energy the smaller thegrain size) milling temperature (the lower the milling tem-perature the smaller the grain size) and alloying additions(the minimum grain size is smaller for solid solutions thanfor pure metals since solid solutions are harder and strongerthan the pure metals on which they are based)

It was suggested that 119889min is determined by the minimumgrain size that can sustain a dislocation pile-up within a grainand by the rate of recovery Based on the dislocation pile-up model the critical equilibrium distance between two edgedislocations in a pile-up L119888 (which could be assumed to bethe crystallite or grain size inmilled powders) was calculated[46] using the equation

119871119888=3119866119887

(1 minus ])119867(2)

where G is the shear modulus 119887 is the Burgers vector ] isthe Poissonrsquos ratio and H is the hardness of the materialAccording to the above equation increased hardness resultsin smaller values of 119871c (grain size) and an approximate linearrelationship was observed between 119871c and the minimumgrain size obtained by milling of a number of metals Otherattempts have also beenmade to theoretically predict 119889min onthe basis of thermodynamic properties of materials [47]

Research 9

6 Nanocomposites

Achievement of a uniform distribution of the reinforcementin a matrix is essential to the achievement of good anduniform mechanical properties in composites Further themechanical properties of the composite tend to improvewith increasing volume fraction andor decreasing particlesize of the reinforcement [48] Traditionally a reasonablylarge volume fraction of the reinforcement could be addedif the size of the reinforcement is large (on a micrometerscale or larger) But if the reinforcement size is very fine(of nanometer dimensions) then the volume fraction addedis typically limited to about 2 to 4 However if a largevolume fraction of nanometer-sized reinforcement can beintroduced the mechanical properties of the composite arelikely to be vastly improved Therefore several investigationshave been undertaken to produce improved nanocompositesthrough MA [20]

Composites are traditionally produced by solidificationprocessing methods since the processing is inexpensive andlarge tonnage quantities can also be produced But it isnot easy to disperse fine reinforcement particles in a liquidmetal since agglomeration of the fine particles occurs andclusters are produced Further wetting of the particles ispoor and consequently even the most vigorous stirring isnot able to break the agglomerates Additionally the finepowders tend to float to the top of the melt during processingof the composites through solidification processing Thussolid-state processing methods such as MA are effective indispersing fine particles (including nanoparticles) An addedadvantage is that since no melting is involved even a largevolume fraction of the reinforcement can be introduced

Some of the specific goals sought in processing ofnanocomposite materials throughMA include incorporationof a high volume percentage of ultrafine reinforcements andachievement of high ductility and even superplasticity insome composites It is also possible that due to the increasedductility the fracture toughness of these composites couldbe higher than that of their coarse-grained counterparts Inline with these goals we have in recent times accomplished(a) achievement of a very uniform distribution of the rein-forcement phases in different types of matrices includingdispersion of a high volume fraction of Al2O3 in Al [49] andof Ti5Si3in 120574ndashTiAl [50 51] and (b) synthesis of MoSi

2+Si3N4

composites for high-temperature applications [52] amongothers However we will describe here only the developmentof Ti5Si3+120574ndashTiAl composites as a typical example

Lightweight intermetallic alloys based on 120574-TiAl arepromising materials for high-temperature structural appli-cations eg in aircraft engines or stationary turbines Eventhough they have many desirable properties such as highspecific strength and modulus both at room and elevatedtemperatures and good corrosion and oxidation resistancethey suffer from inadequate room temperature ductility andinsufficient creep resistance at elevated temperatures espe-cially between 800 and 850∘C an important requirement forelevated temperature applications of these materials There-fore current research programs have been addressing thedevelopment of high-temperature materials with adequate

020406080

100120140160180200

Engi

neer

ing

stres

s (M

Pa)

20 40 60 80 100 120 140 1600Engineering strain ()

-5950

∘C 4x10

-4950

∘C 4x10

-41000

∘C 4x10x10

(a)

(b)

Figure 5 (a) Scanning electronmicrograph of the 120574-TiAl + 60 volTi5Si3composite specimen showing that the twophases are very uni-

formly distributed in the microstructure Such a microstructure isconducive to achieving superplastic deformation under appropriateconditions of testing (b) Tensile stress-strain curve of 120574-TiAl + 60vol Ti

5Si3composite showing that superplastic deformation was

achieved at 950∘C with a strain rate of 4x10minus5 sminus1 and at 1000∘C witha strain rate of 4x10minus4 sminus1

room temperature ductility for easy formability and abilityto increase the high-temperature strength by a suitable heattreatment or alloying additions to obtain sufficient creepresistance at elevated temperatures

Composites of 120574-TiAl and 120585-Ti5Si3phase with the volume

fractions of the 120585-Ti5Si3phase varying from 0 to 60 vol

were produced by MA of a combination of the blendedelemental and prealloyed intermetallic powders Fully denseand porosity-free compacts were produced by hot isostaticpressing with the resulting grain size of each of the phasesbeing about 300-400 nm Figure 5(a) shows scanning elec-tron micrographs of the 120574-TiAl+60 vol 120585-Ti5Si3 compositeshowing that the two phases are very uniformly distributedthroughout the microstructure Materials exhibiting such amicrostructure (roughly equal amounts of the two phasessmall grain sizes and a uniform distribution of the twophases) are expected to be deformed superplastically Totest this hypothesis both compression and tensile testingof these composite specimens were conducted at differenttemperatures and strain rates From the tensile stress-strain

10 Research

Pure Metals Alloysand Compounds

NanostructuredMaterials

MECHANICALALLOYING

ODS Superalloys Supercorroding

Alloys

Synthesis of Novel Phases PVDTargets Solders Paints Food Heaters

Biomaterials Waste Utilization etc

Hydrogen Storage MagneticMaterials Catalysts Sensors etc

Figure 6 An overview of the applications of the MA process

plots of the 120574-TiAl+60 vol 120585-Ti5Si3composite specimens

shown in Figure 5(b) it is clear that the specimens testedat 950∘C with a strain rate of 4times10minus5 sminus1 and at 1000∘Cwith a strain rate of 4times10minus4 sminus1 exhibited large ductilityof nearly 150 and 100 respectively Considering that thiscomposite is based on a ceramic material (Ti

5Si3) this is a

very high amount of deformation suggestive of superplasticdeformation Final proof is provided by TEM investiga-tions that confirm the continued stability of the equiaxedmicrostructure after deformation

7 Applications

As has been frequently pointed out in the literature thetechnique ofMAwas developed out of an industrial necessityin 1965 to produce ODS nickel-based superalloys Sincethen a number of new applications have been exploredfor mechanically alloyed materials (Figure 6) Apart fromthe ODS alloys which continue to be used in the high-temperature applications area MA powders have also beenused for other applications and newer applications are alsobeing explored These novel applications include direct syn-thesis of pure metals and alloys from ores and scrap mate-rials super-corroding alloys for release into ocean bottomsdevelopment of PVD (physical vapor deposition) targetssolders paints etc hydrogen storage materials photovoltaicmaterials catalysts and sensors

A novel and simple application of MA powders is in thedevelopment ofmeals ready to eat (MRE) In this applicationvery finely ground Mg-based alloy powders are used Whenwater is added to these fineMg-Fe powders heat is generatedaccording to the following reaction

Mg (Fe) + 2H2O 997888rarr Mg (OH)2 +H2 +Heat (3)

This heat is utilized to warm the food in remote areas wherecooking or heating facilities may not be available eg in war-torn areas These ldquofood heatersrdquo were widely used during theDesert Stormoperations in the 1990s Let us now look at someof the other applications

71 Pure Metals and Alloys The technique of MA has beenemployed to produce pure metals and alloys from ores andother raw materials such as scrap The process is based onchemical reactions induced by mechanical activation Mostof the reactions studied have been displacement reactions ofthe type

119872119874 + 119877 997888rarr 119872 + 119877119874 (4)

where ametal oxide (MO) is reduced by amore reactivemetal(reductant R) to the pure metal M In addition to oxidesmetal chlorides and sulfides have also been reduced to puremetals this way Similar reactions have also been used toproduce alloys and nanocomposites [21 53]

All solid-state reactions involve the formation of a prod-uct phase at the interface of the component phases Thusin the above example the metal M forms at the interfacebetween the oxide MO and the reductant R and physicallyseparates the reactants Further growth of the product phaseinvolves diffusion of atoms of the reactant phases through theproduct phase which constitutes a barrier layer preventingfurther reaction from occurring In other words the reactioninterface defined as the nominal boundary surface betweenthe reactants continuously decreases during the course ofthe reaction Consequently kinetics of the reaction are slowand elevated temperatures are required to achieve reasonablereaction rates

MA can provide the means to substantially increasethe reaction kinetics of chemical reactions in general andthe reduction reactions in particular This is because therepeated cold welding and fracturing of powder particlesincrease the area of contact between the reactant powderparticles by bringing fresh surfaces to come into contactrepeatedly due to a reduction in particle size during millingThis allows the reaction to proceed without the necessityfor diffusion through the product layer As a consequencereactions that normally require high temperatures will occurat lower temperatures or even without any externally appliedheat In addition the high defect densities induced by MAaccelerate diffusion processes

Research 11

Depending on the milling conditions two entirely differ-ent reaction kinetics are possible [21 54]

(a) the reactionmay extend to a very small volumeduringeach collision resulting in a gradual transformationor

(b) if the reaction enthalpy is sufficiently high (and ifthe adiabatic temperature is above 1800 K) a self-propagating high-temperature synthesis (SHS) reaction(also known as combustion synthesis) can be initi-ated by controlling the milling conditions (eg byadding diluents) to avoid combustion reactions maybe made to proceed in a steady-state manner

The latter type of reactions requires a critical milling time forthe combustion reaction to be initiated If the temperature ofthe vial is recorded during the milling process the tempera-ture initially increases slowly with time After some time thetemperature increases abruptly suggesting that ignition hasoccurred and this is followed by a relatively slow decrease

In the above types of reactions themilled powder consistsof the pure metalM dispersed in the RO phase both usuallyof nanometer dimensions It has been shown that by anintelligent choice of the reductant the RO phase can beeasily leached out Thus by selective removal of the matrixphase by washing with appropriate solvents it is possibleto achieve well-dispersed metal nanoparticles as small as 5nm in size If the crystallinity of these particles is not highit can be improved by subsequent annealing without thefear of agglomeration due to the enclosure of nanoparticlesin a solid matrix Such nanoparticles are structurally andmorphologically uniform having a narrow size distributionThis technique has been used to produce powders of manydifferent metalsmdashAg Cd Co Cr Cu Fe Gd Nb NiTi V W Zn and Zr Several alloys intermetallics andnanocomposites have also been synthesized Figure 7 showstwo micrographs of nanocrystalline ceria particles producedby the conventional vapor synthesis and MA methods Eventhough the grain size is approximately the same in both thecases the nanoparticles produced by MA are very discreteand display a very narrow particle size distribution whilethose produced by vapor deposition are agglomerated andhave a wide particle size distribution

Novel materials have also been synthesized by MA ofblended elemental powders One of the interesting examplesin this category is the synthesis of plain carbon steels andstainless steels [55 56] Twomain advantages of this synthesisroute are that the steels can be manufactured at roomtemperature and that the steels will be nanostructured thusconferring improved properties But these powders needto be consolidated to full density for applications and itis possible that grain growth occurs during this processThe kinetics of grain growth can however be controlled byregulating the time and temperature combinations

72 Hydrogen Storage Materials Another important applica-tion of MA powders is hydrogen storage Mg is a lightweightmetal (the lightest structural material) with a density of174 gcm3 Mg and Mg-based alloys can store a significant

20 nm

50 nm

Figure 7 Transmission electron micrographs of ceria particlesproduced byMA (top) and vapor synthesis (bottom)methods Notethe severe agglomeration of the nanoparticles in the latter case

amount of hydrogen However being a reactive metal Mgalways contains a thin layer of oxide on the surface Thissurface oxide layer which inhibits asorption of hydrogenneeds to be broken down through activation A high-temperature annealing is usually done to achieve this ButMA can overcome these difficulties due to the severe plasticdeformation experienced by the powder particles duringMAA catalyst is also often required to accelerate the kinetics ofhydrogen and dehydrogenation of alloys

Mg can adsorb about 76 wt hydrogen andMg-Ni alloysmuch less However Mg is hard to activate its tempera-ture of operation is high and the sorption and desorptionkinetics are slow Therefore Mg-Ni and other alloys arebeing explored But due to the high vapor pressure of Mgand significant difference in the melting temperatures ofthe constituent elements (Mg = 650∘C and Ni = 1452∘C) itis difficult to prepare these alloys by conventional meltingand solidification processes Therefore MA has been used toconveniently synthesize these alloys since this is a completelysolid-state processing method An added advantage of MA isthat the product will have a nanocrystalline grain structureallowing the improvements described below [57 58]

Figure 8 shows the hydrogenation and dehydrogena-tion plots for conventional and nanocrystalline alloysAn additional plot for a situation where a catalyst isadded to the nanocrystalline material is also shown Fromthese two graphs the following facts become clear Firstlythe nanocrystalline alloys adsorb hydrogen much fasterthan conventional (coarse-grained) alloys even though the

12 Research

0 1 2 3 4 5 6 7 8 9 10time (min)

conventional

p = 8 bar

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

time (min)

nano + catalyst

nano + catalyst

nanocrystallinenanocrystalline

conventional

breakthrough in reaction kinetics due to nanostructure and use of catalyst

0

1

2

3

4

5

6

7

8

hydr

ogen

cont

ent (

wt

)

T = 300∘C p = 10-3 mbar

Figure 8 Comparison of the hydrogen sorption and desorption kinetics of Mg alloys with conventional and nanocrystalline grain sizesSignificant improvement in the desorption kinetics when a catalyst is added is worth noticing

amount of hydrogen adsorbed is the sameWhile the conven-tional alloys do not adsorb any significant amount of hydro-gen in about 10 min the allowable full amount of hydrogenis adsorbed in the nanocrystalline alloy Similar observationsare made by many other authors in many different alloysFurther the kinetics are slightly faster (not much different)when a catalyst is added to the nanocrystalline alloy Onthe other hand the kinetics appear to be very significantlyaffected during the desorption process For example thenanocrystalline alloys desorb hydrogen much faster thanthe conventional alloys Again while the conventional alloysdo not desorb any significant amount of hydrogen thenanocrystalline alloy desorbs over 20 of the hydrogenstored in about 10 min But the most significant effect wasobserved when a catalyst was added to the nanocrystallinealloyAll the adsorbedhydrogenwas desorbed in about 2minwhen a catalyst was added to the nanocrystalline alloy Thusnanostructure processing throughMAappears to be a fruitfulway of developing novel hydrogen storage materials [59]

Many novel complex hydrides based on light metals(Mg Li and Na) are being explored and the importantcategories of materials being investigated include the alanates(also known as aluminohydrides a family of compoundsconsisting of hydrogen and aluminum) borohydrides amideborohydrides amide-imide systems (which have a highhydrogen storage capacity and low operative temperatures)amine borane (compounds of borane groups having ammo-nia addition) and alane

73 Energy Materials The technique of MA has also beenemployed to synthesize materials for energy applicationsThese include synthesis of photovoltaic (PV) materials andultrafine particles for faster ignition and combustion pur-poses

One of the most common PV materials is CIGS acompound of Cu In Ga and Se with the compositionCu(InGa)Se2 Since PV cells have the highest efficiencywhen

the band gap for this semiconductor is 125 eV one will haveto engineer the composition for the ratio of Ga to In This isbecause the band gaps for CuInSe

2 and CuGaSe2 are 102 and170 eV respectively Accordingly a Ga(Ga+In) ratio of about03 is found most appropriate and the final ideal compositionof the PV material will be Cu097In07Ga03Se2

The traditional method of synthesizing this material isto prepare an alloy of Cu Ga and In by vapor depositionmethods and incorporate Se into this by exposure to Seatmosphere Consequently this is a complex process MAhowever can be a simple direct and efficient method ofmanufacturing Blended elemental powders of Cu In Gaand Se weremilled in a copper container for different periodsof time and the phase constitution was monitored throughX-ray diffraction studies It was noted that the tetragonalCu(InGa)Se

2phase has formed even aftermilling the powder

mix for 20 min Process variables such as mill rotation speedball-to-powder weight ratio (BPR) and milling time werevaried and it was noted that the ideal composition wasachieved at lowmilling speeds short milling times and a lowBPR (Table 2) [60] By fine-tuning these parameters it shouldbe possible to achieve the exact composition

Nanostructuredmetal particles have high specific surfaceareas and consequently they are highly reactive and can storeenergy This area has received a great impetus in recentyears since the combustion rates of nanostructured metalparticles aremuch higher than those of coarsemetal particlesThe particle size cannot be exceedingly small because thesurface may be coated with a thin oxide layer and make theparticles passive [61ndash63] MA is very well suited to preparesuch materials because the reactions are easily initiatedby impact or friction and at the same time milling isa simple scalable and controllable technology capable ofmixing reactive components on the nanoscaleThese reactivematerials could be used as additives to propellants explosivesand pyrotechnics as well asmanufacture of reactive structuralcomponents [63]

Research 13

Table 2 Chemical analysis (at) of the Cu-In-Ga-Se powder mechanically alloyed under different processing conditions

BPR Speed (rpm) Time (min) Cu In Ga Se201 300 120 3261 1523 653 4562201 300 240 3173 1563 671 4593101 150 40 2598 1698 725 4978Targeted (Cu

097In07Ga03Se2) 2443 1763 756 5038

Compared to the presently used liquid aviation fuelsmetal particles based on B Al andMg are effective HoweverB and Al have very high boiling points and high heats ofenthalpy On the other hand Mg has a low boiling point butalso low heat of enthalpy Therefore if Al and Mg could bemixed intimately on a nanometer scale one could hope tohave a high heat of enthalpy at reduced boiling temperaturesThat is Mg-Al alloy particles could be practical in achievinga high-energy density and practical ignitionboiling temper-atures MA is a very effective method to accomplish this

Al and Mg powders were blended together in differentproportions and milled Since the milled powders have arange of particle sizes they were sieved to obtain well-defined powder sizes They were then fluidized and thencombusted in a CH4-air premixed flame Both the ignitionand burning times of these metal particles decreased withdecreased particle sizes and were also shorter when the Mgcontent is higher (Figure 9) [64] Similar results have beenreported by other investigators

8 Powder Consolidation

The product of MA is in powder form Except in cases ofchemical applications such as catalysis when the productdoes not require consolidation widespread application ofMA powders requires efficient methods of consolidatingthem into bulk shapes Successful consolidation of MA pow-ders is a nontrivial problem since fully densematerials shouldbe produced while simultaneously retaining the nanometer-sized grains without coarsening or the amorphous phaseswithout crystallizing Conventional consolidation of powdersto full density through processes such as hot extrusion [65]and hot isostatic pressing (HIP) [66] requires use of highpressures and exposure to elevated temperatures for extendedperiods of time [67] Unfortunately however this resultsin significant coarsening of the nanometer-sized grains orcrystallization of amorphous phases and consequently thebenefits of nanostructure processing or amorphization arelost [68] On the other hand retention of finemicrostructuresrequires use of low consolidation temperatures and then itis difficult to achieve full interparticle bonding at these lowtemperatures Therefore novel and innovative methods ofconsolidating MA powders are required The objectives ofconsolidation of these powders are to (a) achieve full densifi-cation (without any porosity) (b) minimize microstructuralcoarsening (ie retention of fine grain size) andor (c)avoid undesirable phase transformations In fact the earlyresults of excellent mechanical properties and most specifi-cally superplasticity at room temperature [69] attributed to

20 Mg (tig)90 Mg (tig)

20 Mg (tb)90 Mg (tb)

20 40 60 80 100 1200Particle Size (m)

0

10

20

30

40

50

60

70

Igni

tion

Burn

ing

Tim

e (m

s)

Figure 9 Ignition and burning times as a function of particle sizefor the mechanically alloyed Al-20 and Al-90 at Mg powdersIn the insert tig and tb represent the ignition and burning timesrespectively

nanocrystalline ceramics have not been successfully repro-duced due to incomplete consolidation of the nanopowdersin the material tested Thus consolidation to full densityassumes even greater importance

Because of the small size of the powder particles (typicallya few micrometers even though the grain size is only afew nanometers) MA powders pose special problems Forexample such nanometric powders are very hard and strongThey also have a high level of interparticle friction Furthersince these powders have a large surface area they also exhibithigh chemical reactivity Therefore special precautions needto be taken to consolidate the metal powders to bulk shapes

Successful consolidation of MA powders has beenachieved by electrodischarge compaction plasma-activatedsintering shock (explosive) consolidation [70] HIP hydro-static extrusion strained powder rolling and sinter forging[71] By utilizing the combination of high temperature andpressure HIP can achieve a particular density at lowerpressure when compared to cold isostatic pressing or at lowertemperature when compared to sintering Because of theincreased diffusivity in nanocrystalline materials sintering(and therefore densification) takes place at temperaturesmuch lower than those in coarse-grained materials This islikely to reduce the grain growth Spark plasma sintering(SPS) has become the most popular technique to consolidatemetal powders especially those with ultrafine grain sizesdue to the fact that full density can be achieved at low

14 Research

SOLID LIQUID GAS

MILLING

USE POWDER AS-ISor Chemical Reaction

Thermal Treatment etcCONSOLIDATE TO SOLID

(Temperature Pressure)

Catalysts Pigments SolderComponents Ceramic Powder etc

Advanced materials with hightolerance to gaseous impurities

Figure 10 Alternatives to consolidation of metal powders Ifpowders are used as catalysts pigments solder components etcthen they need not be consolidated However if powders need tobe consolidated then they could be used in applications with hightolerance to gaseous impurities

temperatures and shorter times in comparison to severalothermethods [72 73] An excellent reviewbyGroza [71]maybe consulted for full details of the methods of consolidationand description of results

9 Powder Contamination

Amajor concern inmetal powders processed byMAmethodsis the nature and amount of impurities that get incorporatedinto the powder and contaminate it The small size of thepowder particles and consequent availability of large surfacearea formation of fresh surfaces during milling and wearand tear of the milling tools and the atmosphere underwhich the powder is milled all contribute to contaminationof the powder In addition to these factors one has toconsider the purity of the starting raw materials and themilling conditions employed (especially the milling atmo-sphere) Thus it appears as though powder contaminationis an inherent drawback of the method and that the milledpowder will always be contaminated with impurities unlessspecial precautions are taken to avoidminimize them Themagnitude of powder contamination appears to depend onthe time of milling intensity of milling atmosphere in whichthe powder is milled nature and size of the milling mediumand differences in the strengthhardness of the powder andthe milling medium and the container

Several attempts have been made in recent years to min-imize powder contamination during milling An importantpoint to remember is that if the starting powders are highlypure then the final product is going to be clean and purewith minimum contamination from other sources providedthat proper precautions are taken during milling It is alsoimportant to keep inmind that all applications do not requireultraclean powders and that the purity desired will depend onthe specific application (Figure 10) But it is a good practiceto minimize powder contamination at every stage and ensurethat additional impurities are not picked up subsequentlyduring handling andor consolidation

One way ofminimizing contamination from the grindingmedium and themilling container is to use the samematerial

for the container and the grinding medium as the powderbeing milled Thus one could use copper balls and coppercontainer formilling copper and copper alloy powders [60] Ifa container of the same material to be milled is not availablethen a thin adherent coating on the internal surface of thecontainer (and also on the surface of the grinding medium)with the material to be milled will minimize contaminationThe idea here is to mill the powder once allowing the powderto be coated onto the grindingmedium and the inner walls ofthe container The milled loose powder is then discarded anda fresh batch of powder is milled but with the old grindingmedia and container In general a simple rule that shouldbe followed to minimize contamination from the millingcontainer and the grinding medium is that the container andgrindingmedium should be harderstronger than the powderbeing milled

Milling atmosphere is perhaps the most important vari-able that needs to be controlled Even though nominally puregases are used to purge the glove box before loading andunloading the powders even small amounts of impurities inthe gases appear to contaminate the powders especially whenthey are reactive The best solution one could think of willbe to place the mill inside a chamber that is evacuated andfilled with high-purity argon gas Since the whole chamberis maintained under argon gas atmosphere (continuouslypurified to keep oxygen and water vapor below 1 ppm each)and the container is inside the mill which is inside thechamber contamination from the atmosphere is minimum

Contamination from PCAs is perhaps the most ubiqui-tous Since most of the PCAs used are organic compoundswhich have low melting and boiling points they decomposeduring milling due to the heat generated The decomposi-tion products consisting of carbon oxygen nitrogen andhydrogen react with the metal atoms and form undesirablecarbides oxides nitrides etc

10 Concluding Remarks and Future Prospects

We have only briefly touched upon the scientific and tech-nological aspects of MA materials along with some of theconcerns such as powder contamination and consolidationissues The historical development of MA during the last 50years or so can be divided into threemajor periods (Figure 11)The first period covering the first twenty years (from 1966up to about 1985) was mostly concerned with the devel-opment and production of oxide-dispersion-strengthened(ODS) superalloys for applications in the aerospace indus-try Several alloys with improved properties based onNi and Fe were developed and found useful applicationsThese included the MA754 MA760 MA956 MA957 andMA6000 [74] A more recent survey of application of MAto steel is also available [75] The second period of aboutfifteen years covering from about 1986 up to about 2000witnessed advances in the fundamental understanding ofthe processes that take place during MA Along with thisa large number of dedicated conferences were held andthere was a burgeoning of publication activity Comparisonshave also been frequently made between MA and othernonequilibriumprocessing techniquesmore specifically RSP

Research 15

1965 19851975 1995 2005

Production of ODS Alloys

Novel Phase SynthesisDevelopment of BasicUnderstanding of MA

Nanostructures andNew Applications

2015

Figure 11 Schematic showing the different periods of development in the field of mechanical alloying (MA) and applications ofMA productssince its inception in 1965

It was shown that themetastable effects achieved by these twononequilibrium processing techniques are similar Revival ofthe mechanochemical processing (MCP) took place and avariety of novel substances were synthesized Several mod-eling studies were also conducted to enable prediction of thephases produced or microstructures obtained although withlimited success The third period starting from about 2001saw a reemergence of the quest for new applications of theMA materials not only as structural materials but also forchemical applications such as catalysis and hydrogen storagewith the realization that contamination of themilled powdersis the limiting factor in the widespread applications of theMA materials Innovative techniques to consolidate the MApowders to full density while retaining the metastable phases(including glassy phases andor nanostructures) in themwerealso developed All these are continuing with the ongoinginvestigations to enhance the scientific understanding of theMA process

At present activities relating to both the science andtechnology of MA have been going on simultaneously Whilesome groups have been focusing on developing a betterunderstanding of the process and optimizing the processvariables to achieve a certain phase or phase combinationin the alloy system others have been concentrating onexploiting this technique for different applications It hasalso come to be realized that the MA process can be usedto recycle used and discarded materials to produce value-added materials for subsequent consumption They can bepulverized separated and used to mix with other powdersto produce composites The bottom line is that newer andimproved materials are being continuously developed fornovel applications

Conflicts of Interest

The author declares that he has no conflicts of interest

References

[1] K H J Buschow R W Cahn M C Flemings B Ilschner E JKramer and SMahajan EdsEncyclopedia ofMaterials Scienceand Technology Elsevier 2001

[2] C Suryanarayana EdNon-equilibrium Processing of MaterialsPergamon Oxford UK 1999

[3] C Suryanarayana ldquoMetallic glassesrdquo Bulletin of Materials Sci-ence vol 6 no 3 pp 579ndash594 1984

[4] C Suryanarayana and A Inoue Bulk Metallic Glasses CRCPress Boca Raton Fla USA 2nd edition 2018

[5] H R Trebin Ed Quasicrystals Structure and Physical Proper-ties Wiley-VCH Weinheim Germany 2003

[6] C Suryanarayana and H Jones ldquoFormation and characteristicsof quasicrystalline phases A reviewrdquo International Journal ofRapid Solidification vol 3 pp 253ndash293 1988

[7] H Gleiter ldquoNanocrystalline materialsrdquo Progress in MaterialsScience vol 33 no 4 pp 223ndash315 1989

[8] C Suryanarayana ldquoRecent developments in nanostructuredmaterialsrdquo Advanced Engineering Materials vol 7 no 11 pp983ndash992 2005

[9] M C Gao J-W Yeh P K Liaw and Y Zhang Eds High-Entropy Alloys Fundamentals and Applications Springer 2016

[10] J S Benjamin ldquoMechanical alloying ndash History and futurepotentialrdquo in Advances in Powder Metallurgy and ParticulateMaterials J M Capus and R M German Eds vol 7 ofNovel Powder Processing pp 155ndash168 Metal Powder IndustriesFederation Princeton NJ USA 1992

[11] M P Phaniraj D-I Kim J-H Shim and Y W CholdquoMicrostructure development in mechanically alloyed yttriadispersed austenitic steelsrdquo Acta Materialia vol 57 no 6 pp1856ndash1864 2009

[12] A Hirata T Fujita Y R Wen J H Schneibel C T Liu and MW Chen ldquoAtomic structure of nanoclusters in oxide-dispersionstrengthened steelsrdquo Nature Materials vol 10 no 12 pp 922ndash926 2011

[13] M K Miller C M Parish and Q Li ldquoAdvanced oxide disper-sion strengthened and nanostructured ferritic alloysrdquoMaterialsScience and Technology (United Kingdom) vol 29 no 10 pp1174ndash1178 2013

[14] N Oono Q X Tang and S Ukai ldquoOxide particle refinementin Ni-based ODS alloyrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 649 pp 250ndash253 2016

[15] D B Witkin and E J Lavernia ldquoSynthesis and mechanicalbehavior of nanostructured materials via cryomillingrdquo Progressin Materials Science vol 51 no 1 pp 1ndash60 2006

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

8 Research

From high-resolution transmission electron microscopy(TEM) observations it was noted that in the early stages ofmilling due to the high deformation rates experienced bythe powder deformation was localized within shear bands[44] These shear bands which contain a high density ofdislocations have a typical width of approximately 05 to 10120583m Small grains with a diameter of 8ndash12 nm were seenwithin the shear bands and electron diffraction patternssuggested significant preferred orientation With continuedmilling the average atomic level strain increased due toincreasing dislocation density and at a certain dislocationdensity within these heavily strained regions the crystaldisintegrated into subgrains that are separated by low-anglegrain boundaries This resulted in a decrease of the latticestrain The subgrains formed this way were of nanometerdimensions and are often between 20 and 30 nm

On further processing deformation occurred in shearbands located in previously unstrained parts of the materialThe grain size decreased steadily and the shear bands coa-lesced The small-angle boundaries are replaced by higherangle grain boundaries implying grain rotation as reflectedby the absence of texture in the electron diffraction patternsand random orientation of the grains observed from thelattice fringes in the high-resolution TEMmicrographs Con-sequently dislocation-free nanocrystalline grains are formedThis is the currently accepted mechanism of nanocrystalformation in MA processed powders

The grain size of the milled materials decreases withmilling time and reaches a saturation level when a balance isestablished between the fracturing and cold welding eventsThis minimum grain size 119889min is different depending on thematerial and milling conditionsThe value of 119889min achievableby milling is determined by the competition between theplastic deformation via dislocation motion that tends todecrease the grain size and the recovery and recrystallizationbehavior of the material that tends to increase the grain sizeThis balance gives a lower bound for the grain size of puremetals and alloys

The 119889min obtained is different for different metals and isalso found to vary with the crystal structure (Figure 4) Inmost of the metals the minimum grain size attained is inthe nanometer dimensions But metals with a body-centeredcubic (BCC) crystal structure reach much smaller valuesin comparison to metals with the other crystal structuresrelated to the difficulty of extensive plastic deformation andconsequent enhanced fracturing tendency during millingCeramics and intermetallic compounds are much harderand usually more brittle than the metals on which they arebased Therefore intuitively one expects that 119889min of thesecompounds is smaller than those of the pure metals but thisdoes not appear to be the case always

It was also reported that 119889min decreases with an increasein the melting temperature of the metal [45] As shown inFigure 4 this trend is amply clear in the case of metalswith close-packed structures (face-centered cubic (FCC) andhexagonal close-packed (HCP)) but not so clear in metalswith a BCC structure Another point of interest is that thedifference in grain size is much less among metals thathave high melting temperatures the minimum grain size is

0

5

10

15

20

25

FCCBCCHCP

Min

imum

Gra

in S

ize (

nm)

0 2000 30001000Melting Temperature (K)

Figure 4 Variation of minimum grain size with temperature inmechanically milled FCC HCP and BCC metals

virtually constant Thus for the HCP metals Co Ti Zr Hfand Ru the minimum grain size is almost the same eventhough themelting temperatures vary between 1495∘C for Coand 2310∘C for Ru

Analysis of the existing data on 119889min of mechanicallymilled pure metals shows that the normalized minimumgrain size 119889minb where b is the Burgers vector of thedislocations decreases with increasing melting temperatureactivation energy for self-diffusion hardness stacking faultenergy bulk modulus and the equilibrium distance betweentwo edge dislocations The 119889min value is also affected bymilling energy (the higher the milling energy the smaller thegrain size) milling temperature (the lower the milling tem-perature the smaller the grain size) and alloying additions(the minimum grain size is smaller for solid solutions thanfor pure metals since solid solutions are harder and strongerthan the pure metals on which they are based)

It was suggested that 119889min is determined by the minimumgrain size that can sustain a dislocation pile-up within a grainand by the rate of recovery Based on the dislocation pile-up model the critical equilibrium distance between two edgedislocations in a pile-up L119888 (which could be assumed to bethe crystallite or grain size inmilled powders) was calculated[46] using the equation

119871119888=3119866119887

(1 minus ])119867(2)

where G is the shear modulus 119887 is the Burgers vector ] isthe Poissonrsquos ratio and H is the hardness of the materialAccording to the above equation increased hardness resultsin smaller values of 119871c (grain size) and an approximate linearrelationship was observed between 119871c and the minimumgrain size obtained by milling of a number of metals Otherattempts have also beenmade to theoretically predict 119889min onthe basis of thermodynamic properties of materials [47]

Research 9

6 Nanocomposites

Achievement of a uniform distribution of the reinforcementin a matrix is essential to the achievement of good anduniform mechanical properties in composites Further themechanical properties of the composite tend to improvewith increasing volume fraction andor decreasing particlesize of the reinforcement [48] Traditionally a reasonablylarge volume fraction of the reinforcement could be addedif the size of the reinforcement is large (on a micrometerscale or larger) But if the reinforcement size is very fine(of nanometer dimensions) then the volume fraction addedis typically limited to about 2 to 4 However if a largevolume fraction of nanometer-sized reinforcement can beintroduced the mechanical properties of the composite arelikely to be vastly improved Therefore several investigationshave been undertaken to produce improved nanocompositesthrough MA [20]

Composites are traditionally produced by solidificationprocessing methods since the processing is inexpensive andlarge tonnage quantities can also be produced But it isnot easy to disperse fine reinforcement particles in a liquidmetal since agglomeration of the fine particles occurs andclusters are produced Further wetting of the particles ispoor and consequently even the most vigorous stirring isnot able to break the agglomerates Additionally the finepowders tend to float to the top of the melt during processingof the composites through solidification processing Thussolid-state processing methods such as MA are effective indispersing fine particles (including nanoparticles) An addedadvantage is that since no melting is involved even a largevolume fraction of the reinforcement can be introduced

Some of the specific goals sought in processing ofnanocomposite materials throughMA include incorporationof a high volume percentage of ultrafine reinforcements andachievement of high ductility and even superplasticity insome composites It is also possible that due to the increasedductility the fracture toughness of these composites couldbe higher than that of their coarse-grained counterparts Inline with these goals we have in recent times accomplished(a) achievement of a very uniform distribution of the rein-forcement phases in different types of matrices includingdispersion of a high volume fraction of Al2O3 in Al [49] andof Ti5Si3in 120574ndashTiAl [50 51] and (b) synthesis of MoSi

2+Si3N4

composites for high-temperature applications [52] amongothers However we will describe here only the developmentof Ti5Si3+120574ndashTiAl composites as a typical example

Lightweight intermetallic alloys based on 120574-TiAl arepromising materials for high-temperature structural appli-cations eg in aircraft engines or stationary turbines Eventhough they have many desirable properties such as highspecific strength and modulus both at room and elevatedtemperatures and good corrosion and oxidation resistancethey suffer from inadequate room temperature ductility andinsufficient creep resistance at elevated temperatures espe-cially between 800 and 850∘C an important requirement forelevated temperature applications of these materials There-fore current research programs have been addressing thedevelopment of high-temperature materials with adequate

020406080

100120140160180200

Engi

neer

ing

stres

s (M

Pa)

20 40 60 80 100 120 140 1600Engineering strain ()

-5950

∘C 4x10

-4950

∘C 4x10

-41000

∘C 4x10x10

(a)

(b)

Figure 5 (a) Scanning electronmicrograph of the 120574-TiAl + 60 volTi5Si3composite specimen showing that the twophases are very uni-

formly distributed in the microstructure Such a microstructure isconducive to achieving superplastic deformation under appropriateconditions of testing (b) Tensile stress-strain curve of 120574-TiAl + 60vol Ti

5Si3composite showing that superplastic deformation was

achieved at 950∘C with a strain rate of 4x10minus5 sminus1 and at 1000∘C witha strain rate of 4x10minus4 sminus1

room temperature ductility for easy formability and abilityto increase the high-temperature strength by a suitable heattreatment or alloying additions to obtain sufficient creepresistance at elevated temperatures

Composites of 120574-TiAl and 120585-Ti5Si3phase with the volume

fractions of the 120585-Ti5Si3phase varying from 0 to 60 vol

were produced by MA of a combination of the blendedelemental and prealloyed intermetallic powders Fully denseand porosity-free compacts were produced by hot isostaticpressing with the resulting grain size of each of the phasesbeing about 300-400 nm Figure 5(a) shows scanning elec-tron micrographs of the 120574-TiAl+60 vol 120585-Ti5Si3 compositeshowing that the two phases are very uniformly distributedthroughout the microstructure Materials exhibiting such amicrostructure (roughly equal amounts of the two phasessmall grain sizes and a uniform distribution of the twophases) are expected to be deformed superplastically Totest this hypothesis both compression and tensile testingof these composite specimens were conducted at differenttemperatures and strain rates From the tensile stress-strain

10 Research

Pure Metals Alloysand Compounds

NanostructuredMaterials

MECHANICALALLOYING

ODS Superalloys Supercorroding

Alloys

Synthesis of Novel Phases PVDTargets Solders Paints Food Heaters

Biomaterials Waste Utilization etc

Hydrogen Storage MagneticMaterials Catalysts Sensors etc

Figure 6 An overview of the applications of the MA process

plots of the 120574-TiAl+60 vol 120585-Ti5Si3composite specimens

shown in Figure 5(b) it is clear that the specimens testedat 950∘C with a strain rate of 4times10minus5 sminus1 and at 1000∘Cwith a strain rate of 4times10minus4 sminus1 exhibited large ductilityof nearly 150 and 100 respectively Considering that thiscomposite is based on a ceramic material (Ti

5Si3) this is a

very high amount of deformation suggestive of superplasticdeformation Final proof is provided by TEM investiga-tions that confirm the continued stability of the equiaxedmicrostructure after deformation

7 Applications

As has been frequently pointed out in the literature thetechnique ofMAwas developed out of an industrial necessityin 1965 to produce ODS nickel-based superalloys Sincethen a number of new applications have been exploredfor mechanically alloyed materials (Figure 6) Apart fromthe ODS alloys which continue to be used in the high-temperature applications area MA powders have also beenused for other applications and newer applications are alsobeing explored These novel applications include direct syn-thesis of pure metals and alloys from ores and scrap mate-rials super-corroding alloys for release into ocean bottomsdevelopment of PVD (physical vapor deposition) targetssolders paints etc hydrogen storage materials photovoltaicmaterials catalysts and sensors

A novel and simple application of MA powders is in thedevelopment ofmeals ready to eat (MRE) In this applicationvery finely ground Mg-based alloy powders are used Whenwater is added to these fineMg-Fe powders heat is generatedaccording to the following reaction

Mg (Fe) + 2H2O 997888rarr Mg (OH)2 +H2 +Heat (3)

This heat is utilized to warm the food in remote areas wherecooking or heating facilities may not be available eg in war-torn areas These ldquofood heatersrdquo were widely used during theDesert Stormoperations in the 1990s Let us now look at someof the other applications

71 Pure Metals and Alloys The technique of MA has beenemployed to produce pure metals and alloys from ores andother raw materials such as scrap The process is based onchemical reactions induced by mechanical activation Mostof the reactions studied have been displacement reactions ofthe type

119872119874 + 119877 997888rarr 119872 + 119877119874 (4)

where ametal oxide (MO) is reduced by amore reactivemetal(reductant R) to the pure metal M In addition to oxidesmetal chlorides and sulfides have also been reduced to puremetals this way Similar reactions have also been used toproduce alloys and nanocomposites [21 53]

All solid-state reactions involve the formation of a prod-uct phase at the interface of the component phases Thusin the above example the metal M forms at the interfacebetween the oxide MO and the reductant R and physicallyseparates the reactants Further growth of the product phaseinvolves diffusion of atoms of the reactant phases through theproduct phase which constitutes a barrier layer preventingfurther reaction from occurring In other words the reactioninterface defined as the nominal boundary surface betweenthe reactants continuously decreases during the course ofthe reaction Consequently kinetics of the reaction are slowand elevated temperatures are required to achieve reasonablereaction rates

MA can provide the means to substantially increasethe reaction kinetics of chemical reactions in general andthe reduction reactions in particular This is because therepeated cold welding and fracturing of powder particlesincrease the area of contact between the reactant powderparticles by bringing fresh surfaces to come into contactrepeatedly due to a reduction in particle size during millingThis allows the reaction to proceed without the necessityfor diffusion through the product layer As a consequencereactions that normally require high temperatures will occurat lower temperatures or even without any externally appliedheat In addition the high defect densities induced by MAaccelerate diffusion processes

Research 11

Depending on the milling conditions two entirely differ-ent reaction kinetics are possible [21 54]

(a) the reactionmay extend to a very small volumeduringeach collision resulting in a gradual transformationor

(b) if the reaction enthalpy is sufficiently high (and ifthe adiabatic temperature is above 1800 K) a self-propagating high-temperature synthesis (SHS) reaction(also known as combustion synthesis) can be initi-ated by controlling the milling conditions (eg byadding diluents) to avoid combustion reactions maybe made to proceed in a steady-state manner

The latter type of reactions requires a critical milling time forthe combustion reaction to be initiated If the temperature ofthe vial is recorded during the milling process the tempera-ture initially increases slowly with time After some time thetemperature increases abruptly suggesting that ignition hasoccurred and this is followed by a relatively slow decrease

In the above types of reactions themilled powder consistsof the pure metalM dispersed in the RO phase both usuallyof nanometer dimensions It has been shown that by anintelligent choice of the reductant the RO phase can beeasily leached out Thus by selective removal of the matrixphase by washing with appropriate solvents it is possibleto achieve well-dispersed metal nanoparticles as small as 5nm in size If the crystallinity of these particles is not highit can be improved by subsequent annealing without thefear of agglomeration due to the enclosure of nanoparticlesin a solid matrix Such nanoparticles are structurally andmorphologically uniform having a narrow size distributionThis technique has been used to produce powders of manydifferent metalsmdashAg Cd Co Cr Cu Fe Gd Nb NiTi V W Zn and Zr Several alloys intermetallics andnanocomposites have also been synthesized Figure 7 showstwo micrographs of nanocrystalline ceria particles producedby the conventional vapor synthesis and MA methods Eventhough the grain size is approximately the same in both thecases the nanoparticles produced by MA are very discreteand display a very narrow particle size distribution whilethose produced by vapor deposition are agglomerated andhave a wide particle size distribution

Novel materials have also been synthesized by MA ofblended elemental powders One of the interesting examplesin this category is the synthesis of plain carbon steels andstainless steels [55 56] Twomain advantages of this synthesisroute are that the steels can be manufactured at roomtemperature and that the steels will be nanostructured thusconferring improved properties But these powders needto be consolidated to full density for applications and itis possible that grain growth occurs during this processThe kinetics of grain growth can however be controlled byregulating the time and temperature combinations

72 Hydrogen Storage Materials Another important applica-tion of MA powders is hydrogen storage Mg is a lightweightmetal (the lightest structural material) with a density of174 gcm3 Mg and Mg-based alloys can store a significant

20 nm

50 nm

Figure 7 Transmission electron micrographs of ceria particlesproduced byMA (top) and vapor synthesis (bottom)methods Notethe severe agglomeration of the nanoparticles in the latter case

amount of hydrogen However being a reactive metal Mgalways contains a thin layer of oxide on the surface Thissurface oxide layer which inhibits asorption of hydrogenneeds to be broken down through activation A high-temperature annealing is usually done to achieve this ButMA can overcome these difficulties due to the severe plasticdeformation experienced by the powder particles duringMAA catalyst is also often required to accelerate the kinetics ofhydrogen and dehydrogenation of alloys

Mg can adsorb about 76 wt hydrogen andMg-Ni alloysmuch less However Mg is hard to activate its tempera-ture of operation is high and the sorption and desorptionkinetics are slow Therefore Mg-Ni and other alloys arebeing explored But due to the high vapor pressure of Mgand significant difference in the melting temperatures ofthe constituent elements (Mg = 650∘C and Ni = 1452∘C) itis difficult to prepare these alloys by conventional meltingand solidification processes Therefore MA has been used toconveniently synthesize these alloys since this is a completelysolid-state processing method An added advantage of MA isthat the product will have a nanocrystalline grain structureallowing the improvements described below [57 58]

Figure 8 shows the hydrogenation and dehydrogena-tion plots for conventional and nanocrystalline alloysAn additional plot for a situation where a catalyst isadded to the nanocrystalline material is also shown Fromthese two graphs the following facts become clear Firstlythe nanocrystalline alloys adsorb hydrogen much fasterthan conventional (coarse-grained) alloys even though the

12 Research

0 1 2 3 4 5 6 7 8 9 10time (min)

conventional

p = 8 bar

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

time (min)

nano + catalyst

nano + catalyst

nanocrystallinenanocrystalline

conventional

breakthrough in reaction kinetics due to nanostructure and use of catalyst

0

1

2

3

4

5

6

7

8

hydr

ogen

cont

ent (

wt

)

T = 300∘C p = 10-3 mbar

Figure 8 Comparison of the hydrogen sorption and desorption kinetics of Mg alloys with conventional and nanocrystalline grain sizesSignificant improvement in the desorption kinetics when a catalyst is added is worth noticing

amount of hydrogen adsorbed is the sameWhile the conven-tional alloys do not adsorb any significant amount of hydro-gen in about 10 min the allowable full amount of hydrogenis adsorbed in the nanocrystalline alloy Similar observationsare made by many other authors in many different alloysFurther the kinetics are slightly faster (not much different)when a catalyst is added to the nanocrystalline alloy Onthe other hand the kinetics appear to be very significantlyaffected during the desorption process For example thenanocrystalline alloys desorb hydrogen much faster thanthe conventional alloys Again while the conventional alloysdo not desorb any significant amount of hydrogen thenanocrystalline alloy desorbs over 20 of the hydrogenstored in about 10 min But the most significant effect wasobserved when a catalyst was added to the nanocrystallinealloyAll the adsorbedhydrogenwas desorbed in about 2minwhen a catalyst was added to the nanocrystalline alloy Thusnanostructure processing throughMAappears to be a fruitfulway of developing novel hydrogen storage materials [59]

Many novel complex hydrides based on light metals(Mg Li and Na) are being explored and the importantcategories of materials being investigated include the alanates(also known as aluminohydrides a family of compoundsconsisting of hydrogen and aluminum) borohydrides amideborohydrides amide-imide systems (which have a highhydrogen storage capacity and low operative temperatures)amine borane (compounds of borane groups having ammo-nia addition) and alane

73 Energy Materials The technique of MA has also beenemployed to synthesize materials for energy applicationsThese include synthesis of photovoltaic (PV) materials andultrafine particles for faster ignition and combustion pur-poses

One of the most common PV materials is CIGS acompound of Cu In Ga and Se with the compositionCu(InGa)Se2 Since PV cells have the highest efficiencywhen

the band gap for this semiconductor is 125 eV one will haveto engineer the composition for the ratio of Ga to In This isbecause the band gaps for CuInSe

2 and CuGaSe2 are 102 and170 eV respectively Accordingly a Ga(Ga+In) ratio of about03 is found most appropriate and the final ideal compositionof the PV material will be Cu097In07Ga03Se2

The traditional method of synthesizing this material isto prepare an alloy of Cu Ga and In by vapor depositionmethods and incorporate Se into this by exposure to Seatmosphere Consequently this is a complex process MAhowever can be a simple direct and efficient method ofmanufacturing Blended elemental powders of Cu In Gaand Se weremilled in a copper container for different periodsof time and the phase constitution was monitored throughX-ray diffraction studies It was noted that the tetragonalCu(InGa)Se

2phase has formed even aftermilling the powder

mix for 20 min Process variables such as mill rotation speedball-to-powder weight ratio (BPR) and milling time werevaried and it was noted that the ideal composition wasachieved at lowmilling speeds short milling times and a lowBPR (Table 2) [60] By fine-tuning these parameters it shouldbe possible to achieve the exact composition

Nanostructuredmetal particles have high specific surfaceareas and consequently they are highly reactive and can storeenergy This area has received a great impetus in recentyears since the combustion rates of nanostructured metalparticles aremuch higher than those of coarsemetal particlesThe particle size cannot be exceedingly small because thesurface may be coated with a thin oxide layer and make theparticles passive [61ndash63] MA is very well suited to preparesuch materials because the reactions are easily initiatedby impact or friction and at the same time milling isa simple scalable and controllable technology capable ofmixing reactive components on the nanoscaleThese reactivematerials could be used as additives to propellants explosivesand pyrotechnics as well asmanufacture of reactive structuralcomponents [63]

Research 13

Table 2 Chemical analysis (at) of the Cu-In-Ga-Se powder mechanically alloyed under different processing conditions

BPR Speed (rpm) Time (min) Cu In Ga Se201 300 120 3261 1523 653 4562201 300 240 3173 1563 671 4593101 150 40 2598 1698 725 4978Targeted (Cu

097In07Ga03Se2) 2443 1763 756 5038

Compared to the presently used liquid aviation fuelsmetal particles based on B Al andMg are effective HoweverB and Al have very high boiling points and high heats ofenthalpy On the other hand Mg has a low boiling point butalso low heat of enthalpy Therefore if Al and Mg could bemixed intimately on a nanometer scale one could hope tohave a high heat of enthalpy at reduced boiling temperaturesThat is Mg-Al alloy particles could be practical in achievinga high-energy density and practical ignitionboiling temper-atures MA is a very effective method to accomplish this

Al and Mg powders were blended together in differentproportions and milled Since the milled powders have arange of particle sizes they were sieved to obtain well-defined powder sizes They were then fluidized and thencombusted in a CH4-air premixed flame Both the ignitionand burning times of these metal particles decreased withdecreased particle sizes and were also shorter when the Mgcontent is higher (Figure 9) [64] Similar results have beenreported by other investigators

8 Powder Consolidation

The product of MA is in powder form Except in cases ofchemical applications such as catalysis when the productdoes not require consolidation widespread application ofMA powders requires efficient methods of consolidatingthem into bulk shapes Successful consolidation of MA pow-ders is a nontrivial problem since fully densematerials shouldbe produced while simultaneously retaining the nanometer-sized grains without coarsening or the amorphous phaseswithout crystallizing Conventional consolidation of powdersto full density through processes such as hot extrusion [65]and hot isostatic pressing (HIP) [66] requires use of highpressures and exposure to elevated temperatures for extendedperiods of time [67] Unfortunately however this resultsin significant coarsening of the nanometer-sized grains orcrystallization of amorphous phases and consequently thebenefits of nanostructure processing or amorphization arelost [68] On the other hand retention of finemicrostructuresrequires use of low consolidation temperatures and then itis difficult to achieve full interparticle bonding at these lowtemperatures Therefore novel and innovative methods ofconsolidating MA powders are required The objectives ofconsolidation of these powders are to (a) achieve full densifi-cation (without any porosity) (b) minimize microstructuralcoarsening (ie retention of fine grain size) andor (c)avoid undesirable phase transformations In fact the earlyresults of excellent mechanical properties and most specifi-cally superplasticity at room temperature [69] attributed to

20 Mg (tig)90 Mg (tig)

20 Mg (tb)90 Mg (tb)

20 40 60 80 100 1200Particle Size (m)

0

10

20

30

40

50

60

70

Igni

tion

Burn

ing

Tim

e (m

s)

Figure 9 Ignition and burning times as a function of particle sizefor the mechanically alloyed Al-20 and Al-90 at Mg powdersIn the insert tig and tb represent the ignition and burning timesrespectively

nanocrystalline ceramics have not been successfully repro-duced due to incomplete consolidation of the nanopowdersin the material tested Thus consolidation to full densityassumes even greater importance

Because of the small size of the powder particles (typicallya few micrometers even though the grain size is only afew nanometers) MA powders pose special problems Forexample such nanometric powders are very hard and strongThey also have a high level of interparticle friction Furthersince these powders have a large surface area they also exhibithigh chemical reactivity Therefore special precautions needto be taken to consolidate the metal powders to bulk shapes

Successful consolidation of MA powders has beenachieved by electrodischarge compaction plasma-activatedsintering shock (explosive) consolidation [70] HIP hydro-static extrusion strained powder rolling and sinter forging[71] By utilizing the combination of high temperature andpressure HIP can achieve a particular density at lowerpressure when compared to cold isostatic pressing or at lowertemperature when compared to sintering Because of theincreased diffusivity in nanocrystalline materials sintering(and therefore densification) takes place at temperaturesmuch lower than those in coarse-grained materials This islikely to reduce the grain growth Spark plasma sintering(SPS) has become the most popular technique to consolidatemetal powders especially those with ultrafine grain sizesdue to the fact that full density can be achieved at low

14 Research

SOLID LIQUID GAS

MILLING

USE POWDER AS-ISor Chemical Reaction

Thermal Treatment etcCONSOLIDATE TO SOLID

(Temperature Pressure)

Catalysts Pigments SolderComponents Ceramic Powder etc

Advanced materials with hightolerance to gaseous impurities

Figure 10 Alternatives to consolidation of metal powders Ifpowders are used as catalysts pigments solder components etcthen they need not be consolidated However if powders need tobe consolidated then they could be used in applications with hightolerance to gaseous impurities

temperatures and shorter times in comparison to severalothermethods [72 73] An excellent reviewbyGroza [71]maybe consulted for full details of the methods of consolidationand description of results

9 Powder Contamination

Amajor concern inmetal powders processed byMAmethodsis the nature and amount of impurities that get incorporatedinto the powder and contaminate it The small size of thepowder particles and consequent availability of large surfacearea formation of fresh surfaces during milling and wearand tear of the milling tools and the atmosphere underwhich the powder is milled all contribute to contaminationof the powder In addition to these factors one has toconsider the purity of the starting raw materials and themilling conditions employed (especially the milling atmo-sphere) Thus it appears as though powder contaminationis an inherent drawback of the method and that the milledpowder will always be contaminated with impurities unlessspecial precautions are taken to avoidminimize them Themagnitude of powder contamination appears to depend onthe time of milling intensity of milling atmosphere in whichthe powder is milled nature and size of the milling mediumand differences in the strengthhardness of the powder andthe milling medium and the container

Several attempts have been made in recent years to min-imize powder contamination during milling An importantpoint to remember is that if the starting powders are highlypure then the final product is going to be clean and purewith minimum contamination from other sources providedthat proper precautions are taken during milling It is alsoimportant to keep inmind that all applications do not requireultraclean powders and that the purity desired will depend onthe specific application (Figure 10) But it is a good practiceto minimize powder contamination at every stage and ensurethat additional impurities are not picked up subsequentlyduring handling andor consolidation

One way ofminimizing contamination from the grindingmedium and themilling container is to use the samematerial

for the container and the grinding medium as the powderbeing milled Thus one could use copper balls and coppercontainer formilling copper and copper alloy powders [60] Ifa container of the same material to be milled is not availablethen a thin adherent coating on the internal surface of thecontainer (and also on the surface of the grinding medium)with the material to be milled will minimize contaminationThe idea here is to mill the powder once allowing the powderto be coated onto the grindingmedium and the inner walls ofthe container The milled loose powder is then discarded anda fresh batch of powder is milled but with the old grindingmedia and container In general a simple rule that shouldbe followed to minimize contamination from the millingcontainer and the grinding medium is that the container andgrindingmedium should be harderstronger than the powderbeing milled

Milling atmosphere is perhaps the most important vari-able that needs to be controlled Even though nominally puregases are used to purge the glove box before loading andunloading the powders even small amounts of impurities inthe gases appear to contaminate the powders especially whenthey are reactive The best solution one could think of willbe to place the mill inside a chamber that is evacuated andfilled with high-purity argon gas Since the whole chamberis maintained under argon gas atmosphere (continuouslypurified to keep oxygen and water vapor below 1 ppm each)and the container is inside the mill which is inside thechamber contamination from the atmosphere is minimum

Contamination from PCAs is perhaps the most ubiqui-tous Since most of the PCAs used are organic compoundswhich have low melting and boiling points they decomposeduring milling due to the heat generated The decomposi-tion products consisting of carbon oxygen nitrogen andhydrogen react with the metal atoms and form undesirablecarbides oxides nitrides etc

10 Concluding Remarks and Future Prospects

We have only briefly touched upon the scientific and tech-nological aspects of MA materials along with some of theconcerns such as powder contamination and consolidationissues The historical development of MA during the last 50years or so can be divided into threemajor periods (Figure 11)The first period covering the first twenty years (from 1966up to about 1985) was mostly concerned with the devel-opment and production of oxide-dispersion-strengthened(ODS) superalloys for applications in the aerospace indus-try Several alloys with improved properties based onNi and Fe were developed and found useful applicationsThese included the MA754 MA760 MA956 MA957 andMA6000 [74] A more recent survey of application of MAto steel is also available [75] The second period of aboutfifteen years covering from about 1986 up to about 2000witnessed advances in the fundamental understanding ofthe processes that take place during MA Along with thisa large number of dedicated conferences were held andthere was a burgeoning of publication activity Comparisonshave also been frequently made between MA and othernonequilibriumprocessing techniquesmore specifically RSP

Research 15

1965 19851975 1995 2005

Production of ODS Alloys

Novel Phase SynthesisDevelopment of BasicUnderstanding of MA

Nanostructures andNew Applications

2015

Figure 11 Schematic showing the different periods of development in the field of mechanical alloying (MA) and applications ofMA productssince its inception in 1965

It was shown that themetastable effects achieved by these twononequilibrium processing techniques are similar Revival ofthe mechanochemical processing (MCP) took place and avariety of novel substances were synthesized Several mod-eling studies were also conducted to enable prediction of thephases produced or microstructures obtained although withlimited success The third period starting from about 2001saw a reemergence of the quest for new applications of theMA materials not only as structural materials but also forchemical applications such as catalysis and hydrogen storagewith the realization that contamination of themilled powdersis the limiting factor in the widespread applications of theMA materials Innovative techniques to consolidate the MApowders to full density while retaining the metastable phases(including glassy phases andor nanostructures) in themwerealso developed All these are continuing with the ongoinginvestigations to enhance the scientific understanding of theMA process

At present activities relating to both the science andtechnology of MA have been going on simultaneously Whilesome groups have been focusing on developing a betterunderstanding of the process and optimizing the processvariables to achieve a certain phase or phase combinationin the alloy system others have been concentrating onexploiting this technique for different applications It hasalso come to be realized that the MA process can be usedto recycle used and discarded materials to produce value-added materials for subsequent consumption They can bepulverized separated and used to mix with other powdersto produce composites The bottom line is that newer andimproved materials are being continuously developed fornovel applications

Conflicts of Interest

The author declares that he has no conflicts of interest

References

[1] K H J Buschow R W Cahn M C Flemings B Ilschner E JKramer and SMahajan EdsEncyclopedia ofMaterials Scienceand Technology Elsevier 2001

[2] C Suryanarayana EdNon-equilibrium Processing of MaterialsPergamon Oxford UK 1999

[3] C Suryanarayana ldquoMetallic glassesrdquo Bulletin of Materials Sci-ence vol 6 no 3 pp 579ndash594 1984

[4] C Suryanarayana and A Inoue Bulk Metallic Glasses CRCPress Boca Raton Fla USA 2nd edition 2018

[5] H R Trebin Ed Quasicrystals Structure and Physical Proper-ties Wiley-VCH Weinheim Germany 2003

[6] C Suryanarayana and H Jones ldquoFormation and characteristicsof quasicrystalline phases A reviewrdquo International Journal ofRapid Solidification vol 3 pp 253ndash293 1988

[7] H Gleiter ldquoNanocrystalline materialsrdquo Progress in MaterialsScience vol 33 no 4 pp 223ndash315 1989

[8] C Suryanarayana ldquoRecent developments in nanostructuredmaterialsrdquo Advanced Engineering Materials vol 7 no 11 pp983ndash992 2005

[9] M C Gao J-W Yeh P K Liaw and Y Zhang Eds High-Entropy Alloys Fundamentals and Applications Springer 2016

[10] J S Benjamin ldquoMechanical alloying ndash History and futurepotentialrdquo in Advances in Powder Metallurgy and ParticulateMaterials J M Capus and R M German Eds vol 7 ofNovel Powder Processing pp 155ndash168 Metal Powder IndustriesFederation Princeton NJ USA 1992

[11] M P Phaniraj D-I Kim J-H Shim and Y W CholdquoMicrostructure development in mechanically alloyed yttriadispersed austenitic steelsrdquo Acta Materialia vol 57 no 6 pp1856ndash1864 2009

[12] A Hirata T Fujita Y R Wen J H Schneibel C T Liu and MW Chen ldquoAtomic structure of nanoclusters in oxide-dispersionstrengthened steelsrdquo Nature Materials vol 10 no 12 pp 922ndash926 2011

[13] M K Miller C M Parish and Q Li ldquoAdvanced oxide disper-sion strengthened and nanostructured ferritic alloysrdquoMaterialsScience and Technology (United Kingdom) vol 29 no 10 pp1174ndash1178 2013

[14] N Oono Q X Tang and S Ukai ldquoOxide particle refinementin Ni-based ODS alloyrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 649 pp 250ndash253 2016

[15] D B Witkin and E J Lavernia ldquoSynthesis and mechanicalbehavior of nanostructured materials via cryomillingrdquo Progressin Materials Science vol 51 no 1 pp 1ndash60 2006

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

Research 9

6 Nanocomposites

Achievement of a uniform distribution of the reinforcementin a matrix is essential to the achievement of good anduniform mechanical properties in composites Further themechanical properties of the composite tend to improvewith increasing volume fraction andor decreasing particlesize of the reinforcement [48] Traditionally a reasonablylarge volume fraction of the reinforcement could be addedif the size of the reinforcement is large (on a micrometerscale or larger) But if the reinforcement size is very fine(of nanometer dimensions) then the volume fraction addedis typically limited to about 2 to 4 However if a largevolume fraction of nanometer-sized reinforcement can beintroduced the mechanical properties of the composite arelikely to be vastly improved Therefore several investigationshave been undertaken to produce improved nanocompositesthrough MA [20]

Composites are traditionally produced by solidificationprocessing methods since the processing is inexpensive andlarge tonnage quantities can also be produced But it isnot easy to disperse fine reinforcement particles in a liquidmetal since agglomeration of the fine particles occurs andclusters are produced Further wetting of the particles ispoor and consequently even the most vigorous stirring isnot able to break the agglomerates Additionally the finepowders tend to float to the top of the melt during processingof the composites through solidification processing Thussolid-state processing methods such as MA are effective indispersing fine particles (including nanoparticles) An addedadvantage is that since no melting is involved even a largevolume fraction of the reinforcement can be introduced

Some of the specific goals sought in processing ofnanocomposite materials throughMA include incorporationof a high volume percentage of ultrafine reinforcements andachievement of high ductility and even superplasticity insome composites It is also possible that due to the increasedductility the fracture toughness of these composites couldbe higher than that of their coarse-grained counterparts Inline with these goals we have in recent times accomplished(a) achievement of a very uniform distribution of the rein-forcement phases in different types of matrices includingdispersion of a high volume fraction of Al2O3 in Al [49] andof Ti5Si3in 120574ndashTiAl [50 51] and (b) synthesis of MoSi

2+Si3N4

composites for high-temperature applications [52] amongothers However we will describe here only the developmentof Ti5Si3+120574ndashTiAl composites as a typical example

Lightweight intermetallic alloys based on 120574-TiAl arepromising materials for high-temperature structural appli-cations eg in aircraft engines or stationary turbines Eventhough they have many desirable properties such as highspecific strength and modulus both at room and elevatedtemperatures and good corrosion and oxidation resistancethey suffer from inadequate room temperature ductility andinsufficient creep resistance at elevated temperatures espe-cially between 800 and 850∘C an important requirement forelevated temperature applications of these materials There-fore current research programs have been addressing thedevelopment of high-temperature materials with adequate

020406080

100120140160180200

Engi

neer

ing

stres

s (M

Pa)

20 40 60 80 100 120 140 1600Engineering strain ()

-5950

∘C 4x10

-4950

∘C 4x10

-41000

∘C 4x10x10

(a)

(b)

Figure 5 (a) Scanning electronmicrograph of the 120574-TiAl + 60 volTi5Si3composite specimen showing that the twophases are very uni-

formly distributed in the microstructure Such a microstructure isconducive to achieving superplastic deformation under appropriateconditions of testing (b) Tensile stress-strain curve of 120574-TiAl + 60vol Ti

5Si3composite showing that superplastic deformation was

achieved at 950∘C with a strain rate of 4x10minus5 sminus1 and at 1000∘C witha strain rate of 4x10minus4 sminus1

room temperature ductility for easy formability and abilityto increase the high-temperature strength by a suitable heattreatment or alloying additions to obtain sufficient creepresistance at elevated temperatures

Composites of 120574-TiAl and 120585-Ti5Si3phase with the volume

fractions of the 120585-Ti5Si3phase varying from 0 to 60 vol

were produced by MA of a combination of the blendedelemental and prealloyed intermetallic powders Fully denseand porosity-free compacts were produced by hot isostaticpressing with the resulting grain size of each of the phasesbeing about 300-400 nm Figure 5(a) shows scanning elec-tron micrographs of the 120574-TiAl+60 vol 120585-Ti5Si3 compositeshowing that the two phases are very uniformly distributedthroughout the microstructure Materials exhibiting such amicrostructure (roughly equal amounts of the two phasessmall grain sizes and a uniform distribution of the twophases) are expected to be deformed superplastically Totest this hypothesis both compression and tensile testingof these composite specimens were conducted at differenttemperatures and strain rates From the tensile stress-strain

10 Research

Pure Metals Alloysand Compounds

NanostructuredMaterials

MECHANICALALLOYING

ODS Superalloys Supercorroding

Alloys

Synthesis of Novel Phases PVDTargets Solders Paints Food Heaters

Biomaterials Waste Utilization etc

Hydrogen Storage MagneticMaterials Catalysts Sensors etc

Figure 6 An overview of the applications of the MA process

plots of the 120574-TiAl+60 vol 120585-Ti5Si3composite specimens

shown in Figure 5(b) it is clear that the specimens testedat 950∘C with a strain rate of 4times10minus5 sminus1 and at 1000∘Cwith a strain rate of 4times10minus4 sminus1 exhibited large ductilityof nearly 150 and 100 respectively Considering that thiscomposite is based on a ceramic material (Ti

5Si3) this is a

very high amount of deformation suggestive of superplasticdeformation Final proof is provided by TEM investiga-tions that confirm the continued stability of the equiaxedmicrostructure after deformation

7 Applications

As has been frequently pointed out in the literature thetechnique ofMAwas developed out of an industrial necessityin 1965 to produce ODS nickel-based superalloys Sincethen a number of new applications have been exploredfor mechanically alloyed materials (Figure 6) Apart fromthe ODS alloys which continue to be used in the high-temperature applications area MA powders have also beenused for other applications and newer applications are alsobeing explored These novel applications include direct syn-thesis of pure metals and alloys from ores and scrap mate-rials super-corroding alloys for release into ocean bottomsdevelopment of PVD (physical vapor deposition) targetssolders paints etc hydrogen storage materials photovoltaicmaterials catalysts and sensors

A novel and simple application of MA powders is in thedevelopment ofmeals ready to eat (MRE) In this applicationvery finely ground Mg-based alloy powders are used Whenwater is added to these fineMg-Fe powders heat is generatedaccording to the following reaction

Mg (Fe) + 2H2O 997888rarr Mg (OH)2 +H2 +Heat (3)

This heat is utilized to warm the food in remote areas wherecooking or heating facilities may not be available eg in war-torn areas These ldquofood heatersrdquo were widely used during theDesert Stormoperations in the 1990s Let us now look at someof the other applications

71 Pure Metals and Alloys The technique of MA has beenemployed to produce pure metals and alloys from ores andother raw materials such as scrap The process is based onchemical reactions induced by mechanical activation Mostof the reactions studied have been displacement reactions ofthe type

119872119874 + 119877 997888rarr 119872 + 119877119874 (4)

where ametal oxide (MO) is reduced by amore reactivemetal(reductant R) to the pure metal M In addition to oxidesmetal chlorides and sulfides have also been reduced to puremetals this way Similar reactions have also been used toproduce alloys and nanocomposites [21 53]

All solid-state reactions involve the formation of a prod-uct phase at the interface of the component phases Thusin the above example the metal M forms at the interfacebetween the oxide MO and the reductant R and physicallyseparates the reactants Further growth of the product phaseinvolves diffusion of atoms of the reactant phases through theproduct phase which constitutes a barrier layer preventingfurther reaction from occurring In other words the reactioninterface defined as the nominal boundary surface betweenthe reactants continuously decreases during the course ofthe reaction Consequently kinetics of the reaction are slowand elevated temperatures are required to achieve reasonablereaction rates

MA can provide the means to substantially increasethe reaction kinetics of chemical reactions in general andthe reduction reactions in particular This is because therepeated cold welding and fracturing of powder particlesincrease the area of contact between the reactant powderparticles by bringing fresh surfaces to come into contactrepeatedly due to a reduction in particle size during millingThis allows the reaction to proceed without the necessityfor diffusion through the product layer As a consequencereactions that normally require high temperatures will occurat lower temperatures or even without any externally appliedheat In addition the high defect densities induced by MAaccelerate diffusion processes

Research 11

Depending on the milling conditions two entirely differ-ent reaction kinetics are possible [21 54]

(a) the reactionmay extend to a very small volumeduringeach collision resulting in a gradual transformationor

(b) if the reaction enthalpy is sufficiently high (and ifthe adiabatic temperature is above 1800 K) a self-propagating high-temperature synthesis (SHS) reaction(also known as combustion synthesis) can be initi-ated by controlling the milling conditions (eg byadding diluents) to avoid combustion reactions maybe made to proceed in a steady-state manner

The latter type of reactions requires a critical milling time forthe combustion reaction to be initiated If the temperature ofthe vial is recorded during the milling process the tempera-ture initially increases slowly with time After some time thetemperature increases abruptly suggesting that ignition hasoccurred and this is followed by a relatively slow decrease

In the above types of reactions themilled powder consistsof the pure metalM dispersed in the RO phase both usuallyof nanometer dimensions It has been shown that by anintelligent choice of the reductant the RO phase can beeasily leached out Thus by selective removal of the matrixphase by washing with appropriate solvents it is possibleto achieve well-dispersed metal nanoparticles as small as 5nm in size If the crystallinity of these particles is not highit can be improved by subsequent annealing without thefear of agglomeration due to the enclosure of nanoparticlesin a solid matrix Such nanoparticles are structurally andmorphologically uniform having a narrow size distributionThis technique has been used to produce powders of manydifferent metalsmdashAg Cd Co Cr Cu Fe Gd Nb NiTi V W Zn and Zr Several alloys intermetallics andnanocomposites have also been synthesized Figure 7 showstwo micrographs of nanocrystalline ceria particles producedby the conventional vapor synthesis and MA methods Eventhough the grain size is approximately the same in both thecases the nanoparticles produced by MA are very discreteand display a very narrow particle size distribution whilethose produced by vapor deposition are agglomerated andhave a wide particle size distribution

Novel materials have also been synthesized by MA ofblended elemental powders One of the interesting examplesin this category is the synthesis of plain carbon steels andstainless steels [55 56] Twomain advantages of this synthesisroute are that the steels can be manufactured at roomtemperature and that the steels will be nanostructured thusconferring improved properties But these powders needto be consolidated to full density for applications and itis possible that grain growth occurs during this processThe kinetics of grain growth can however be controlled byregulating the time and temperature combinations

72 Hydrogen Storage Materials Another important applica-tion of MA powders is hydrogen storage Mg is a lightweightmetal (the lightest structural material) with a density of174 gcm3 Mg and Mg-based alloys can store a significant

20 nm

50 nm

Figure 7 Transmission electron micrographs of ceria particlesproduced byMA (top) and vapor synthesis (bottom)methods Notethe severe agglomeration of the nanoparticles in the latter case

amount of hydrogen However being a reactive metal Mgalways contains a thin layer of oxide on the surface Thissurface oxide layer which inhibits asorption of hydrogenneeds to be broken down through activation A high-temperature annealing is usually done to achieve this ButMA can overcome these difficulties due to the severe plasticdeformation experienced by the powder particles duringMAA catalyst is also often required to accelerate the kinetics ofhydrogen and dehydrogenation of alloys

Mg can adsorb about 76 wt hydrogen andMg-Ni alloysmuch less However Mg is hard to activate its tempera-ture of operation is high and the sorption and desorptionkinetics are slow Therefore Mg-Ni and other alloys arebeing explored But due to the high vapor pressure of Mgand significant difference in the melting temperatures ofthe constituent elements (Mg = 650∘C and Ni = 1452∘C) itis difficult to prepare these alloys by conventional meltingand solidification processes Therefore MA has been used toconveniently synthesize these alloys since this is a completelysolid-state processing method An added advantage of MA isthat the product will have a nanocrystalline grain structureallowing the improvements described below [57 58]

Figure 8 shows the hydrogenation and dehydrogena-tion plots for conventional and nanocrystalline alloysAn additional plot for a situation where a catalyst isadded to the nanocrystalline material is also shown Fromthese two graphs the following facts become clear Firstlythe nanocrystalline alloys adsorb hydrogen much fasterthan conventional (coarse-grained) alloys even though the

12 Research

0 1 2 3 4 5 6 7 8 9 10time (min)

conventional

p = 8 bar

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

time (min)

nano + catalyst

nano + catalyst

nanocrystallinenanocrystalline

conventional

breakthrough in reaction kinetics due to nanostructure and use of catalyst

0

1

2

3

4

5

6

7

8

hydr

ogen

cont

ent (

wt

)

T = 300∘C p = 10-3 mbar

Figure 8 Comparison of the hydrogen sorption and desorption kinetics of Mg alloys with conventional and nanocrystalline grain sizesSignificant improvement in the desorption kinetics when a catalyst is added is worth noticing

amount of hydrogen adsorbed is the sameWhile the conven-tional alloys do not adsorb any significant amount of hydro-gen in about 10 min the allowable full amount of hydrogenis adsorbed in the nanocrystalline alloy Similar observationsare made by many other authors in many different alloysFurther the kinetics are slightly faster (not much different)when a catalyst is added to the nanocrystalline alloy Onthe other hand the kinetics appear to be very significantlyaffected during the desorption process For example thenanocrystalline alloys desorb hydrogen much faster thanthe conventional alloys Again while the conventional alloysdo not desorb any significant amount of hydrogen thenanocrystalline alloy desorbs over 20 of the hydrogenstored in about 10 min But the most significant effect wasobserved when a catalyst was added to the nanocrystallinealloyAll the adsorbedhydrogenwas desorbed in about 2minwhen a catalyst was added to the nanocrystalline alloy Thusnanostructure processing throughMAappears to be a fruitfulway of developing novel hydrogen storage materials [59]

Many novel complex hydrides based on light metals(Mg Li and Na) are being explored and the importantcategories of materials being investigated include the alanates(also known as aluminohydrides a family of compoundsconsisting of hydrogen and aluminum) borohydrides amideborohydrides amide-imide systems (which have a highhydrogen storage capacity and low operative temperatures)amine borane (compounds of borane groups having ammo-nia addition) and alane

73 Energy Materials The technique of MA has also beenemployed to synthesize materials for energy applicationsThese include synthesis of photovoltaic (PV) materials andultrafine particles for faster ignition and combustion pur-poses

One of the most common PV materials is CIGS acompound of Cu In Ga and Se with the compositionCu(InGa)Se2 Since PV cells have the highest efficiencywhen

the band gap for this semiconductor is 125 eV one will haveto engineer the composition for the ratio of Ga to In This isbecause the band gaps for CuInSe

2 and CuGaSe2 are 102 and170 eV respectively Accordingly a Ga(Ga+In) ratio of about03 is found most appropriate and the final ideal compositionof the PV material will be Cu097In07Ga03Se2

The traditional method of synthesizing this material isto prepare an alloy of Cu Ga and In by vapor depositionmethods and incorporate Se into this by exposure to Seatmosphere Consequently this is a complex process MAhowever can be a simple direct and efficient method ofmanufacturing Blended elemental powders of Cu In Gaand Se weremilled in a copper container for different periodsof time and the phase constitution was monitored throughX-ray diffraction studies It was noted that the tetragonalCu(InGa)Se

2phase has formed even aftermilling the powder

mix for 20 min Process variables such as mill rotation speedball-to-powder weight ratio (BPR) and milling time werevaried and it was noted that the ideal composition wasachieved at lowmilling speeds short milling times and a lowBPR (Table 2) [60] By fine-tuning these parameters it shouldbe possible to achieve the exact composition

Nanostructuredmetal particles have high specific surfaceareas and consequently they are highly reactive and can storeenergy This area has received a great impetus in recentyears since the combustion rates of nanostructured metalparticles aremuch higher than those of coarsemetal particlesThe particle size cannot be exceedingly small because thesurface may be coated with a thin oxide layer and make theparticles passive [61ndash63] MA is very well suited to preparesuch materials because the reactions are easily initiatedby impact or friction and at the same time milling isa simple scalable and controllable technology capable ofmixing reactive components on the nanoscaleThese reactivematerials could be used as additives to propellants explosivesand pyrotechnics as well asmanufacture of reactive structuralcomponents [63]

Research 13

Table 2 Chemical analysis (at) of the Cu-In-Ga-Se powder mechanically alloyed under different processing conditions

BPR Speed (rpm) Time (min) Cu In Ga Se201 300 120 3261 1523 653 4562201 300 240 3173 1563 671 4593101 150 40 2598 1698 725 4978Targeted (Cu

097In07Ga03Se2) 2443 1763 756 5038

Compared to the presently used liquid aviation fuelsmetal particles based on B Al andMg are effective HoweverB and Al have very high boiling points and high heats ofenthalpy On the other hand Mg has a low boiling point butalso low heat of enthalpy Therefore if Al and Mg could bemixed intimately on a nanometer scale one could hope tohave a high heat of enthalpy at reduced boiling temperaturesThat is Mg-Al alloy particles could be practical in achievinga high-energy density and practical ignitionboiling temper-atures MA is a very effective method to accomplish this

Al and Mg powders were blended together in differentproportions and milled Since the milled powders have arange of particle sizes they were sieved to obtain well-defined powder sizes They were then fluidized and thencombusted in a CH4-air premixed flame Both the ignitionand burning times of these metal particles decreased withdecreased particle sizes and were also shorter when the Mgcontent is higher (Figure 9) [64] Similar results have beenreported by other investigators

8 Powder Consolidation

The product of MA is in powder form Except in cases ofchemical applications such as catalysis when the productdoes not require consolidation widespread application ofMA powders requires efficient methods of consolidatingthem into bulk shapes Successful consolidation of MA pow-ders is a nontrivial problem since fully densematerials shouldbe produced while simultaneously retaining the nanometer-sized grains without coarsening or the amorphous phaseswithout crystallizing Conventional consolidation of powdersto full density through processes such as hot extrusion [65]and hot isostatic pressing (HIP) [66] requires use of highpressures and exposure to elevated temperatures for extendedperiods of time [67] Unfortunately however this resultsin significant coarsening of the nanometer-sized grains orcrystallization of amorphous phases and consequently thebenefits of nanostructure processing or amorphization arelost [68] On the other hand retention of finemicrostructuresrequires use of low consolidation temperatures and then itis difficult to achieve full interparticle bonding at these lowtemperatures Therefore novel and innovative methods ofconsolidating MA powders are required The objectives ofconsolidation of these powders are to (a) achieve full densifi-cation (without any porosity) (b) minimize microstructuralcoarsening (ie retention of fine grain size) andor (c)avoid undesirable phase transformations In fact the earlyresults of excellent mechanical properties and most specifi-cally superplasticity at room temperature [69] attributed to

20 Mg (tig)90 Mg (tig)

20 Mg (tb)90 Mg (tb)

20 40 60 80 100 1200Particle Size (m)

0

10

20

30

40

50

60

70

Igni

tion

Burn

ing

Tim

e (m

s)

Figure 9 Ignition and burning times as a function of particle sizefor the mechanically alloyed Al-20 and Al-90 at Mg powdersIn the insert tig and tb represent the ignition and burning timesrespectively

nanocrystalline ceramics have not been successfully repro-duced due to incomplete consolidation of the nanopowdersin the material tested Thus consolidation to full densityassumes even greater importance

Because of the small size of the powder particles (typicallya few micrometers even though the grain size is only afew nanometers) MA powders pose special problems Forexample such nanometric powders are very hard and strongThey also have a high level of interparticle friction Furthersince these powders have a large surface area they also exhibithigh chemical reactivity Therefore special precautions needto be taken to consolidate the metal powders to bulk shapes

Successful consolidation of MA powders has beenachieved by electrodischarge compaction plasma-activatedsintering shock (explosive) consolidation [70] HIP hydro-static extrusion strained powder rolling and sinter forging[71] By utilizing the combination of high temperature andpressure HIP can achieve a particular density at lowerpressure when compared to cold isostatic pressing or at lowertemperature when compared to sintering Because of theincreased diffusivity in nanocrystalline materials sintering(and therefore densification) takes place at temperaturesmuch lower than those in coarse-grained materials This islikely to reduce the grain growth Spark plasma sintering(SPS) has become the most popular technique to consolidatemetal powders especially those with ultrafine grain sizesdue to the fact that full density can be achieved at low

14 Research

SOLID LIQUID GAS

MILLING

USE POWDER AS-ISor Chemical Reaction

Thermal Treatment etcCONSOLIDATE TO SOLID

(Temperature Pressure)

Catalysts Pigments SolderComponents Ceramic Powder etc

Advanced materials with hightolerance to gaseous impurities

Figure 10 Alternatives to consolidation of metal powders Ifpowders are used as catalysts pigments solder components etcthen they need not be consolidated However if powders need tobe consolidated then they could be used in applications with hightolerance to gaseous impurities

temperatures and shorter times in comparison to severalothermethods [72 73] An excellent reviewbyGroza [71]maybe consulted for full details of the methods of consolidationand description of results

9 Powder Contamination

Amajor concern inmetal powders processed byMAmethodsis the nature and amount of impurities that get incorporatedinto the powder and contaminate it The small size of thepowder particles and consequent availability of large surfacearea formation of fresh surfaces during milling and wearand tear of the milling tools and the atmosphere underwhich the powder is milled all contribute to contaminationof the powder In addition to these factors one has toconsider the purity of the starting raw materials and themilling conditions employed (especially the milling atmo-sphere) Thus it appears as though powder contaminationis an inherent drawback of the method and that the milledpowder will always be contaminated with impurities unlessspecial precautions are taken to avoidminimize them Themagnitude of powder contamination appears to depend onthe time of milling intensity of milling atmosphere in whichthe powder is milled nature and size of the milling mediumand differences in the strengthhardness of the powder andthe milling medium and the container

Several attempts have been made in recent years to min-imize powder contamination during milling An importantpoint to remember is that if the starting powders are highlypure then the final product is going to be clean and purewith minimum contamination from other sources providedthat proper precautions are taken during milling It is alsoimportant to keep inmind that all applications do not requireultraclean powders and that the purity desired will depend onthe specific application (Figure 10) But it is a good practiceto minimize powder contamination at every stage and ensurethat additional impurities are not picked up subsequentlyduring handling andor consolidation

One way ofminimizing contamination from the grindingmedium and themilling container is to use the samematerial

for the container and the grinding medium as the powderbeing milled Thus one could use copper balls and coppercontainer formilling copper and copper alloy powders [60] Ifa container of the same material to be milled is not availablethen a thin adherent coating on the internal surface of thecontainer (and also on the surface of the grinding medium)with the material to be milled will minimize contaminationThe idea here is to mill the powder once allowing the powderto be coated onto the grindingmedium and the inner walls ofthe container The milled loose powder is then discarded anda fresh batch of powder is milled but with the old grindingmedia and container In general a simple rule that shouldbe followed to minimize contamination from the millingcontainer and the grinding medium is that the container andgrindingmedium should be harderstronger than the powderbeing milled

Milling atmosphere is perhaps the most important vari-able that needs to be controlled Even though nominally puregases are used to purge the glove box before loading andunloading the powders even small amounts of impurities inthe gases appear to contaminate the powders especially whenthey are reactive The best solution one could think of willbe to place the mill inside a chamber that is evacuated andfilled with high-purity argon gas Since the whole chamberis maintained under argon gas atmosphere (continuouslypurified to keep oxygen and water vapor below 1 ppm each)and the container is inside the mill which is inside thechamber contamination from the atmosphere is minimum

Contamination from PCAs is perhaps the most ubiqui-tous Since most of the PCAs used are organic compoundswhich have low melting and boiling points they decomposeduring milling due to the heat generated The decomposi-tion products consisting of carbon oxygen nitrogen andhydrogen react with the metal atoms and form undesirablecarbides oxides nitrides etc

10 Concluding Remarks and Future Prospects

We have only briefly touched upon the scientific and tech-nological aspects of MA materials along with some of theconcerns such as powder contamination and consolidationissues The historical development of MA during the last 50years or so can be divided into threemajor periods (Figure 11)The first period covering the first twenty years (from 1966up to about 1985) was mostly concerned with the devel-opment and production of oxide-dispersion-strengthened(ODS) superalloys for applications in the aerospace indus-try Several alloys with improved properties based onNi and Fe were developed and found useful applicationsThese included the MA754 MA760 MA956 MA957 andMA6000 [74] A more recent survey of application of MAto steel is also available [75] The second period of aboutfifteen years covering from about 1986 up to about 2000witnessed advances in the fundamental understanding ofthe processes that take place during MA Along with thisa large number of dedicated conferences were held andthere was a burgeoning of publication activity Comparisonshave also been frequently made between MA and othernonequilibriumprocessing techniquesmore specifically RSP

Research 15

1965 19851975 1995 2005

Production of ODS Alloys

Novel Phase SynthesisDevelopment of BasicUnderstanding of MA

Nanostructures andNew Applications

2015

Figure 11 Schematic showing the different periods of development in the field of mechanical alloying (MA) and applications ofMA productssince its inception in 1965

It was shown that themetastable effects achieved by these twononequilibrium processing techniques are similar Revival ofthe mechanochemical processing (MCP) took place and avariety of novel substances were synthesized Several mod-eling studies were also conducted to enable prediction of thephases produced or microstructures obtained although withlimited success The third period starting from about 2001saw a reemergence of the quest for new applications of theMA materials not only as structural materials but also forchemical applications such as catalysis and hydrogen storagewith the realization that contamination of themilled powdersis the limiting factor in the widespread applications of theMA materials Innovative techniques to consolidate the MApowders to full density while retaining the metastable phases(including glassy phases andor nanostructures) in themwerealso developed All these are continuing with the ongoinginvestigations to enhance the scientific understanding of theMA process

At present activities relating to both the science andtechnology of MA have been going on simultaneously Whilesome groups have been focusing on developing a betterunderstanding of the process and optimizing the processvariables to achieve a certain phase or phase combinationin the alloy system others have been concentrating onexploiting this technique for different applications It hasalso come to be realized that the MA process can be usedto recycle used and discarded materials to produce value-added materials for subsequent consumption They can bepulverized separated and used to mix with other powdersto produce composites The bottom line is that newer andimproved materials are being continuously developed fornovel applications

Conflicts of Interest

The author declares that he has no conflicts of interest

References

[1] K H J Buschow R W Cahn M C Flemings B Ilschner E JKramer and SMahajan EdsEncyclopedia ofMaterials Scienceand Technology Elsevier 2001

[2] C Suryanarayana EdNon-equilibrium Processing of MaterialsPergamon Oxford UK 1999

[3] C Suryanarayana ldquoMetallic glassesrdquo Bulletin of Materials Sci-ence vol 6 no 3 pp 579ndash594 1984

[4] C Suryanarayana and A Inoue Bulk Metallic Glasses CRCPress Boca Raton Fla USA 2nd edition 2018

[5] H R Trebin Ed Quasicrystals Structure and Physical Proper-ties Wiley-VCH Weinheim Germany 2003

[6] C Suryanarayana and H Jones ldquoFormation and characteristicsof quasicrystalline phases A reviewrdquo International Journal ofRapid Solidification vol 3 pp 253ndash293 1988

[7] H Gleiter ldquoNanocrystalline materialsrdquo Progress in MaterialsScience vol 33 no 4 pp 223ndash315 1989

[8] C Suryanarayana ldquoRecent developments in nanostructuredmaterialsrdquo Advanced Engineering Materials vol 7 no 11 pp983ndash992 2005

[9] M C Gao J-W Yeh P K Liaw and Y Zhang Eds High-Entropy Alloys Fundamentals and Applications Springer 2016

[10] J S Benjamin ldquoMechanical alloying ndash History and futurepotentialrdquo in Advances in Powder Metallurgy and ParticulateMaterials J M Capus and R M German Eds vol 7 ofNovel Powder Processing pp 155ndash168 Metal Powder IndustriesFederation Princeton NJ USA 1992

[11] M P Phaniraj D-I Kim J-H Shim and Y W CholdquoMicrostructure development in mechanically alloyed yttriadispersed austenitic steelsrdquo Acta Materialia vol 57 no 6 pp1856ndash1864 2009

[12] A Hirata T Fujita Y R Wen J H Schneibel C T Liu and MW Chen ldquoAtomic structure of nanoclusters in oxide-dispersionstrengthened steelsrdquo Nature Materials vol 10 no 12 pp 922ndash926 2011

[13] M K Miller C M Parish and Q Li ldquoAdvanced oxide disper-sion strengthened and nanostructured ferritic alloysrdquoMaterialsScience and Technology (United Kingdom) vol 29 no 10 pp1174ndash1178 2013

[14] N Oono Q X Tang and S Ukai ldquoOxide particle refinementin Ni-based ODS alloyrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 649 pp 250ndash253 2016

[15] D B Witkin and E J Lavernia ldquoSynthesis and mechanicalbehavior of nanostructured materials via cryomillingrdquo Progressin Materials Science vol 51 no 1 pp 1ndash60 2006

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

10 Research

Pure Metals Alloysand Compounds

NanostructuredMaterials

MECHANICALALLOYING

ODS Superalloys Supercorroding

Alloys

Synthesis of Novel Phases PVDTargets Solders Paints Food Heaters

Biomaterials Waste Utilization etc

Hydrogen Storage MagneticMaterials Catalysts Sensors etc

Figure 6 An overview of the applications of the MA process

plots of the 120574-TiAl+60 vol 120585-Ti5Si3composite specimens

shown in Figure 5(b) it is clear that the specimens testedat 950∘C with a strain rate of 4times10minus5 sminus1 and at 1000∘Cwith a strain rate of 4times10minus4 sminus1 exhibited large ductilityof nearly 150 and 100 respectively Considering that thiscomposite is based on a ceramic material (Ti

5Si3) this is a

very high amount of deformation suggestive of superplasticdeformation Final proof is provided by TEM investiga-tions that confirm the continued stability of the equiaxedmicrostructure after deformation

7 Applications

As has been frequently pointed out in the literature thetechnique ofMAwas developed out of an industrial necessityin 1965 to produce ODS nickel-based superalloys Sincethen a number of new applications have been exploredfor mechanically alloyed materials (Figure 6) Apart fromthe ODS alloys which continue to be used in the high-temperature applications area MA powders have also beenused for other applications and newer applications are alsobeing explored These novel applications include direct syn-thesis of pure metals and alloys from ores and scrap mate-rials super-corroding alloys for release into ocean bottomsdevelopment of PVD (physical vapor deposition) targetssolders paints etc hydrogen storage materials photovoltaicmaterials catalysts and sensors

A novel and simple application of MA powders is in thedevelopment ofmeals ready to eat (MRE) In this applicationvery finely ground Mg-based alloy powders are used Whenwater is added to these fineMg-Fe powders heat is generatedaccording to the following reaction

Mg (Fe) + 2H2O 997888rarr Mg (OH)2 +H2 +Heat (3)

This heat is utilized to warm the food in remote areas wherecooking or heating facilities may not be available eg in war-torn areas These ldquofood heatersrdquo were widely used during theDesert Stormoperations in the 1990s Let us now look at someof the other applications

71 Pure Metals and Alloys The technique of MA has beenemployed to produce pure metals and alloys from ores andother raw materials such as scrap The process is based onchemical reactions induced by mechanical activation Mostof the reactions studied have been displacement reactions ofthe type

119872119874 + 119877 997888rarr 119872 + 119877119874 (4)

where ametal oxide (MO) is reduced by amore reactivemetal(reductant R) to the pure metal M In addition to oxidesmetal chlorides and sulfides have also been reduced to puremetals this way Similar reactions have also been used toproduce alloys and nanocomposites [21 53]

All solid-state reactions involve the formation of a prod-uct phase at the interface of the component phases Thusin the above example the metal M forms at the interfacebetween the oxide MO and the reductant R and physicallyseparates the reactants Further growth of the product phaseinvolves diffusion of atoms of the reactant phases through theproduct phase which constitutes a barrier layer preventingfurther reaction from occurring In other words the reactioninterface defined as the nominal boundary surface betweenthe reactants continuously decreases during the course ofthe reaction Consequently kinetics of the reaction are slowand elevated temperatures are required to achieve reasonablereaction rates

MA can provide the means to substantially increasethe reaction kinetics of chemical reactions in general andthe reduction reactions in particular This is because therepeated cold welding and fracturing of powder particlesincrease the area of contact between the reactant powderparticles by bringing fresh surfaces to come into contactrepeatedly due to a reduction in particle size during millingThis allows the reaction to proceed without the necessityfor diffusion through the product layer As a consequencereactions that normally require high temperatures will occurat lower temperatures or even without any externally appliedheat In addition the high defect densities induced by MAaccelerate diffusion processes

Research 11

Depending on the milling conditions two entirely differ-ent reaction kinetics are possible [21 54]

(a) the reactionmay extend to a very small volumeduringeach collision resulting in a gradual transformationor

(b) if the reaction enthalpy is sufficiently high (and ifthe adiabatic temperature is above 1800 K) a self-propagating high-temperature synthesis (SHS) reaction(also known as combustion synthesis) can be initi-ated by controlling the milling conditions (eg byadding diluents) to avoid combustion reactions maybe made to proceed in a steady-state manner

The latter type of reactions requires a critical milling time forthe combustion reaction to be initiated If the temperature ofthe vial is recorded during the milling process the tempera-ture initially increases slowly with time After some time thetemperature increases abruptly suggesting that ignition hasoccurred and this is followed by a relatively slow decrease

In the above types of reactions themilled powder consistsof the pure metalM dispersed in the RO phase both usuallyof nanometer dimensions It has been shown that by anintelligent choice of the reductant the RO phase can beeasily leached out Thus by selective removal of the matrixphase by washing with appropriate solvents it is possibleto achieve well-dispersed metal nanoparticles as small as 5nm in size If the crystallinity of these particles is not highit can be improved by subsequent annealing without thefear of agglomeration due to the enclosure of nanoparticlesin a solid matrix Such nanoparticles are structurally andmorphologically uniform having a narrow size distributionThis technique has been used to produce powders of manydifferent metalsmdashAg Cd Co Cr Cu Fe Gd Nb NiTi V W Zn and Zr Several alloys intermetallics andnanocomposites have also been synthesized Figure 7 showstwo micrographs of nanocrystalline ceria particles producedby the conventional vapor synthesis and MA methods Eventhough the grain size is approximately the same in both thecases the nanoparticles produced by MA are very discreteand display a very narrow particle size distribution whilethose produced by vapor deposition are agglomerated andhave a wide particle size distribution

Novel materials have also been synthesized by MA ofblended elemental powders One of the interesting examplesin this category is the synthesis of plain carbon steels andstainless steels [55 56] Twomain advantages of this synthesisroute are that the steels can be manufactured at roomtemperature and that the steels will be nanostructured thusconferring improved properties But these powders needto be consolidated to full density for applications and itis possible that grain growth occurs during this processThe kinetics of grain growth can however be controlled byregulating the time and temperature combinations

72 Hydrogen Storage Materials Another important applica-tion of MA powders is hydrogen storage Mg is a lightweightmetal (the lightest structural material) with a density of174 gcm3 Mg and Mg-based alloys can store a significant

20 nm

50 nm

Figure 7 Transmission electron micrographs of ceria particlesproduced byMA (top) and vapor synthesis (bottom)methods Notethe severe agglomeration of the nanoparticles in the latter case

amount of hydrogen However being a reactive metal Mgalways contains a thin layer of oxide on the surface Thissurface oxide layer which inhibits asorption of hydrogenneeds to be broken down through activation A high-temperature annealing is usually done to achieve this ButMA can overcome these difficulties due to the severe plasticdeformation experienced by the powder particles duringMAA catalyst is also often required to accelerate the kinetics ofhydrogen and dehydrogenation of alloys

Mg can adsorb about 76 wt hydrogen andMg-Ni alloysmuch less However Mg is hard to activate its tempera-ture of operation is high and the sorption and desorptionkinetics are slow Therefore Mg-Ni and other alloys arebeing explored But due to the high vapor pressure of Mgand significant difference in the melting temperatures ofthe constituent elements (Mg = 650∘C and Ni = 1452∘C) itis difficult to prepare these alloys by conventional meltingand solidification processes Therefore MA has been used toconveniently synthesize these alloys since this is a completelysolid-state processing method An added advantage of MA isthat the product will have a nanocrystalline grain structureallowing the improvements described below [57 58]

Figure 8 shows the hydrogenation and dehydrogena-tion plots for conventional and nanocrystalline alloysAn additional plot for a situation where a catalyst isadded to the nanocrystalline material is also shown Fromthese two graphs the following facts become clear Firstlythe nanocrystalline alloys adsorb hydrogen much fasterthan conventional (coarse-grained) alloys even though the

12 Research

0 1 2 3 4 5 6 7 8 9 10time (min)

conventional

p = 8 bar

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

time (min)

nano + catalyst

nano + catalyst

nanocrystallinenanocrystalline

conventional

breakthrough in reaction kinetics due to nanostructure and use of catalyst

0

1

2

3

4

5

6

7

8

hydr

ogen

cont

ent (

wt

)

T = 300∘C p = 10-3 mbar

Figure 8 Comparison of the hydrogen sorption and desorption kinetics of Mg alloys with conventional and nanocrystalline grain sizesSignificant improvement in the desorption kinetics when a catalyst is added is worth noticing

amount of hydrogen adsorbed is the sameWhile the conven-tional alloys do not adsorb any significant amount of hydro-gen in about 10 min the allowable full amount of hydrogenis adsorbed in the nanocrystalline alloy Similar observationsare made by many other authors in many different alloysFurther the kinetics are slightly faster (not much different)when a catalyst is added to the nanocrystalline alloy Onthe other hand the kinetics appear to be very significantlyaffected during the desorption process For example thenanocrystalline alloys desorb hydrogen much faster thanthe conventional alloys Again while the conventional alloysdo not desorb any significant amount of hydrogen thenanocrystalline alloy desorbs over 20 of the hydrogenstored in about 10 min But the most significant effect wasobserved when a catalyst was added to the nanocrystallinealloyAll the adsorbedhydrogenwas desorbed in about 2minwhen a catalyst was added to the nanocrystalline alloy Thusnanostructure processing throughMAappears to be a fruitfulway of developing novel hydrogen storage materials [59]

Many novel complex hydrides based on light metals(Mg Li and Na) are being explored and the importantcategories of materials being investigated include the alanates(also known as aluminohydrides a family of compoundsconsisting of hydrogen and aluminum) borohydrides amideborohydrides amide-imide systems (which have a highhydrogen storage capacity and low operative temperatures)amine borane (compounds of borane groups having ammo-nia addition) and alane

73 Energy Materials The technique of MA has also beenemployed to synthesize materials for energy applicationsThese include synthesis of photovoltaic (PV) materials andultrafine particles for faster ignition and combustion pur-poses

One of the most common PV materials is CIGS acompound of Cu In Ga and Se with the compositionCu(InGa)Se2 Since PV cells have the highest efficiencywhen

the band gap for this semiconductor is 125 eV one will haveto engineer the composition for the ratio of Ga to In This isbecause the band gaps for CuInSe

2 and CuGaSe2 are 102 and170 eV respectively Accordingly a Ga(Ga+In) ratio of about03 is found most appropriate and the final ideal compositionof the PV material will be Cu097In07Ga03Se2

The traditional method of synthesizing this material isto prepare an alloy of Cu Ga and In by vapor depositionmethods and incorporate Se into this by exposure to Seatmosphere Consequently this is a complex process MAhowever can be a simple direct and efficient method ofmanufacturing Blended elemental powders of Cu In Gaand Se weremilled in a copper container for different periodsof time and the phase constitution was monitored throughX-ray diffraction studies It was noted that the tetragonalCu(InGa)Se

2phase has formed even aftermilling the powder

mix for 20 min Process variables such as mill rotation speedball-to-powder weight ratio (BPR) and milling time werevaried and it was noted that the ideal composition wasachieved at lowmilling speeds short milling times and a lowBPR (Table 2) [60] By fine-tuning these parameters it shouldbe possible to achieve the exact composition

Nanostructuredmetal particles have high specific surfaceareas and consequently they are highly reactive and can storeenergy This area has received a great impetus in recentyears since the combustion rates of nanostructured metalparticles aremuch higher than those of coarsemetal particlesThe particle size cannot be exceedingly small because thesurface may be coated with a thin oxide layer and make theparticles passive [61ndash63] MA is very well suited to preparesuch materials because the reactions are easily initiatedby impact or friction and at the same time milling isa simple scalable and controllable technology capable ofmixing reactive components on the nanoscaleThese reactivematerials could be used as additives to propellants explosivesand pyrotechnics as well asmanufacture of reactive structuralcomponents [63]

Research 13

Table 2 Chemical analysis (at) of the Cu-In-Ga-Se powder mechanically alloyed under different processing conditions

BPR Speed (rpm) Time (min) Cu In Ga Se201 300 120 3261 1523 653 4562201 300 240 3173 1563 671 4593101 150 40 2598 1698 725 4978Targeted (Cu

097In07Ga03Se2) 2443 1763 756 5038

Compared to the presently used liquid aviation fuelsmetal particles based on B Al andMg are effective HoweverB and Al have very high boiling points and high heats ofenthalpy On the other hand Mg has a low boiling point butalso low heat of enthalpy Therefore if Al and Mg could bemixed intimately on a nanometer scale one could hope tohave a high heat of enthalpy at reduced boiling temperaturesThat is Mg-Al alloy particles could be practical in achievinga high-energy density and practical ignitionboiling temper-atures MA is a very effective method to accomplish this

Al and Mg powders were blended together in differentproportions and milled Since the milled powders have arange of particle sizes they were sieved to obtain well-defined powder sizes They were then fluidized and thencombusted in a CH4-air premixed flame Both the ignitionand burning times of these metal particles decreased withdecreased particle sizes and were also shorter when the Mgcontent is higher (Figure 9) [64] Similar results have beenreported by other investigators

8 Powder Consolidation

The product of MA is in powder form Except in cases ofchemical applications such as catalysis when the productdoes not require consolidation widespread application ofMA powders requires efficient methods of consolidatingthem into bulk shapes Successful consolidation of MA pow-ders is a nontrivial problem since fully densematerials shouldbe produced while simultaneously retaining the nanometer-sized grains without coarsening or the amorphous phaseswithout crystallizing Conventional consolidation of powdersto full density through processes such as hot extrusion [65]and hot isostatic pressing (HIP) [66] requires use of highpressures and exposure to elevated temperatures for extendedperiods of time [67] Unfortunately however this resultsin significant coarsening of the nanometer-sized grains orcrystallization of amorphous phases and consequently thebenefits of nanostructure processing or amorphization arelost [68] On the other hand retention of finemicrostructuresrequires use of low consolidation temperatures and then itis difficult to achieve full interparticle bonding at these lowtemperatures Therefore novel and innovative methods ofconsolidating MA powders are required The objectives ofconsolidation of these powders are to (a) achieve full densifi-cation (without any porosity) (b) minimize microstructuralcoarsening (ie retention of fine grain size) andor (c)avoid undesirable phase transformations In fact the earlyresults of excellent mechanical properties and most specifi-cally superplasticity at room temperature [69] attributed to

20 Mg (tig)90 Mg (tig)

20 Mg (tb)90 Mg (tb)

20 40 60 80 100 1200Particle Size (m)

0

10

20

30

40

50

60

70

Igni

tion

Burn

ing

Tim

e (m

s)

Figure 9 Ignition and burning times as a function of particle sizefor the mechanically alloyed Al-20 and Al-90 at Mg powdersIn the insert tig and tb represent the ignition and burning timesrespectively

nanocrystalline ceramics have not been successfully repro-duced due to incomplete consolidation of the nanopowdersin the material tested Thus consolidation to full densityassumes even greater importance

Because of the small size of the powder particles (typicallya few micrometers even though the grain size is only afew nanometers) MA powders pose special problems Forexample such nanometric powders are very hard and strongThey also have a high level of interparticle friction Furthersince these powders have a large surface area they also exhibithigh chemical reactivity Therefore special precautions needto be taken to consolidate the metal powders to bulk shapes

Successful consolidation of MA powders has beenachieved by electrodischarge compaction plasma-activatedsintering shock (explosive) consolidation [70] HIP hydro-static extrusion strained powder rolling and sinter forging[71] By utilizing the combination of high temperature andpressure HIP can achieve a particular density at lowerpressure when compared to cold isostatic pressing or at lowertemperature when compared to sintering Because of theincreased diffusivity in nanocrystalline materials sintering(and therefore densification) takes place at temperaturesmuch lower than those in coarse-grained materials This islikely to reduce the grain growth Spark plasma sintering(SPS) has become the most popular technique to consolidatemetal powders especially those with ultrafine grain sizesdue to the fact that full density can be achieved at low

14 Research

SOLID LIQUID GAS

MILLING

USE POWDER AS-ISor Chemical Reaction

Thermal Treatment etcCONSOLIDATE TO SOLID

(Temperature Pressure)

Catalysts Pigments SolderComponents Ceramic Powder etc

Advanced materials with hightolerance to gaseous impurities

Figure 10 Alternatives to consolidation of metal powders Ifpowders are used as catalysts pigments solder components etcthen they need not be consolidated However if powders need tobe consolidated then they could be used in applications with hightolerance to gaseous impurities

temperatures and shorter times in comparison to severalothermethods [72 73] An excellent reviewbyGroza [71]maybe consulted for full details of the methods of consolidationand description of results

9 Powder Contamination

Amajor concern inmetal powders processed byMAmethodsis the nature and amount of impurities that get incorporatedinto the powder and contaminate it The small size of thepowder particles and consequent availability of large surfacearea formation of fresh surfaces during milling and wearand tear of the milling tools and the atmosphere underwhich the powder is milled all contribute to contaminationof the powder In addition to these factors one has toconsider the purity of the starting raw materials and themilling conditions employed (especially the milling atmo-sphere) Thus it appears as though powder contaminationis an inherent drawback of the method and that the milledpowder will always be contaminated with impurities unlessspecial precautions are taken to avoidminimize them Themagnitude of powder contamination appears to depend onthe time of milling intensity of milling atmosphere in whichthe powder is milled nature and size of the milling mediumand differences in the strengthhardness of the powder andthe milling medium and the container

Several attempts have been made in recent years to min-imize powder contamination during milling An importantpoint to remember is that if the starting powders are highlypure then the final product is going to be clean and purewith minimum contamination from other sources providedthat proper precautions are taken during milling It is alsoimportant to keep inmind that all applications do not requireultraclean powders and that the purity desired will depend onthe specific application (Figure 10) But it is a good practiceto minimize powder contamination at every stage and ensurethat additional impurities are not picked up subsequentlyduring handling andor consolidation

One way ofminimizing contamination from the grindingmedium and themilling container is to use the samematerial

for the container and the grinding medium as the powderbeing milled Thus one could use copper balls and coppercontainer formilling copper and copper alloy powders [60] Ifa container of the same material to be milled is not availablethen a thin adherent coating on the internal surface of thecontainer (and also on the surface of the grinding medium)with the material to be milled will minimize contaminationThe idea here is to mill the powder once allowing the powderto be coated onto the grindingmedium and the inner walls ofthe container The milled loose powder is then discarded anda fresh batch of powder is milled but with the old grindingmedia and container In general a simple rule that shouldbe followed to minimize contamination from the millingcontainer and the grinding medium is that the container andgrindingmedium should be harderstronger than the powderbeing milled

Milling atmosphere is perhaps the most important vari-able that needs to be controlled Even though nominally puregases are used to purge the glove box before loading andunloading the powders even small amounts of impurities inthe gases appear to contaminate the powders especially whenthey are reactive The best solution one could think of willbe to place the mill inside a chamber that is evacuated andfilled with high-purity argon gas Since the whole chamberis maintained under argon gas atmosphere (continuouslypurified to keep oxygen and water vapor below 1 ppm each)and the container is inside the mill which is inside thechamber contamination from the atmosphere is minimum

Contamination from PCAs is perhaps the most ubiqui-tous Since most of the PCAs used are organic compoundswhich have low melting and boiling points they decomposeduring milling due to the heat generated The decomposi-tion products consisting of carbon oxygen nitrogen andhydrogen react with the metal atoms and form undesirablecarbides oxides nitrides etc

10 Concluding Remarks and Future Prospects

We have only briefly touched upon the scientific and tech-nological aspects of MA materials along with some of theconcerns such as powder contamination and consolidationissues The historical development of MA during the last 50years or so can be divided into threemajor periods (Figure 11)The first period covering the first twenty years (from 1966up to about 1985) was mostly concerned with the devel-opment and production of oxide-dispersion-strengthened(ODS) superalloys for applications in the aerospace indus-try Several alloys with improved properties based onNi and Fe were developed and found useful applicationsThese included the MA754 MA760 MA956 MA957 andMA6000 [74] A more recent survey of application of MAto steel is also available [75] The second period of aboutfifteen years covering from about 1986 up to about 2000witnessed advances in the fundamental understanding ofthe processes that take place during MA Along with thisa large number of dedicated conferences were held andthere was a burgeoning of publication activity Comparisonshave also been frequently made between MA and othernonequilibriumprocessing techniquesmore specifically RSP

Research 15

1965 19851975 1995 2005

Production of ODS Alloys

Novel Phase SynthesisDevelopment of BasicUnderstanding of MA

Nanostructures andNew Applications

2015

Figure 11 Schematic showing the different periods of development in the field of mechanical alloying (MA) and applications ofMA productssince its inception in 1965

It was shown that themetastable effects achieved by these twononequilibrium processing techniques are similar Revival ofthe mechanochemical processing (MCP) took place and avariety of novel substances were synthesized Several mod-eling studies were also conducted to enable prediction of thephases produced or microstructures obtained although withlimited success The third period starting from about 2001saw a reemergence of the quest for new applications of theMA materials not only as structural materials but also forchemical applications such as catalysis and hydrogen storagewith the realization that contamination of themilled powdersis the limiting factor in the widespread applications of theMA materials Innovative techniques to consolidate the MApowders to full density while retaining the metastable phases(including glassy phases andor nanostructures) in themwerealso developed All these are continuing with the ongoinginvestigations to enhance the scientific understanding of theMA process

At present activities relating to both the science andtechnology of MA have been going on simultaneously Whilesome groups have been focusing on developing a betterunderstanding of the process and optimizing the processvariables to achieve a certain phase or phase combinationin the alloy system others have been concentrating onexploiting this technique for different applications It hasalso come to be realized that the MA process can be usedto recycle used and discarded materials to produce value-added materials for subsequent consumption They can bepulverized separated and used to mix with other powdersto produce composites The bottom line is that newer andimproved materials are being continuously developed fornovel applications

Conflicts of Interest

The author declares that he has no conflicts of interest

References

[1] K H J Buschow R W Cahn M C Flemings B Ilschner E JKramer and SMahajan EdsEncyclopedia ofMaterials Scienceand Technology Elsevier 2001

[2] C Suryanarayana EdNon-equilibrium Processing of MaterialsPergamon Oxford UK 1999

[3] C Suryanarayana ldquoMetallic glassesrdquo Bulletin of Materials Sci-ence vol 6 no 3 pp 579ndash594 1984

[4] C Suryanarayana and A Inoue Bulk Metallic Glasses CRCPress Boca Raton Fla USA 2nd edition 2018

[5] H R Trebin Ed Quasicrystals Structure and Physical Proper-ties Wiley-VCH Weinheim Germany 2003

[6] C Suryanarayana and H Jones ldquoFormation and characteristicsof quasicrystalline phases A reviewrdquo International Journal ofRapid Solidification vol 3 pp 253ndash293 1988

[7] H Gleiter ldquoNanocrystalline materialsrdquo Progress in MaterialsScience vol 33 no 4 pp 223ndash315 1989

[8] C Suryanarayana ldquoRecent developments in nanostructuredmaterialsrdquo Advanced Engineering Materials vol 7 no 11 pp983ndash992 2005

[9] M C Gao J-W Yeh P K Liaw and Y Zhang Eds High-Entropy Alloys Fundamentals and Applications Springer 2016

[10] J S Benjamin ldquoMechanical alloying ndash History and futurepotentialrdquo in Advances in Powder Metallurgy and ParticulateMaterials J M Capus and R M German Eds vol 7 ofNovel Powder Processing pp 155ndash168 Metal Powder IndustriesFederation Princeton NJ USA 1992

[11] M P Phaniraj D-I Kim J-H Shim and Y W CholdquoMicrostructure development in mechanically alloyed yttriadispersed austenitic steelsrdquo Acta Materialia vol 57 no 6 pp1856ndash1864 2009

[12] A Hirata T Fujita Y R Wen J H Schneibel C T Liu and MW Chen ldquoAtomic structure of nanoclusters in oxide-dispersionstrengthened steelsrdquo Nature Materials vol 10 no 12 pp 922ndash926 2011

[13] M K Miller C M Parish and Q Li ldquoAdvanced oxide disper-sion strengthened and nanostructured ferritic alloysrdquoMaterialsScience and Technology (United Kingdom) vol 29 no 10 pp1174ndash1178 2013

[14] N Oono Q X Tang and S Ukai ldquoOxide particle refinementin Ni-based ODS alloyrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 649 pp 250ndash253 2016

[15] D B Witkin and E J Lavernia ldquoSynthesis and mechanicalbehavior of nanostructured materials via cryomillingrdquo Progressin Materials Science vol 51 no 1 pp 1ndash60 2006

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

Research 11

Depending on the milling conditions two entirely differ-ent reaction kinetics are possible [21 54]

(a) the reactionmay extend to a very small volumeduringeach collision resulting in a gradual transformationor

(b) if the reaction enthalpy is sufficiently high (and ifthe adiabatic temperature is above 1800 K) a self-propagating high-temperature synthesis (SHS) reaction(also known as combustion synthesis) can be initi-ated by controlling the milling conditions (eg byadding diluents) to avoid combustion reactions maybe made to proceed in a steady-state manner

The latter type of reactions requires a critical milling time forthe combustion reaction to be initiated If the temperature ofthe vial is recorded during the milling process the tempera-ture initially increases slowly with time After some time thetemperature increases abruptly suggesting that ignition hasoccurred and this is followed by a relatively slow decrease

In the above types of reactions themilled powder consistsof the pure metalM dispersed in the RO phase both usuallyof nanometer dimensions It has been shown that by anintelligent choice of the reductant the RO phase can beeasily leached out Thus by selective removal of the matrixphase by washing with appropriate solvents it is possibleto achieve well-dispersed metal nanoparticles as small as 5nm in size If the crystallinity of these particles is not highit can be improved by subsequent annealing without thefear of agglomeration due to the enclosure of nanoparticlesin a solid matrix Such nanoparticles are structurally andmorphologically uniform having a narrow size distributionThis technique has been used to produce powders of manydifferent metalsmdashAg Cd Co Cr Cu Fe Gd Nb NiTi V W Zn and Zr Several alloys intermetallics andnanocomposites have also been synthesized Figure 7 showstwo micrographs of nanocrystalline ceria particles producedby the conventional vapor synthesis and MA methods Eventhough the grain size is approximately the same in both thecases the nanoparticles produced by MA are very discreteand display a very narrow particle size distribution whilethose produced by vapor deposition are agglomerated andhave a wide particle size distribution

Novel materials have also been synthesized by MA ofblended elemental powders One of the interesting examplesin this category is the synthesis of plain carbon steels andstainless steels [55 56] Twomain advantages of this synthesisroute are that the steels can be manufactured at roomtemperature and that the steels will be nanostructured thusconferring improved properties But these powders needto be consolidated to full density for applications and itis possible that grain growth occurs during this processThe kinetics of grain growth can however be controlled byregulating the time and temperature combinations

72 Hydrogen Storage Materials Another important applica-tion of MA powders is hydrogen storage Mg is a lightweightmetal (the lightest structural material) with a density of174 gcm3 Mg and Mg-based alloys can store a significant

20 nm

50 nm

Figure 7 Transmission electron micrographs of ceria particlesproduced byMA (top) and vapor synthesis (bottom)methods Notethe severe agglomeration of the nanoparticles in the latter case

amount of hydrogen However being a reactive metal Mgalways contains a thin layer of oxide on the surface Thissurface oxide layer which inhibits asorption of hydrogenneeds to be broken down through activation A high-temperature annealing is usually done to achieve this ButMA can overcome these difficulties due to the severe plasticdeformation experienced by the powder particles duringMAA catalyst is also often required to accelerate the kinetics ofhydrogen and dehydrogenation of alloys

Mg can adsorb about 76 wt hydrogen andMg-Ni alloysmuch less However Mg is hard to activate its tempera-ture of operation is high and the sorption and desorptionkinetics are slow Therefore Mg-Ni and other alloys arebeing explored But due to the high vapor pressure of Mgand significant difference in the melting temperatures ofthe constituent elements (Mg = 650∘C and Ni = 1452∘C) itis difficult to prepare these alloys by conventional meltingand solidification processes Therefore MA has been used toconveniently synthesize these alloys since this is a completelysolid-state processing method An added advantage of MA isthat the product will have a nanocrystalline grain structureallowing the improvements described below [57 58]

Figure 8 shows the hydrogenation and dehydrogena-tion plots for conventional and nanocrystalline alloysAn additional plot for a situation where a catalyst isadded to the nanocrystalline material is also shown Fromthese two graphs the following facts become clear Firstlythe nanocrystalline alloys adsorb hydrogen much fasterthan conventional (coarse-grained) alloys even though the

12 Research

0 1 2 3 4 5 6 7 8 9 10time (min)

conventional

p = 8 bar

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

time (min)

nano + catalyst

nano + catalyst

nanocrystallinenanocrystalline

conventional

breakthrough in reaction kinetics due to nanostructure and use of catalyst

0

1

2

3

4

5

6

7

8

hydr

ogen

cont

ent (

wt

)

T = 300∘C p = 10-3 mbar

Figure 8 Comparison of the hydrogen sorption and desorption kinetics of Mg alloys with conventional and nanocrystalline grain sizesSignificant improvement in the desorption kinetics when a catalyst is added is worth noticing

amount of hydrogen adsorbed is the sameWhile the conven-tional alloys do not adsorb any significant amount of hydro-gen in about 10 min the allowable full amount of hydrogenis adsorbed in the nanocrystalline alloy Similar observationsare made by many other authors in many different alloysFurther the kinetics are slightly faster (not much different)when a catalyst is added to the nanocrystalline alloy Onthe other hand the kinetics appear to be very significantlyaffected during the desorption process For example thenanocrystalline alloys desorb hydrogen much faster thanthe conventional alloys Again while the conventional alloysdo not desorb any significant amount of hydrogen thenanocrystalline alloy desorbs over 20 of the hydrogenstored in about 10 min But the most significant effect wasobserved when a catalyst was added to the nanocrystallinealloyAll the adsorbedhydrogenwas desorbed in about 2minwhen a catalyst was added to the nanocrystalline alloy Thusnanostructure processing throughMAappears to be a fruitfulway of developing novel hydrogen storage materials [59]

Many novel complex hydrides based on light metals(Mg Li and Na) are being explored and the importantcategories of materials being investigated include the alanates(also known as aluminohydrides a family of compoundsconsisting of hydrogen and aluminum) borohydrides amideborohydrides amide-imide systems (which have a highhydrogen storage capacity and low operative temperatures)amine borane (compounds of borane groups having ammo-nia addition) and alane

73 Energy Materials The technique of MA has also beenemployed to synthesize materials for energy applicationsThese include synthesis of photovoltaic (PV) materials andultrafine particles for faster ignition and combustion pur-poses

One of the most common PV materials is CIGS acompound of Cu In Ga and Se with the compositionCu(InGa)Se2 Since PV cells have the highest efficiencywhen

the band gap for this semiconductor is 125 eV one will haveto engineer the composition for the ratio of Ga to In This isbecause the band gaps for CuInSe

2 and CuGaSe2 are 102 and170 eV respectively Accordingly a Ga(Ga+In) ratio of about03 is found most appropriate and the final ideal compositionof the PV material will be Cu097In07Ga03Se2

The traditional method of synthesizing this material isto prepare an alloy of Cu Ga and In by vapor depositionmethods and incorporate Se into this by exposure to Seatmosphere Consequently this is a complex process MAhowever can be a simple direct and efficient method ofmanufacturing Blended elemental powders of Cu In Gaand Se weremilled in a copper container for different periodsof time and the phase constitution was monitored throughX-ray diffraction studies It was noted that the tetragonalCu(InGa)Se

2phase has formed even aftermilling the powder

mix for 20 min Process variables such as mill rotation speedball-to-powder weight ratio (BPR) and milling time werevaried and it was noted that the ideal composition wasachieved at lowmilling speeds short milling times and a lowBPR (Table 2) [60] By fine-tuning these parameters it shouldbe possible to achieve the exact composition

Nanostructuredmetal particles have high specific surfaceareas and consequently they are highly reactive and can storeenergy This area has received a great impetus in recentyears since the combustion rates of nanostructured metalparticles aremuch higher than those of coarsemetal particlesThe particle size cannot be exceedingly small because thesurface may be coated with a thin oxide layer and make theparticles passive [61ndash63] MA is very well suited to preparesuch materials because the reactions are easily initiatedby impact or friction and at the same time milling isa simple scalable and controllable technology capable ofmixing reactive components on the nanoscaleThese reactivematerials could be used as additives to propellants explosivesand pyrotechnics as well asmanufacture of reactive structuralcomponents [63]

Research 13

Table 2 Chemical analysis (at) of the Cu-In-Ga-Se powder mechanically alloyed under different processing conditions

BPR Speed (rpm) Time (min) Cu In Ga Se201 300 120 3261 1523 653 4562201 300 240 3173 1563 671 4593101 150 40 2598 1698 725 4978Targeted (Cu

097In07Ga03Se2) 2443 1763 756 5038

Compared to the presently used liquid aviation fuelsmetal particles based on B Al andMg are effective HoweverB and Al have very high boiling points and high heats ofenthalpy On the other hand Mg has a low boiling point butalso low heat of enthalpy Therefore if Al and Mg could bemixed intimately on a nanometer scale one could hope tohave a high heat of enthalpy at reduced boiling temperaturesThat is Mg-Al alloy particles could be practical in achievinga high-energy density and practical ignitionboiling temper-atures MA is a very effective method to accomplish this

Al and Mg powders were blended together in differentproportions and milled Since the milled powders have arange of particle sizes they were sieved to obtain well-defined powder sizes They were then fluidized and thencombusted in a CH4-air premixed flame Both the ignitionand burning times of these metal particles decreased withdecreased particle sizes and were also shorter when the Mgcontent is higher (Figure 9) [64] Similar results have beenreported by other investigators

8 Powder Consolidation

The product of MA is in powder form Except in cases ofchemical applications such as catalysis when the productdoes not require consolidation widespread application ofMA powders requires efficient methods of consolidatingthem into bulk shapes Successful consolidation of MA pow-ders is a nontrivial problem since fully densematerials shouldbe produced while simultaneously retaining the nanometer-sized grains without coarsening or the amorphous phaseswithout crystallizing Conventional consolidation of powdersto full density through processes such as hot extrusion [65]and hot isostatic pressing (HIP) [66] requires use of highpressures and exposure to elevated temperatures for extendedperiods of time [67] Unfortunately however this resultsin significant coarsening of the nanometer-sized grains orcrystallization of amorphous phases and consequently thebenefits of nanostructure processing or amorphization arelost [68] On the other hand retention of finemicrostructuresrequires use of low consolidation temperatures and then itis difficult to achieve full interparticle bonding at these lowtemperatures Therefore novel and innovative methods ofconsolidating MA powders are required The objectives ofconsolidation of these powders are to (a) achieve full densifi-cation (without any porosity) (b) minimize microstructuralcoarsening (ie retention of fine grain size) andor (c)avoid undesirable phase transformations In fact the earlyresults of excellent mechanical properties and most specifi-cally superplasticity at room temperature [69] attributed to

20 Mg (tig)90 Mg (tig)

20 Mg (tb)90 Mg (tb)

20 40 60 80 100 1200Particle Size (m)

0

10

20

30

40

50

60

70

Igni

tion

Burn

ing

Tim

e (m

s)

Figure 9 Ignition and burning times as a function of particle sizefor the mechanically alloyed Al-20 and Al-90 at Mg powdersIn the insert tig and tb represent the ignition and burning timesrespectively

nanocrystalline ceramics have not been successfully repro-duced due to incomplete consolidation of the nanopowdersin the material tested Thus consolidation to full densityassumes even greater importance

Because of the small size of the powder particles (typicallya few micrometers even though the grain size is only afew nanometers) MA powders pose special problems Forexample such nanometric powders are very hard and strongThey also have a high level of interparticle friction Furthersince these powders have a large surface area they also exhibithigh chemical reactivity Therefore special precautions needto be taken to consolidate the metal powders to bulk shapes

Successful consolidation of MA powders has beenachieved by electrodischarge compaction plasma-activatedsintering shock (explosive) consolidation [70] HIP hydro-static extrusion strained powder rolling and sinter forging[71] By utilizing the combination of high temperature andpressure HIP can achieve a particular density at lowerpressure when compared to cold isostatic pressing or at lowertemperature when compared to sintering Because of theincreased diffusivity in nanocrystalline materials sintering(and therefore densification) takes place at temperaturesmuch lower than those in coarse-grained materials This islikely to reduce the grain growth Spark plasma sintering(SPS) has become the most popular technique to consolidatemetal powders especially those with ultrafine grain sizesdue to the fact that full density can be achieved at low

14 Research

SOLID LIQUID GAS

MILLING

USE POWDER AS-ISor Chemical Reaction

Thermal Treatment etcCONSOLIDATE TO SOLID

(Temperature Pressure)

Catalysts Pigments SolderComponents Ceramic Powder etc

Advanced materials with hightolerance to gaseous impurities

Figure 10 Alternatives to consolidation of metal powders Ifpowders are used as catalysts pigments solder components etcthen they need not be consolidated However if powders need tobe consolidated then they could be used in applications with hightolerance to gaseous impurities

temperatures and shorter times in comparison to severalothermethods [72 73] An excellent reviewbyGroza [71]maybe consulted for full details of the methods of consolidationand description of results

9 Powder Contamination

Amajor concern inmetal powders processed byMAmethodsis the nature and amount of impurities that get incorporatedinto the powder and contaminate it The small size of thepowder particles and consequent availability of large surfacearea formation of fresh surfaces during milling and wearand tear of the milling tools and the atmosphere underwhich the powder is milled all contribute to contaminationof the powder In addition to these factors one has toconsider the purity of the starting raw materials and themilling conditions employed (especially the milling atmo-sphere) Thus it appears as though powder contaminationis an inherent drawback of the method and that the milledpowder will always be contaminated with impurities unlessspecial precautions are taken to avoidminimize them Themagnitude of powder contamination appears to depend onthe time of milling intensity of milling atmosphere in whichthe powder is milled nature and size of the milling mediumand differences in the strengthhardness of the powder andthe milling medium and the container

Several attempts have been made in recent years to min-imize powder contamination during milling An importantpoint to remember is that if the starting powders are highlypure then the final product is going to be clean and purewith minimum contamination from other sources providedthat proper precautions are taken during milling It is alsoimportant to keep inmind that all applications do not requireultraclean powders and that the purity desired will depend onthe specific application (Figure 10) But it is a good practiceto minimize powder contamination at every stage and ensurethat additional impurities are not picked up subsequentlyduring handling andor consolidation

One way ofminimizing contamination from the grindingmedium and themilling container is to use the samematerial

for the container and the grinding medium as the powderbeing milled Thus one could use copper balls and coppercontainer formilling copper and copper alloy powders [60] Ifa container of the same material to be milled is not availablethen a thin adherent coating on the internal surface of thecontainer (and also on the surface of the grinding medium)with the material to be milled will minimize contaminationThe idea here is to mill the powder once allowing the powderto be coated onto the grindingmedium and the inner walls ofthe container The milled loose powder is then discarded anda fresh batch of powder is milled but with the old grindingmedia and container In general a simple rule that shouldbe followed to minimize contamination from the millingcontainer and the grinding medium is that the container andgrindingmedium should be harderstronger than the powderbeing milled

Milling atmosphere is perhaps the most important vari-able that needs to be controlled Even though nominally puregases are used to purge the glove box before loading andunloading the powders even small amounts of impurities inthe gases appear to contaminate the powders especially whenthey are reactive The best solution one could think of willbe to place the mill inside a chamber that is evacuated andfilled with high-purity argon gas Since the whole chamberis maintained under argon gas atmosphere (continuouslypurified to keep oxygen and water vapor below 1 ppm each)and the container is inside the mill which is inside thechamber contamination from the atmosphere is minimum

Contamination from PCAs is perhaps the most ubiqui-tous Since most of the PCAs used are organic compoundswhich have low melting and boiling points they decomposeduring milling due to the heat generated The decomposi-tion products consisting of carbon oxygen nitrogen andhydrogen react with the metal atoms and form undesirablecarbides oxides nitrides etc

10 Concluding Remarks and Future Prospects

We have only briefly touched upon the scientific and tech-nological aspects of MA materials along with some of theconcerns such as powder contamination and consolidationissues The historical development of MA during the last 50years or so can be divided into threemajor periods (Figure 11)The first period covering the first twenty years (from 1966up to about 1985) was mostly concerned with the devel-opment and production of oxide-dispersion-strengthened(ODS) superalloys for applications in the aerospace indus-try Several alloys with improved properties based onNi and Fe were developed and found useful applicationsThese included the MA754 MA760 MA956 MA957 andMA6000 [74] A more recent survey of application of MAto steel is also available [75] The second period of aboutfifteen years covering from about 1986 up to about 2000witnessed advances in the fundamental understanding ofthe processes that take place during MA Along with thisa large number of dedicated conferences were held andthere was a burgeoning of publication activity Comparisonshave also been frequently made between MA and othernonequilibriumprocessing techniquesmore specifically RSP

Research 15

1965 19851975 1995 2005

Production of ODS Alloys

Novel Phase SynthesisDevelopment of BasicUnderstanding of MA

Nanostructures andNew Applications

2015

Figure 11 Schematic showing the different periods of development in the field of mechanical alloying (MA) and applications ofMA productssince its inception in 1965

It was shown that themetastable effects achieved by these twononequilibrium processing techniques are similar Revival ofthe mechanochemical processing (MCP) took place and avariety of novel substances were synthesized Several mod-eling studies were also conducted to enable prediction of thephases produced or microstructures obtained although withlimited success The third period starting from about 2001saw a reemergence of the quest for new applications of theMA materials not only as structural materials but also forchemical applications such as catalysis and hydrogen storagewith the realization that contamination of themilled powdersis the limiting factor in the widespread applications of theMA materials Innovative techniques to consolidate the MApowders to full density while retaining the metastable phases(including glassy phases andor nanostructures) in themwerealso developed All these are continuing with the ongoinginvestigations to enhance the scientific understanding of theMA process

At present activities relating to both the science andtechnology of MA have been going on simultaneously Whilesome groups have been focusing on developing a betterunderstanding of the process and optimizing the processvariables to achieve a certain phase or phase combinationin the alloy system others have been concentrating onexploiting this technique for different applications It hasalso come to be realized that the MA process can be usedto recycle used and discarded materials to produce value-added materials for subsequent consumption They can bepulverized separated and used to mix with other powdersto produce composites The bottom line is that newer andimproved materials are being continuously developed fornovel applications

Conflicts of Interest

The author declares that he has no conflicts of interest

References

[1] K H J Buschow R W Cahn M C Flemings B Ilschner E JKramer and SMahajan EdsEncyclopedia ofMaterials Scienceand Technology Elsevier 2001

[2] C Suryanarayana EdNon-equilibrium Processing of MaterialsPergamon Oxford UK 1999

[3] C Suryanarayana ldquoMetallic glassesrdquo Bulletin of Materials Sci-ence vol 6 no 3 pp 579ndash594 1984

[4] C Suryanarayana and A Inoue Bulk Metallic Glasses CRCPress Boca Raton Fla USA 2nd edition 2018

[5] H R Trebin Ed Quasicrystals Structure and Physical Proper-ties Wiley-VCH Weinheim Germany 2003

[6] C Suryanarayana and H Jones ldquoFormation and characteristicsof quasicrystalline phases A reviewrdquo International Journal ofRapid Solidification vol 3 pp 253ndash293 1988

[7] H Gleiter ldquoNanocrystalline materialsrdquo Progress in MaterialsScience vol 33 no 4 pp 223ndash315 1989

[8] C Suryanarayana ldquoRecent developments in nanostructuredmaterialsrdquo Advanced Engineering Materials vol 7 no 11 pp983ndash992 2005

[9] M C Gao J-W Yeh P K Liaw and Y Zhang Eds High-Entropy Alloys Fundamentals and Applications Springer 2016

[10] J S Benjamin ldquoMechanical alloying ndash History and futurepotentialrdquo in Advances in Powder Metallurgy and ParticulateMaterials J M Capus and R M German Eds vol 7 ofNovel Powder Processing pp 155ndash168 Metal Powder IndustriesFederation Princeton NJ USA 1992

[11] M P Phaniraj D-I Kim J-H Shim and Y W CholdquoMicrostructure development in mechanically alloyed yttriadispersed austenitic steelsrdquo Acta Materialia vol 57 no 6 pp1856ndash1864 2009

[12] A Hirata T Fujita Y R Wen J H Schneibel C T Liu and MW Chen ldquoAtomic structure of nanoclusters in oxide-dispersionstrengthened steelsrdquo Nature Materials vol 10 no 12 pp 922ndash926 2011

[13] M K Miller C M Parish and Q Li ldquoAdvanced oxide disper-sion strengthened and nanostructured ferritic alloysrdquoMaterialsScience and Technology (United Kingdom) vol 29 no 10 pp1174ndash1178 2013

[14] N Oono Q X Tang and S Ukai ldquoOxide particle refinementin Ni-based ODS alloyrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 649 pp 250ndash253 2016

[15] D B Witkin and E J Lavernia ldquoSynthesis and mechanicalbehavior of nanostructured materials via cryomillingrdquo Progressin Materials Science vol 51 no 1 pp 1ndash60 2006

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

12 Research

0 1 2 3 4 5 6 7 8 9 10time (min)

conventional

p = 8 bar

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

7

8

time (min)

nano + catalyst

nano + catalyst

nanocrystallinenanocrystalline

conventional

breakthrough in reaction kinetics due to nanostructure and use of catalyst

0

1

2

3

4

5

6

7

8

hydr

ogen

cont

ent (

wt

)

T = 300∘C p = 10-3 mbar

Figure 8 Comparison of the hydrogen sorption and desorption kinetics of Mg alloys with conventional and nanocrystalline grain sizesSignificant improvement in the desorption kinetics when a catalyst is added is worth noticing

amount of hydrogen adsorbed is the sameWhile the conven-tional alloys do not adsorb any significant amount of hydro-gen in about 10 min the allowable full amount of hydrogenis adsorbed in the nanocrystalline alloy Similar observationsare made by many other authors in many different alloysFurther the kinetics are slightly faster (not much different)when a catalyst is added to the nanocrystalline alloy Onthe other hand the kinetics appear to be very significantlyaffected during the desorption process For example thenanocrystalline alloys desorb hydrogen much faster thanthe conventional alloys Again while the conventional alloysdo not desorb any significant amount of hydrogen thenanocrystalline alloy desorbs over 20 of the hydrogenstored in about 10 min But the most significant effect wasobserved when a catalyst was added to the nanocrystallinealloyAll the adsorbedhydrogenwas desorbed in about 2minwhen a catalyst was added to the nanocrystalline alloy Thusnanostructure processing throughMAappears to be a fruitfulway of developing novel hydrogen storage materials [59]

Many novel complex hydrides based on light metals(Mg Li and Na) are being explored and the importantcategories of materials being investigated include the alanates(also known as aluminohydrides a family of compoundsconsisting of hydrogen and aluminum) borohydrides amideborohydrides amide-imide systems (which have a highhydrogen storage capacity and low operative temperatures)amine borane (compounds of borane groups having ammo-nia addition) and alane

73 Energy Materials The technique of MA has also beenemployed to synthesize materials for energy applicationsThese include synthesis of photovoltaic (PV) materials andultrafine particles for faster ignition and combustion pur-poses

One of the most common PV materials is CIGS acompound of Cu In Ga and Se with the compositionCu(InGa)Se2 Since PV cells have the highest efficiencywhen

the band gap for this semiconductor is 125 eV one will haveto engineer the composition for the ratio of Ga to In This isbecause the band gaps for CuInSe

2 and CuGaSe2 are 102 and170 eV respectively Accordingly a Ga(Ga+In) ratio of about03 is found most appropriate and the final ideal compositionof the PV material will be Cu097In07Ga03Se2

The traditional method of synthesizing this material isto prepare an alloy of Cu Ga and In by vapor depositionmethods and incorporate Se into this by exposure to Seatmosphere Consequently this is a complex process MAhowever can be a simple direct and efficient method ofmanufacturing Blended elemental powders of Cu In Gaand Se weremilled in a copper container for different periodsof time and the phase constitution was monitored throughX-ray diffraction studies It was noted that the tetragonalCu(InGa)Se

2phase has formed even aftermilling the powder

mix for 20 min Process variables such as mill rotation speedball-to-powder weight ratio (BPR) and milling time werevaried and it was noted that the ideal composition wasachieved at lowmilling speeds short milling times and a lowBPR (Table 2) [60] By fine-tuning these parameters it shouldbe possible to achieve the exact composition

Nanostructuredmetal particles have high specific surfaceareas and consequently they are highly reactive and can storeenergy This area has received a great impetus in recentyears since the combustion rates of nanostructured metalparticles aremuch higher than those of coarsemetal particlesThe particle size cannot be exceedingly small because thesurface may be coated with a thin oxide layer and make theparticles passive [61ndash63] MA is very well suited to preparesuch materials because the reactions are easily initiatedby impact or friction and at the same time milling isa simple scalable and controllable technology capable ofmixing reactive components on the nanoscaleThese reactivematerials could be used as additives to propellants explosivesand pyrotechnics as well asmanufacture of reactive structuralcomponents [63]

Research 13

Table 2 Chemical analysis (at) of the Cu-In-Ga-Se powder mechanically alloyed under different processing conditions

BPR Speed (rpm) Time (min) Cu In Ga Se201 300 120 3261 1523 653 4562201 300 240 3173 1563 671 4593101 150 40 2598 1698 725 4978Targeted (Cu

097In07Ga03Se2) 2443 1763 756 5038

Compared to the presently used liquid aviation fuelsmetal particles based on B Al andMg are effective HoweverB and Al have very high boiling points and high heats ofenthalpy On the other hand Mg has a low boiling point butalso low heat of enthalpy Therefore if Al and Mg could bemixed intimately on a nanometer scale one could hope tohave a high heat of enthalpy at reduced boiling temperaturesThat is Mg-Al alloy particles could be practical in achievinga high-energy density and practical ignitionboiling temper-atures MA is a very effective method to accomplish this

Al and Mg powders were blended together in differentproportions and milled Since the milled powders have arange of particle sizes they were sieved to obtain well-defined powder sizes They were then fluidized and thencombusted in a CH4-air premixed flame Both the ignitionand burning times of these metal particles decreased withdecreased particle sizes and were also shorter when the Mgcontent is higher (Figure 9) [64] Similar results have beenreported by other investigators

8 Powder Consolidation

The product of MA is in powder form Except in cases ofchemical applications such as catalysis when the productdoes not require consolidation widespread application ofMA powders requires efficient methods of consolidatingthem into bulk shapes Successful consolidation of MA pow-ders is a nontrivial problem since fully densematerials shouldbe produced while simultaneously retaining the nanometer-sized grains without coarsening or the amorphous phaseswithout crystallizing Conventional consolidation of powdersto full density through processes such as hot extrusion [65]and hot isostatic pressing (HIP) [66] requires use of highpressures and exposure to elevated temperatures for extendedperiods of time [67] Unfortunately however this resultsin significant coarsening of the nanometer-sized grains orcrystallization of amorphous phases and consequently thebenefits of nanostructure processing or amorphization arelost [68] On the other hand retention of finemicrostructuresrequires use of low consolidation temperatures and then itis difficult to achieve full interparticle bonding at these lowtemperatures Therefore novel and innovative methods ofconsolidating MA powders are required The objectives ofconsolidation of these powders are to (a) achieve full densifi-cation (without any porosity) (b) minimize microstructuralcoarsening (ie retention of fine grain size) andor (c)avoid undesirable phase transformations In fact the earlyresults of excellent mechanical properties and most specifi-cally superplasticity at room temperature [69] attributed to

20 Mg (tig)90 Mg (tig)

20 Mg (tb)90 Mg (tb)

20 40 60 80 100 1200Particle Size (m)

0

10

20

30

40

50

60

70

Igni

tion

Burn

ing

Tim

e (m

s)

Figure 9 Ignition and burning times as a function of particle sizefor the mechanically alloyed Al-20 and Al-90 at Mg powdersIn the insert tig and tb represent the ignition and burning timesrespectively

nanocrystalline ceramics have not been successfully repro-duced due to incomplete consolidation of the nanopowdersin the material tested Thus consolidation to full densityassumes even greater importance

Because of the small size of the powder particles (typicallya few micrometers even though the grain size is only afew nanometers) MA powders pose special problems Forexample such nanometric powders are very hard and strongThey also have a high level of interparticle friction Furthersince these powders have a large surface area they also exhibithigh chemical reactivity Therefore special precautions needto be taken to consolidate the metal powders to bulk shapes

Successful consolidation of MA powders has beenachieved by electrodischarge compaction plasma-activatedsintering shock (explosive) consolidation [70] HIP hydro-static extrusion strained powder rolling and sinter forging[71] By utilizing the combination of high temperature andpressure HIP can achieve a particular density at lowerpressure when compared to cold isostatic pressing or at lowertemperature when compared to sintering Because of theincreased diffusivity in nanocrystalline materials sintering(and therefore densification) takes place at temperaturesmuch lower than those in coarse-grained materials This islikely to reduce the grain growth Spark plasma sintering(SPS) has become the most popular technique to consolidatemetal powders especially those with ultrafine grain sizesdue to the fact that full density can be achieved at low

14 Research

SOLID LIQUID GAS

MILLING

USE POWDER AS-ISor Chemical Reaction

Thermal Treatment etcCONSOLIDATE TO SOLID

(Temperature Pressure)

Catalysts Pigments SolderComponents Ceramic Powder etc

Advanced materials with hightolerance to gaseous impurities

Figure 10 Alternatives to consolidation of metal powders Ifpowders are used as catalysts pigments solder components etcthen they need not be consolidated However if powders need tobe consolidated then they could be used in applications with hightolerance to gaseous impurities

temperatures and shorter times in comparison to severalothermethods [72 73] An excellent reviewbyGroza [71]maybe consulted for full details of the methods of consolidationand description of results

9 Powder Contamination

Amajor concern inmetal powders processed byMAmethodsis the nature and amount of impurities that get incorporatedinto the powder and contaminate it The small size of thepowder particles and consequent availability of large surfacearea formation of fresh surfaces during milling and wearand tear of the milling tools and the atmosphere underwhich the powder is milled all contribute to contaminationof the powder In addition to these factors one has toconsider the purity of the starting raw materials and themilling conditions employed (especially the milling atmo-sphere) Thus it appears as though powder contaminationis an inherent drawback of the method and that the milledpowder will always be contaminated with impurities unlessspecial precautions are taken to avoidminimize them Themagnitude of powder contamination appears to depend onthe time of milling intensity of milling atmosphere in whichthe powder is milled nature and size of the milling mediumand differences in the strengthhardness of the powder andthe milling medium and the container

Several attempts have been made in recent years to min-imize powder contamination during milling An importantpoint to remember is that if the starting powders are highlypure then the final product is going to be clean and purewith minimum contamination from other sources providedthat proper precautions are taken during milling It is alsoimportant to keep inmind that all applications do not requireultraclean powders and that the purity desired will depend onthe specific application (Figure 10) But it is a good practiceto minimize powder contamination at every stage and ensurethat additional impurities are not picked up subsequentlyduring handling andor consolidation

One way ofminimizing contamination from the grindingmedium and themilling container is to use the samematerial

for the container and the grinding medium as the powderbeing milled Thus one could use copper balls and coppercontainer formilling copper and copper alloy powders [60] Ifa container of the same material to be milled is not availablethen a thin adherent coating on the internal surface of thecontainer (and also on the surface of the grinding medium)with the material to be milled will minimize contaminationThe idea here is to mill the powder once allowing the powderto be coated onto the grindingmedium and the inner walls ofthe container The milled loose powder is then discarded anda fresh batch of powder is milled but with the old grindingmedia and container In general a simple rule that shouldbe followed to minimize contamination from the millingcontainer and the grinding medium is that the container andgrindingmedium should be harderstronger than the powderbeing milled

Milling atmosphere is perhaps the most important vari-able that needs to be controlled Even though nominally puregases are used to purge the glove box before loading andunloading the powders even small amounts of impurities inthe gases appear to contaminate the powders especially whenthey are reactive The best solution one could think of willbe to place the mill inside a chamber that is evacuated andfilled with high-purity argon gas Since the whole chamberis maintained under argon gas atmosphere (continuouslypurified to keep oxygen and water vapor below 1 ppm each)and the container is inside the mill which is inside thechamber contamination from the atmosphere is minimum

Contamination from PCAs is perhaps the most ubiqui-tous Since most of the PCAs used are organic compoundswhich have low melting and boiling points they decomposeduring milling due to the heat generated The decomposi-tion products consisting of carbon oxygen nitrogen andhydrogen react with the metal atoms and form undesirablecarbides oxides nitrides etc

10 Concluding Remarks and Future Prospects

We have only briefly touched upon the scientific and tech-nological aspects of MA materials along with some of theconcerns such as powder contamination and consolidationissues The historical development of MA during the last 50years or so can be divided into threemajor periods (Figure 11)The first period covering the first twenty years (from 1966up to about 1985) was mostly concerned with the devel-opment and production of oxide-dispersion-strengthened(ODS) superalloys for applications in the aerospace indus-try Several alloys with improved properties based onNi and Fe were developed and found useful applicationsThese included the MA754 MA760 MA956 MA957 andMA6000 [74] A more recent survey of application of MAto steel is also available [75] The second period of aboutfifteen years covering from about 1986 up to about 2000witnessed advances in the fundamental understanding ofthe processes that take place during MA Along with thisa large number of dedicated conferences were held andthere was a burgeoning of publication activity Comparisonshave also been frequently made between MA and othernonequilibriumprocessing techniquesmore specifically RSP

Research 15

1965 19851975 1995 2005

Production of ODS Alloys

Novel Phase SynthesisDevelopment of BasicUnderstanding of MA

Nanostructures andNew Applications

2015

Figure 11 Schematic showing the different periods of development in the field of mechanical alloying (MA) and applications ofMA productssince its inception in 1965

It was shown that themetastable effects achieved by these twononequilibrium processing techniques are similar Revival ofthe mechanochemical processing (MCP) took place and avariety of novel substances were synthesized Several mod-eling studies were also conducted to enable prediction of thephases produced or microstructures obtained although withlimited success The third period starting from about 2001saw a reemergence of the quest for new applications of theMA materials not only as structural materials but also forchemical applications such as catalysis and hydrogen storagewith the realization that contamination of themilled powdersis the limiting factor in the widespread applications of theMA materials Innovative techniques to consolidate the MApowders to full density while retaining the metastable phases(including glassy phases andor nanostructures) in themwerealso developed All these are continuing with the ongoinginvestigations to enhance the scientific understanding of theMA process

At present activities relating to both the science andtechnology of MA have been going on simultaneously Whilesome groups have been focusing on developing a betterunderstanding of the process and optimizing the processvariables to achieve a certain phase or phase combinationin the alloy system others have been concentrating onexploiting this technique for different applications It hasalso come to be realized that the MA process can be usedto recycle used and discarded materials to produce value-added materials for subsequent consumption They can bepulverized separated and used to mix with other powdersto produce composites The bottom line is that newer andimproved materials are being continuously developed fornovel applications

Conflicts of Interest

The author declares that he has no conflicts of interest

References

[1] K H J Buschow R W Cahn M C Flemings B Ilschner E JKramer and SMahajan EdsEncyclopedia ofMaterials Scienceand Technology Elsevier 2001

[2] C Suryanarayana EdNon-equilibrium Processing of MaterialsPergamon Oxford UK 1999

[3] C Suryanarayana ldquoMetallic glassesrdquo Bulletin of Materials Sci-ence vol 6 no 3 pp 579ndash594 1984

[4] C Suryanarayana and A Inoue Bulk Metallic Glasses CRCPress Boca Raton Fla USA 2nd edition 2018

[5] H R Trebin Ed Quasicrystals Structure and Physical Proper-ties Wiley-VCH Weinheim Germany 2003

[6] C Suryanarayana and H Jones ldquoFormation and characteristicsof quasicrystalline phases A reviewrdquo International Journal ofRapid Solidification vol 3 pp 253ndash293 1988

[7] H Gleiter ldquoNanocrystalline materialsrdquo Progress in MaterialsScience vol 33 no 4 pp 223ndash315 1989

[8] C Suryanarayana ldquoRecent developments in nanostructuredmaterialsrdquo Advanced Engineering Materials vol 7 no 11 pp983ndash992 2005

[9] M C Gao J-W Yeh P K Liaw and Y Zhang Eds High-Entropy Alloys Fundamentals and Applications Springer 2016

[10] J S Benjamin ldquoMechanical alloying ndash History and futurepotentialrdquo in Advances in Powder Metallurgy and ParticulateMaterials J M Capus and R M German Eds vol 7 ofNovel Powder Processing pp 155ndash168 Metal Powder IndustriesFederation Princeton NJ USA 1992

[11] M P Phaniraj D-I Kim J-H Shim and Y W CholdquoMicrostructure development in mechanically alloyed yttriadispersed austenitic steelsrdquo Acta Materialia vol 57 no 6 pp1856ndash1864 2009

[12] A Hirata T Fujita Y R Wen J H Schneibel C T Liu and MW Chen ldquoAtomic structure of nanoclusters in oxide-dispersionstrengthened steelsrdquo Nature Materials vol 10 no 12 pp 922ndash926 2011

[13] M K Miller C M Parish and Q Li ldquoAdvanced oxide disper-sion strengthened and nanostructured ferritic alloysrdquoMaterialsScience and Technology (United Kingdom) vol 29 no 10 pp1174ndash1178 2013

[14] N Oono Q X Tang and S Ukai ldquoOxide particle refinementin Ni-based ODS alloyrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 649 pp 250ndash253 2016

[15] D B Witkin and E J Lavernia ldquoSynthesis and mechanicalbehavior of nanostructured materials via cryomillingrdquo Progressin Materials Science vol 51 no 1 pp 1ndash60 2006

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

Research 13

Table 2 Chemical analysis (at) of the Cu-In-Ga-Se powder mechanically alloyed under different processing conditions

BPR Speed (rpm) Time (min) Cu In Ga Se201 300 120 3261 1523 653 4562201 300 240 3173 1563 671 4593101 150 40 2598 1698 725 4978Targeted (Cu

097In07Ga03Se2) 2443 1763 756 5038

Compared to the presently used liquid aviation fuelsmetal particles based on B Al andMg are effective HoweverB and Al have very high boiling points and high heats ofenthalpy On the other hand Mg has a low boiling point butalso low heat of enthalpy Therefore if Al and Mg could bemixed intimately on a nanometer scale one could hope tohave a high heat of enthalpy at reduced boiling temperaturesThat is Mg-Al alloy particles could be practical in achievinga high-energy density and practical ignitionboiling temper-atures MA is a very effective method to accomplish this

Al and Mg powders were blended together in differentproportions and milled Since the milled powders have arange of particle sizes they were sieved to obtain well-defined powder sizes They were then fluidized and thencombusted in a CH4-air premixed flame Both the ignitionand burning times of these metal particles decreased withdecreased particle sizes and were also shorter when the Mgcontent is higher (Figure 9) [64] Similar results have beenreported by other investigators

8 Powder Consolidation

The product of MA is in powder form Except in cases ofchemical applications such as catalysis when the productdoes not require consolidation widespread application ofMA powders requires efficient methods of consolidatingthem into bulk shapes Successful consolidation of MA pow-ders is a nontrivial problem since fully densematerials shouldbe produced while simultaneously retaining the nanometer-sized grains without coarsening or the amorphous phaseswithout crystallizing Conventional consolidation of powdersto full density through processes such as hot extrusion [65]and hot isostatic pressing (HIP) [66] requires use of highpressures and exposure to elevated temperatures for extendedperiods of time [67] Unfortunately however this resultsin significant coarsening of the nanometer-sized grains orcrystallization of amorphous phases and consequently thebenefits of nanostructure processing or amorphization arelost [68] On the other hand retention of finemicrostructuresrequires use of low consolidation temperatures and then itis difficult to achieve full interparticle bonding at these lowtemperatures Therefore novel and innovative methods ofconsolidating MA powders are required The objectives ofconsolidation of these powders are to (a) achieve full densifi-cation (without any porosity) (b) minimize microstructuralcoarsening (ie retention of fine grain size) andor (c)avoid undesirable phase transformations In fact the earlyresults of excellent mechanical properties and most specifi-cally superplasticity at room temperature [69] attributed to

20 Mg (tig)90 Mg (tig)

20 Mg (tb)90 Mg (tb)

20 40 60 80 100 1200Particle Size (m)

0

10

20

30

40

50

60

70

Igni

tion

Burn

ing

Tim

e (m

s)

Figure 9 Ignition and burning times as a function of particle sizefor the mechanically alloyed Al-20 and Al-90 at Mg powdersIn the insert tig and tb represent the ignition and burning timesrespectively

nanocrystalline ceramics have not been successfully repro-duced due to incomplete consolidation of the nanopowdersin the material tested Thus consolidation to full densityassumes even greater importance

Because of the small size of the powder particles (typicallya few micrometers even though the grain size is only afew nanometers) MA powders pose special problems Forexample such nanometric powders are very hard and strongThey also have a high level of interparticle friction Furthersince these powders have a large surface area they also exhibithigh chemical reactivity Therefore special precautions needto be taken to consolidate the metal powders to bulk shapes

Successful consolidation of MA powders has beenachieved by electrodischarge compaction plasma-activatedsintering shock (explosive) consolidation [70] HIP hydro-static extrusion strained powder rolling and sinter forging[71] By utilizing the combination of high temperature andpressure HIP can achieve a particular density at lowerpressure when compared to cold isostatic pressing or at lowertemperature when compared to sintering Because of theincreased diffusivity in nanocrystalline materials sintering(and therefore densification) takes place at temperaturesmuch lower than those in coarse-grained materials This islikely to reduce the grain growth Spark plasma sintering(SPS) has become the most popular technique to consolidatemetal powders especially those with ultrafine grain sizesdue to the fact that full density can be achieved at low

14 Research

SOLID LIQUID GAS

MILLING

USE POWDER AS-ISor Chemical Reaction

Thermal Treatment etcCONSOLIDATE TO SOLID

(Temperature Pressure)

Catalysts Pigments SolderComponents Ceramic Powder etc

Advanced materials with hightolerance to gaseous impurities

Figure 10 Alternatives to consolidation of metal powders Ifpowders are used as catalysts pigments solder components etcthen they need not be consolidated However if powders need tobe consolidated then they could be used in applications with hightolerance to gaseous impurities

temperatures and shorter times in comparison to severalothermethods [72 73] An excellent reviewbyGroza [71]maybe consulted for full details of the methods of consolidationand description of results

9 Powder Contamination

Amajor concern inmetal powders processed byMAmethodsis the nature and amount of impurities that get incorporatedinto the powder and contaminate it The small size of thepowder particles and consequent availability of large surfacearea formation of fresh surfaces during milling and wearand tear of the milling tools and the atmosphere underwhich the powder is milled all contribute to contaminationof the powder In addition to these factors one has toconsider the purity of the starting raw materials and themilling conditions employed (especially the milling atmo-sphere) Thus it appears as though powder contaminationis an inherent drawback of the method and that the milledpowder will always be contaminated with impurities unlessspecial precautions are taken to avoidminimize them Themagnitude of powder contamination appears to depend onthe time of milling intensity of milling atmosphere in whichthe powder is milled nature and size of the milling mediumand differences in the strengthhardness of the powder andthe milling medium and the container

Several attempts have been made in recent years to min-imize powder contamination during milling An importantpoint to remember is that if the starting powders are highlypure then the final product is going to be clean and purewith minimum contamination from other sources providedthat proper precautions are taken during milling It is alsoimportant to keep inmind that all applications do not requireultraclean powders and that the purity desired will depend onthe specific application (Figure 10) But it is a good practiceto minimize powder contamination at every stage and ensurethat additional impurities are not picked up subsequentlyduring handling andor consolidation

One way ofminimizing contamination from the grindingmedium and themilling container is to use the samematerial

for the container and the grinding medium as the powderbeing milled Thus one could use copper balls and coppercontainer formilling copper and copper alloy powders [60] Ifa container of the same material to be milled is not availablethen a thin adherent coating on the internal surface of thecontainer (and also on the surface of the grinding medium)with the material to be milled will minimize contaminationThe idea here is to mill the powder once allowing the powderto be coated onto the grindingmedium and the inner walls ofthe container The milled loose powder is then discarded anda fresh batch of powder is milled but with the old grindingmedia and container In general a simple rule that shouldbe followed to minimize contamination from the millingcontainer and the grinding medium is that the container andgrindingmedium should be harderstronger than the powderbeing milled

Milling atmosphere is perhaps the most important vari-able that needs to be controlled Even though nominally puregases are used to purge the glove box before loading andunloading the powders even small amounts of impurities inthe gases appear to contaminate the powders especially whenthey are reactive The best solution one could think of willbe to place the mill inside a chamber that is evacuated andfilled with high-purity argon gas Since the whole chamberis maintained under argon gas atmosphere (continuouslypurified to keep oxygen and water vapor below 1 ppm each)and the container is inside the mill which is inside thechamber contamination from the atmosphere is minimum

Contamination from PCAs is perhaps the most ubiqui-tous Since most of the PCAs used are organic compoundswhich have low melting and boiling points they decomposeduring milling due to the heat generated The decomposi-tion products consisting of carbon oxygen nitrogen andhydrogen react with the metal atoms and form undesirablecarbides oxides nitrides etc

10 Concluding Remarks and Future Prospects

We have only briefly touched upon the scientific and tech-nological aspects of MA materials along with some of theconcerns such as powder contamination and consolidationissues The historical development of MA during the last 50years or so can be divided into threemajor periods (Figure 11)The first period covering the first twenty years (from 1966up to about 1985) was mostly concerned with the devel-opment and production of oxide-dispersion-strengthened(ODS) superalloys for applications in the aerospace indus-try Several alloys with improved properties based onNi and Fe were developed and found useful applicationsThese included the MA754 MA760 MA956 MA957 andMA6000 [74] A more recent survey of application of MAto steel is also available [75] The second period of aboutfifteen years covering from about 1986 up to about 2000witnessed advances in the fundamental understanding ofthe processes that take place during MA Along with thisa large number of dedicated conferences were held andthere was a burgeoning of publication activity Comparisonshave also been frequently made between MA and othernonequilibriumprocessing techniquesmore specifically RSP

Research 15

1965 19851975 1995 2005

Production of ODS Alloys

Novel Phase SynthesisDevelopment of BasicUnderstanding of MA

Nanostructures andNew Applications

2015

Figure 11 Schematic showing the different periods of development in the field of mechanical alloying (MA) and applications ofMA productssince its inception in 1965

It was shown that themetastable effects achieved by these twononequilibrium processing techniques are similar Revival ofthe mechanochemical processing (MCP) took place and avariety of novel substances were synthesized Several mod-eling studies were also conducted to enable prediction of thephases produced or microstructures obtained although withlimited success The third period starting from about 2001saw a reemergence of the quest for new applications of theMA materials not only as structural materials but also forchemical applications such as catalysis and hydrogen storagewith the realization that contamination of themilled powdersis the limiting factor in the widespread applications of theMA materials Innovative techniques to consolidate the MApowders to full density while retaining the metastable phases(including glassy phases andor nanostructures) in themwerealso developed All these are continuing with the ongoinginvestigations to enhance the scientific understanding of theMA process

At present activities relating to both the science andtechnology of MA have been going on simultaneously Whilesome groups have been focusing on developing a betterunderstanding of the process and optimizing the processvariables to achieve a certain phase or phase combinationin the alloy system others have been concentrating onexploiting this technique for different applications It hasalso come to be realized that the MA process can be usedto recycle used and discarded materials to produce value-added materials for subsequent consumption They can bepulverized separated and used to mix with other powdersto produce composites The bottom line is that newer andimproved materials are being continuously developed fornovel applications

Conflicts of Interest

The author declares that he has no conflicts of interest

References

[1] K H J Buschow R W Cahn M C Flemings B Ilschner E JKramer and SMahajan EdsEncyclopedia ofMaterials Scienceand Technology Elsevier 2001

[2] C Suryanarayana EdNon-equilibrium Processing of MaterialsPergamon Oxford UK 1999

[3] C Suryanarayana ldquoMetallic glassesrdquo Bulletin of Materials Sci-ence vol 6 no 3 pp 579ndash594 1984

[4] C Suryanarayana and A Inoue Bulk Metallic Glasses CRCPress Boca Raton Fla USA 2nd edition 2018

[5] H R Trebin Ed Quasicrystals Structure and Physical Proper-ties Wiley-VCH Weinheim Germany 2003

[6] C Suryanarayana and H Jones ldquoFormation and characteristicsof quasicrystalline phases A reviewrdquo International Journal ofRapid Solidification vol 3 pp 253ndash293 1988

[7] H Gleiter ldquoNanocrystalline materialsrdquo Progress in MaterialsScience vol 33 no 4 pp 223ndash315 1989

[8] C Suryanarayana ldquoRecent developments in nanostructuredmaterialsrdquo Advanced Engineering Materials vol 7 no 11 pp983ndash992 2005

[9] M C Gao J-W Yeh P K Liaw and Y Zhang Eds High-Entropy Alloys Fundamentals and Applications Springer 2016

[10] J S Benjamin ldquoMechanical alloying ndash History and futurepotentialrdquo in Advances in Powder Metallurgy and ParticulateMaterials J M Capus and R M German Eds vol 7 ofNovel Powder Processing pp 155ndash168 Metal Powder IndustriesFederation Princeton NJ USA 1992

[11] M P Phaniraj D-I Kim J-H Shim and Y W CholdquoMicrostructure development in mechanically alloyed yttriadispersed austenitic steelsrdquo Acta Materialia vol 57 no 6 pp1856ndash1864 2009

[12] A Hirata T Fujita Y R Wen J H Schneibel C T Liu and MW Chen ldquoAtomic structure of nanoclusters in oxide-dispersionstrengthened steelsrdquo Nature Materials vol 10 no 12 pp 922ndash926 2011

[13] M K Miller C M Parish and Q Li ldquoAdvanced oxide disper-sion strengthened and nanostructured ferritic alloysrdquoMaterialsScience and Technology (United Kingdom) vol 29 no 10 pp1174ndash1178 2013

[14] N Oono Q X Tang and S Ukai ldquoOxide particle refinementin Ni-based ODS alloyrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 649 pp 250ndash253 2016

[15] D B Witkin and E J Lavernia ldquoSynthesis and mechanicalbehavior of nanostructured materials via cryomillingrdquo Progressin Materials Science vol 51 no 1 pp 1ndash60 2006

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

14 Research

SOLID LIQUID GAS

MILLING

USE POWDER AS-ISor Chemical Reaction

Thermal Treatment etcCONSOLIDATE TO SOLID

(Temperature Pressure)

Catalysts Pigments SolderComponents Ceramic Powder etc

Advanced materials with hightolerance to gaseous impurities

Figure 10 Alternatives to consolidation of metal powders Ifpowders are used as catalysts pigments solder components etcthen they need not be consolidated However if powders need tobe consolidated then they could be used in applications with hightolerance to gaseous impurities

temperatures and shorter times in comparison to severalothermethods [72 73] An excellent reviewbyGroza [71]maybe consulted for full details of the methods of consolidationand description of results

9 Powder Contamination

Amajor concern inmetal powders processed byMAmethodsis the nature and amount of impurities that get incorporatedinto the powder and contaminate it The small size of thepowder particles and consequent availability of large surfacearea formation of fresh surfaces during milling and wearand tear of the milling tools and the atmosphere underwhich the powder is milled all contribute to contaminationof the powder In addition to these factors one has toconsider the purity of the starting raw materials and themilling conditions employed (especially the milling atmo-sphere) Thus it appears as though powder contaminationis an inherent drawback of the method and that the milledpowder will always be contaminated with impurities unlessspecial precautions are taken to avoidminimize them Themagnitude of powder contamination appears to depend onthe time of milling intensity of milling atmosphere in whichthe powder is milled nature and size of the milling mediumand differences in the strengthhardness of the powder andthe milling medium and the container

Several attempts have been made in recent years to min-imize powder contamination during milling An importantpoint to remember is that if the starting powders are highlypure then the final product is going to be clean and purewith minimum contamination from other sources providedthat proper precautions are taken during milling It is alsoimportant to keep inmind that all applications do not requireultraclean powders and that the purity desired will depend onthe specific application (Figure 10) But it is a good practiceto minimize powder contamination at every stage and ensurethat additional impurities are not picked up subsequentlyduring handling andor consolidation

One way ofminimizing contamination from the grindingmedium and themilling container is to use the samematerial

for the container and the grinding medium as the powderbeing milled Thus one could use copper balls and coppercontainer formilling copper and copper alloy powders [60] Ifa container of the same material to be milled is not availablethen a thin adherent coating on the internal surface of thecontainer (and also on the surface of the grinding medium)with the material to be milled will minimize contaminationThe idea here is to mill the powder once allowing the powderto be coated onto the grindingmedium and the inner walls ofthe container The milled loose powder is then discarded anda fresh batch of powder is milled but with the old grindingmedia and container In general a simple rule that shouldbe followed to minimize contamination from the millingcontainer and the grinding medium is that the container andgrindingmedium should be harderstronger than the powderbeing milled

Milling atmosphere is perhaps the most important vari-able that needs to be controlled Even though nominally puregases are used to purge the glove box before loading andunloading the powders even small amounts of impurities inthe gases appear to contaminate the powders especially whenthey are reactive The best solution one could think of willbe to place the mill inside a chamber that is evacuated andfilled with high-purity argon gas Since the whole chamberis maintained under argon gas atmosphere (continuouslypurified to keep oxygen and water vapor below 1 ppm each)and the container is inside the mill which is inside thechamber contamination from the atmosphere is minimum

Contamination from PCAs is perhaps the most ubiqui-tous Since most of the PCAs used are organic compoundswhich have low melting and boiling points they decomposeduring milling due to the heat generated The decomposi-tion products consisting of carbon oxygen nitrogen andhydrogen react with the metal atoms and form undesirablecarbides oxides nitrides etc

10 Concluding Remarks and Future Prospects

We have only briefly touched upon the scientific and tech-nological aspects of MA materials along with some of theconcerns such as powder contamination and consolidationissues The historical development of MA during the last 50years or so can be divided into threemajor periods (Figure 11)The first period covering the first twenty years (from 1966up to about 1985) was mostly concerned with the devel-opment and production of oxide-dispersion-strengthened(ODS) superalloys for applications in the aerospace indus-try Several alloys with improved properties based onNi and Fe were developed and found useful applicationsThese included the MA754 MA760 MA956 MA957 andMA6000 [74] A more recent survey of application of MAto steel is also available [75] The second period of aboutfifteen years covering from about 1986 up to about 2000witnessed advances in the fundamental understanding ofthe processes that take place during MA Along with thisa large number of dedicated conferences were held andthere was a burgeoning of publication activity Comparisonshave also been frequently made between MA and othernonequilibriumprocessing techniquesmore specifically RSP

Research 15

1965 19851975 1995 2005

Production of ODS Alloys

Novel Phase SynthesisDevelopment of BasicUnderstanding of MA

Nanostructures andNew Applications

2015

Figure 11 Schematic showing the different periods of development in the field of mechanical alloying (MA) and applications ofMA productssince its inception in 1965

It was shown that themetastable effects achieved by these twononequilibrium processing techniques are similar Revival ofthe mechanochemical processing (MCP) took place and avariety of novel substances were synthesized Several mod-eling studies were also conducted to enable prediction of thephases produced or microstructures obtained although withlimited success The third period starting from about 2001saw a reemergence of the quest for new applications of theMA materials not only as structural materials but also forchemical applications such as catalysis and hydrogen storagewith the realization that contamination of themilled powdersis the limiting factor in the widespread applications of theMA materials Innovative techniques to consolidate the MApowders to full density while retaining the metastable phases(including glassy phases andor nanostructures) in themwerealso developed All these are continuing with the ongoinginvestigations to enhance the scientific understanding of theMA process

At present activities relating to both the science andtechnology of MA have been going on simultaneously Whilesome groups have been focusing on developing a betterunderstanding of the process and optimizing the processvariables to achieve a certain phase or phase combinationin the alloy system others have been concentrating onexploiting this technique for different applications It hasalso come to be realized that the MA process can be usedto recycle used and discarded materials to produce value-added materials for subsequent consumption They can bepulverized separated and used to mix with other powdersto produce composites The bottom line is that newer andimproved materials are being continuously developed fornovel applications

Conflicts of Interest

The author declares that he has no conflicts of interest

References

[1] K H J Buschow R W Cahn M C Flemings B Ilschner E JKramer and SMahajan EdsEncyclopedia ofMaterials Scienceand Technology Elsevier 2001

[2] C Suryanarayana EdNon-equilibrium Processing of MaterialsPergamon Oxford UK 1999

[3] C Suryanarayana ldquoMetallic glassesrdquo Bulletin of Materials Sci-ence vol 6 no 3 pp 579ndash594 1984

[4] C Suryanarayana and A Inoue Bulk Metallic Glasses CRCPress Boca Raton Fla USA 2nd edition 2018

[5] H R Trebin Ed Quasicrystals Structure and Physical Proper-ties Wiley-VCH Weinheim Germany 2003

[6] C Suryanarayana and H Jones ldquoFormation and characteristicsof quasicrystalline phases A reviewrdquo International Journal ofRapid Solidification vol 3 pp 253ndash293 1988

[7] H Gleiter ldquoNanocrystalline materialsrdquo Progress in MaterialsScience vol 33 no 4 pp 223ndash315 1989

[8] C Suryanarayana ldquoRecent developments in nanostructuredmaterialsrdquo Advanced Engineering Materials vol 7 no 11 pp983ndash992 2005

[9] M C Gao J-W Yeh P K Liaw and Y Zhang Eds High-Entropy Alloys Fundamentals and Applications Springer 2016

[10] J S Benjamin ldquoMechanical alloying ndash History and futurepotentialrdquo in Advances in Powder Metallurgy and ParticulateMaterials J M Capus and R M German Eds vol 7 ofNovel Powder Processing pp 155ndash168 Metal Powder IndustriesFederation Princeton NJ USA 1992

[11] M P Phaniraj D-I Kim J-H Shim and Y W CholdquoMicrostructure development in mechanically alloyed yttriadispersed austenitic steelsrdquo Acta Materialia vol 57 no 6 pp1856ndash1864 2009

[12] A Hirata T Fujita Y R Wen J H Schneibel C T Liu and MW Chen ldquoAtomic structure of nanoclusters in oxide-dispersionstrengthened steelsrdquo Nature Materials vol 10 no 12 pp 922ndash926 2011

[13] M K Miller C M Parish and Q Li ldquoAdvanced oxide disper-sion strengthened and nanostructured ferritic alloysrdquoMaterialsScience and Technology (United Kingdom) vol 29 no 10 pp1174ndash1178 2013

[14] N Oono Q X Tang and S Ukai ldquoOxide particle refinementin Ni-based ODS alloyrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 649 pp 250ndash253 2016

[15] D B Witkin and E J Lavernia ldquoSynthesis and mechanicalbehavior of nanostructured materials via cryomillingrdquo Progressin Materials Science vol 51 no 1 pp 1ndash60 2006

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

Research 15

1965 19851975 1995 2005

Production of ODS Alloys

Novel Phase SynthesisDevelopment of BasicUnderstanding of MA

Nanostructures andNew Applications

2015

Figure 11 Schematic showing the different periods of development in the field of mechanical alloying (MA) and applications ofMA productssince its inception in 1965

It was shown that themetastable effects achieved by these twononequilibrium processing techniques are similar Revival ofthe mechanochemical processing (MCP) took place and avariety of novel substances were synthesized Several mod-eling studies were also conducted to enable prediction of thephases produced or microstructures obtained although withlimited success The third period starting from about 2001saw a reemergence of the quest for new applications of theMA materials not only as structural materials but also forchemical applications such as catalysis and hydrogen storagewith the realization that contamination of themilled powdersis the limiting factor in the widespread applications of theMA materials Innovative techniques to consolidate the MApowders to full density while retaining the metastable phases(including glassy phases andor nanostructures) in themwerealso developed All these are continuing with the ongoinginvestigations to enhance the scientific understanding of theMA process

At present activities relating to both the science andtechnology of MA have been going on simultaneously Whilesome groups have been focusing on developing a betterunderstanding of the process and optimizing the processvariables to achieve a certain phase or phase combinationin the alloy system others have been concentrating onexploiting this technique for different applications It hasalso come to be realized that the MA process can be usedto recycle used and discarded materials to produce value-added materials for subsequent consumption They can bepulverized separated and used to mix with other powdersto produce composites The bottom line is that newer andimproved materials are being continuously developed fornovel applications

Conflicts of Interest

The author declares that he has no conflicts of interest

References

[1] K H J Buschow R W Cahn M C Flemings B Ilschner E JKramer and SMahajan EdsEncyclopedia ofMaterials Scienceand Technology Elsevier 2001

[2] C Suryanarayana EdNon-equilibrium Processing of MaterialsPergamon Oxford UK 1999

[3] C Suryanarayana ldquoMetallic glassesrdquo Bulletin of Materials Sci-ence vol 6 no 3 pp 579ndash594 1984

[4] C Suryanarayana and A Inoue Bulk Metallic Glasses CRCPress Boca Raton Fla USA 2nd edition 2018

[5] H R Trebin Ed Quasicrystals Structure and Physical Proper-ties Wiley-VCH Weinheim Germany 2003

[6] C Suryanarayana and H Jones ldquoFormation and characteristicsof quasicrystalline phases A reviewrdquo International Journal ofRapid Solidification vol 3 pp 253ndash293 1988

[7] H Gleiter ldquoNanocrystalline materialsrdquo Progress in MaterialsScience vol 33 no 4 pp 223ndash315 1989

[8] C Suryanarayana ldquoRecent developments in nanostructuredmaterialsrdquo Advanced Engineering Materials vol 7 no 11 pp983ndash992 2005

[9] M C Gao J-W Yeh P K Liaw and Y Zhang Eds High-Entropy Alloys Fundamentals and Applications Springer 2016

[10] J S Benjamin ldquoMechanical alloying ndash History and futurepotentialrdquo in Advances in Powder Metallurgy and ParticulateMaterials J M Capus and R M German Eds vol 7 ofNovel Powder Processing pp 155ndash168 Metal Powder IndustriesFederation Princeton NJ USA 1992

[11] M P Phaniraj D-I Kim J-H Shim and Y W CholdquoMicrostructure development in mechanically alloyed yttriadispersed austenitic steelsrdquo Acta Materialia vol 57 no 6 pp1856ndash1864 2009

[12] A Hirata T Fujita Y R Wen J H Schneibel C T Liu and MW Chen ldquoAtomic structure of nanoclusters in oxide-dispersionstrengthened steelsrdquo Nature Materials vol 10 no 12 pp 922ndash926 2011

[13] M K Miller C M Parish and Q Li ldquoAdvanced oxide disper-sion strengthened and nanostructured ferritic alloysrdquoMaterialsScience and Technology (United Kingdom) vol 29 no 10 pp1174ndash1178 2013

[14] N Oono Q X Tang and S Ukai ldquoOxide particle refinementin Ni-based ODS alloyrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 649 pp 250ndash253 2016

[15] D B Witkin and E J Lavernia ldquoSynthesis and mechanicalbehavior of nanostructured materials via cryomillingrdquo Progressin Materials Science vol 51 no 1 pp 1ndash60 2006

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

16 Research

[16] C Suryanarayana ldquoMechanical alloying and millingrdquo Progressin Materials Science vol 46 no 1-2 pp 1ndash184 2001

[17] C Suryanarayana Mechanical Alloying and Milling MarcelDekker Inc New York NY USA 2004

[18] E Ma ldquoAlloys created between immiscible metalsrdquo Progress inMaterials Science vol 50 pp 413ndash509 2005

[19] C Suryanarayana and J Liu ldquoProcessing and characterizationof mechanically alloyed immiscible metalsrdquo International Jour-nal of Materials Research vol 103 no 9 pp 1125ndash1129 2012

[20] C Suryanarayana and N Al-Aqeeli ldquoMechanically alloyednanocompositesrdquo Progress in Materials Science vol 58 no 4pp 383ndash502 2013

[21] L Takacs ldquoSelf-sustaining reactions induced by ball millingrdquoProgress in Materials Science vol 47 no 4 pp 355ndash414 2002

[22] B Bergk U Muhle I Povstugar et al ldquoNon-equilibrium solidsolution of molybdenum and sodium Atomic scale experimen-tal and first principles studiesrdquo Acta Materialia vol 144 pp700ndash706 2018

[23] C Aguilar P Guzman S Lascano et al ldquoSolid solution andamorphous phase in TindashNbndashTandashMn systems synthesized bymechanical alloyingrdquo Journal of Alloys and Compounds vol670 pp 346ndash355 2016

[24] A A Al-Joubori and C Suryanarayana ldquoSynthesis of stable andmetastable phases in the Ni-Si system by mechanical alloyingrdquoPowder Technology vol 302 pp 8ndash14 2016

[25] C Suryanarayana E Ivanov and V V Boldyrev ldquoThe scienceand technology of mechanical alloyingrdquo Materials Science andEngineering A Structural Materials Properties Microstructureand Processing vol 304-306 no 1-2 pp 151ndash158 2001

[26] C Suryanarayana ldquoPhase formation under non-equilibriumprocessing conditions rapid solidification processing andmechanical alloyingrdquo Journal of Materials Science vol 53 no19 pp 13364ndash13379 2018

[27] C Suryanarayana ldquoNanocrystalline materialsrdquo InternationalMaterials Reviews vol 40 pp 41ndash64 1995

[28] K Gschneidner Jr A Russell A Pecharsky et al ldquoA family ofductile intermetallic compoundsrdquo Nature Materials vol 2 no9 pp 587ndash590 2003

[29] K A Gschneidner Jr M Ji C Z Wang et al ldquoInfluence of theelectronic structure on the ductile behavior of B2 CsCl-type ABintermetallicsrdquo Acta Materialia vol 57 no 19 pp 5876ndash58812009

[30] A A Al-Joubori and C Suryanarayana ldquoSynthesis ofmetastable NiGe

2by mechanical alloyingrdquo Materials amp

Design vol 87 pp 520ndash526 2015[31] T Tominaga A Takasaki T Shibato and K Swierczek ldquoHREM

observation and high-pressure composition isotherm mea-surement of Ti

45Zr38Ni17quasicrystal powders synthesized by

mechanical alloyingrdquo Journal of Alloys and Compounds vol645 pp S292ndashS294 2015

[32] P J SteinhardtTheSecondKind of ImpossibleTheExtraordinaryQuest for a New Form of Matter Simon amp Schuster 2018

[33] H H Liebermann Ed Rapidly Solidified Alloys ProcessesStructures Properties Applications Marcel Dekker Inc NewYork NY USA 1993

[34] W Klement R H Willens and P Duwez ldquoNon-crystallinestructure in solidified Gold-Silicon alloysrdquo Nature vol 187 no4740 pp 869-870 1960

[35] N Nishiyama K Takenaka H Miura N Saidoh Y Zengand A Inoue ldquoThe worldrsquos biggest glassy alloy ever maderdquoIntermetallics vol 30 pp 19ndash24 2012

[36] AYe Yermakov Ye Ye Yurchikov andVA Barinov ldquoMagneticproperties of amorphous powders of Y-Co alloys produced bygrindingrdquo Physics ofMetals andMetallography vol 52 no 6 pp50ndash58 1981

[37] C C Koch O B Cavin C G McKamey and J O ScarbroughldquoPreparation of ldquoamorphousrdquoNi

60Nb40bymechanical alloyingrdquo

Applied Physics Letters vol 43 pp 1017ndash1019 1983[38] C Suryanarayana Bibliography on Mechanical Alloying and

Milling Cambridge International Science Publishing Cam-bridge UK 1995

[39] C Suryanarayana I Seki and A Inoue ldquoA critical analysis ofthe glass-forming ability of alloysrdquo Journal of Non-CrystallineSolids vol 355 no 6 pp 355ndash360 2009

[40] S Sharma R Vaidyanathan and C Suryanarayana ldquoCriterionfor predicting the glass-forming ability of alloysrdquo AppliedPhysics Letters vol 90 pp 111915-1ndash111915-3 2007

[41] U Patil S-J Hong and C Suryanarayana ldquoAn unusual phasetransformation during mechanical alloying of an Fe-based bulkmetallic glass compositionrdquo Journal of Alloys and Compoundsvol 389 no 1-2 pp 121ndash126 2005

[42] S Sharma and C Suryanarayana ldquoMechanical crystallization ofFe-based amorphous alloysrdquo Journal of Applied Physics vol 102pp 083544-1ndash083544-7 2007

[43] C C Koch Nanostructured Materials Processing Propertiesand Applications William Andrew Norwich NY USA 2ndedition 2007

[44] E Hellstern H J Fecht C Garland and W L JohnsonldquoMechanism of achieving nanocrystalline AlRu by ball millingrdquoinMulticomponent Ultrafine Microstructures L E McCandlishD E Polk R W Siegel and B H Kear Eds vol 132 of MaterRes Soc pp 137ndash142 Pittsburgh Pa USA 1989

[45] J Eckert J C Holzer C E Krill andW L Johnson ldquoStructuraland thermodynamic properties of nanocrystalline fcc metalsprepared bymechanical attritionrdquo Journal ofMaterials Researchvol 7 no 7 pp 1751ndash1761 1992

[46] T G Nieh and J Wadsworth ldquoHall-Petch relation in nanocrys-talline solidsrdquo Scripta Metallurgica et Materialia vol 25 pp955ndash958 1991

[47] N X Sun and K Lu ldquoGrain size limit of polycrystallinematerialsrdquo Physical Review B Condensed Matter and MaterialsPhysics vol B59 pp 5987ndash5989 1999

[48] T W Clyne and P J Withers An Introduction to Metal MatrixComposites Cambridge University Press Cambridge UK 1995

[49] B Prabhu C Suryanarayana L An and R VaidyanathanldquoSynthesis and characterization of high volume fraction Al-Al2O3nanocomposite powders by high-energy millingrdquo Mate-

rials Science and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 425 pp 192ndash200 2006

[50] T Klassen C Suryanarayana and R Bormann ldquoLow-temperature superplasticity in ultrafine-grained Ti

5Si3-TiAl

compositesrdquo Scripta Materialia vol 59 pp 455ndash458 2008[51] C Suryanarayana R Behn T Klassen and R Bormann

ldquoMechanical characterization ofmechanically alloyed ultrafine-grained Ti

5Si3+40vol120574-TiAl compositesrdquo Materials Science

and Engineering A Structural Materials Properties Microstruc-ture and Processing vol 579 pp 18ndash25 2013

[52] C Suryanarayana ldquoStructure and properties of ultrafine-grained MoSi

2+Si3N4composites synthesized by mechanical

alloyingrdquoMaterials Science and Engineering A Structural Mate-rials Properties Microstructure and Processing vol 479 pp 23ndash30 2008

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016

Research 17

[53] C Suryanarayana ldquoRecent advances in the synthesis of alloyphases by mechanical alloyingmillingrdquo Metals and Materialsvol 2 pp 195ndash209 1996

[54] G B Schaffer and P G McCormick ldquoDisplacement reactionsduring mechanical alloyingrdquo Metallurgical Transactions A vol21 no 10 pp 2789ndash2794 1990

[55] A A Al-Joubori and C Suryanarayana ldquoSynthesis of austeniticstainless steel powder alloys by mechanical alloyingrdquo Journal ofMaterials Science vol 52 no 20 pp 11919ndash11932 2017

[56] A A Al-Joubori and C Suryanarayana ldquoSynthesis and thermalstability of homogeneous nanostructured Fe

3C (cementite)rdquo

Journal of Materials Science vol 53 no 10 pp 7877ndash7890 2018[57] M Dornheim N Eigen G Barkhordarian T Klassen and

R Bormann ldquoTailoring hydrogen storage materials towardsapplicationrdquo Advanced Engineering Materials vol 8 no 5 pp377ndash385 2006

[58] I P Jain P Jain and A Jain ldquoNovel hydrogen storage materialsA review of lightweight complex hydridesrdquo Journal of Alloys andCompounds vol 503 pp 303ndash339 2010

[59] G Barkhordarian T Klassen and R Bormann ldquoFast hydrogensorption kinetics of nanocrystalline Mg using Nb

2O5as cata-

lystrdquo Scripta Materialia vol 49 no 3 pp 213ndash217 2003[60] C Suryanarayana E Ivanov R Noufi M A Contreras and J

J Moore ldquoPhase selection in a mechanically alloyed Cu-In-Ga-Se powder mixturerdquo Journal of Materials Research vol 14 no 2pp 377ndash383 1999

[61] D L Hastings and E L Dreizin ldquoReactive structural materialspreparation and characterizationrdquo Advanced Engineering Mate-rials vol 20 pp 1700631-1ndash1700631-20 2018

[62] R A Yetter G A Risha and S F Son ldquoMetal particle com-bustion and nanotechnologyrdquo Proceedings of the CombustionInstitute vol 32 pp 1819ndash1838 2009

[63] E L Dreizin and M Schoenitz ldquoMechanochemically preparedreactive and energetic materials a reviewrdquo Journal of MaterialsScience vol 52 no 20 pp 11789ndash11809 2017

[64] R-H Chen C Suryanarayana and M Chaos ldquoCombustioncharacteristics of mechanically alloyed ultrafine-grained Al-MgpowdersrdquoAdvanced EngineeringMaterials vol 8 no 6 pp 563ndash567 2006

[65] E Basiri Tochaee H R Madaah Hosseini and S M Seyed Rei-hani ldquoFabrication of high strength in-situ Al-Al

3Ti nanocom-

posite by mechanical alloying and hot extrusion Investigationof fracture toughnessrdquo Materials Science and Engineering AStructural Materials Properties Microstructure and Processingvol 658 pp 246ndash254 2016

[66] R Zheng Z Zhang M Nakatani et al ldquoEnhanced ductilityin harmonic structure designed SUS 316L produced by highenergy ballmilling andhot isostatic sinteringrdquoMaterials Scienceand Engineering A Structural Materials Properties Microstruc-ture and Processing vol 674 pp 212ndash220 2016

[67] M Krasnowski S Gierlotka and T Kulik ldquoNanocrystallineAl5Fe2intermetallic and Al

5Fe2-Al composites manufactured

by high-pressure consolidation of milled powderrdquo Journal ofAlloys and Compounds vol 656 pp 82ndash87 2016

[68] ZWang K Prashanth K Surreddi C Suryanarayana J Eckertand S Scudino ldquoPressure-assisted sintering of Al-Gd-Ni-Coamorphous alloy powdersrdquoMaterialia vol 2 pp 157ndash166 2018

[69] J Karch R Birringer and H Gleiter ldquoCeramics ductile at lowtemperaturerdquo Nature vol 330 no 6148 pp 556ndash558 1987

[70] A Delgado S Cordova I Lopez D Nemir and E ShafirovichldquoMechanically activated combustion synthesis and shockwave

consolidation of magnesium siliciderdquo Journal of Alloys andCompounds vol 658 pp 422ndash429 2016

[71] J R Groza ldquoNanocrystalline powder consolidation methodsrdquoinNanostructuredMaterials Processing Properties andApplica-tions C C Koch Ed pp 173ndash233WilliamAndrew PublishingNorwich NY USA 2007

[72] C Keller K Tabalaiev G Marnier J Noudem X Sauvage andE Hug ldquoInfluence of spark plasma sintering conditions on thesintering and functional properties of an ultra-fine grained 316Lstainless steel obtained from ball-milled powderrdquo MaterialsScience and Engineering A Structural Materials PropertiesMicrostructure and Processing vol 665 pp 125ndash134 2016

[73] H Deng A Chen L Chen Y Wei Z Xia and J Tang ldquoBulknanostructured Ti-45Al-8Nb alloy fabricated by cryomillingand spark plasma sinteringrdquo Journal of Alloys and Compoundsvol 772 pp 140ndash149 2019

[74] J de Barbadillo and G D Smith ldquoRecent developments andchallenges in the application of mechanically alloyed oxidedispersion strengthened alloysrdquo Materials Science Forum vol88-90 pp 167ndash174 1992

[75] C Suryanarayana and A A Al-Joubori ldquoAlloyed steelsmechanicallyrdquo in Encyclopedia of Iron Steel and Their AlloysR Colas and G Totten Eds pp 159ndash177 Taylor amp Francis NewYork NY USA 2016