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Research ArticleSynthesis and Characterization of NanocrystallineNi50Al50−xMox (x = 0–5) Intermetallic Compound duringMechanical Alloying Process

A. Khajesarvi and G. H. Akbari

Department of Materials Science and Engineering, Shahid Bahonar University, Kerman 76135-133, Iran

Correspondence should be addressed to A. Khajesarvi; [email protected]

Received 13 March 2015; Accepted 9 August 2015

Academic Editor: Ipsita Banerjee

Copyright © 2015 A. Khajesarvi and G. H. Akbari. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

Nanocrystalline Ni50Al50−𝑥Mo𝑥(𝑥 = 0, 0.5, 1, 2.5, 5) intermetallic compound was produced through mechanical alloying of nickel,

aluminum, and molybdenum powders. Powders produced frommilling were analyzed using scanning electron microscopy (SEM)and X-ray diffractometry (XRD). Results showed that, with increasing the atomic percent of molybdenum, average grain sizedecreased from 3 to 0.5 𝜇m. Parameter lattice and lattice strain increased with increasing the atomic percent of molybdenum,while the crystal structure became finer up to 10 nm. Also, maximum microhardness was obtained for NiAl

49Mo1alloy.

1. Introduction

There is an increasing interest in producing nanocrystalsbecause of their excellent physical andmechanical propertiesin comparison to coarse-grain materials [1]. NiAl intermetal-lic compound can be used as a high-temperature materialowing to its highmelting point, low density, excellent thermalconduct, good temperature stability, and good oxidationresistance [2–5]. Unfortunately, this compound has a big dis-advantage: low toughness at low temperature and low creepstrength at high temperatures, which limits the applicationof this intermetallic compound [6]. Considerable efforts havebeen made in terms of increasing the mechanical propertiesof NiAl via reducing the size of materials microstructureto nanometer dimension, micro- and macroalloying, andusing alloying elements [7–9]. Although extensive researchhas been done on this alloy, recent investigations on revealinginteresting properties of NiAl system are still continuingso that promising results have been obtained in terms ofimproving fragility through modifying grain size [10–14].

Severalmethods like powdermetallurgy, self-combustionsynthesis, andmechanical alloying have been used for synthe-sizing intermetallic compounds. Mechanical alloying is oneof the solid state methods, which is suitable for producing

compounds with high steam pressure and elements withdifferent melting points that are not produced using theconventional methods [15, 16]. Also, this method can be usedto directly produce nanocrystalline structures [17].

Formation of NiAl intermetallic compound duringmechanical alloying is a self-expanding reaction in separateparticles, which is accompanied by a sudden release of energyand thermodynamic factors play an effective role in theformation of final phase. One of the elements which promotesthe deficiencies of intermetallic compounds is molybdenumand its most useful effect is in terms of increasing roomtemperature ductility of this compound [18]. Main effect ofnegligible element such as Fe, Ga, and Mo on microstructureof NiAl alloy is the formation of a solid solution with NiAlintermetallic compound. Adding Mo leads to a differentbehavior from adding Fe and Ga. Also, adding Mo has slighttendency tomodify grain size ofNiAl alloy. Average grain sizein the presence of Mo is less than 20 nm, while it is 20–50 nmin NiAl alloy Mo-free [19].

Therefore, to promote mechanical properties of NiAlintermetallic compound, this paper sought to examine theeffect of Mo microalloy on the production process ofnanocrystalline Ni-Al intermetallic compound. All the alloyswere obtained from pure elements with a combination close

Hindawi Publishing CorporationJournal of NanoparticlesVolume 2015, Article ID 651852, 6 pageshttp://dx.doi.org/10.1155/2015/651852

2 Journal of Nanoparticles

to the stoichiometric NiAl compound using mechanicalalloying.

2. Experimental Procedure

In this study, aluminumpowders with the purity ofmore than99%andparticle size of less than 200𝜇m,nickel powderswiththe purity of more than 99.9% and particle size of less than10 𝜇m, and molybdenum powders with the purity of morethan 99.99% and particle size of less than 150𝜇m were used.The powder mixtures were milled in a planetary ball mill.The mill atmosphere was protected by argon gas to preventthe oxidation of powder particles. In all the experiments, 4large and 12 small balls with the respective diameters of 2 and1 cm were used. By selecting different sizes, the balls chose arandom motion [20] and more energy was transferred to thepowder [21]. Vial and balls were made of hardened chromesteel. Total weight of the used powderwas 12 g. Rotation speedof milling vial was constant in all experiments and equal to250 rpm. Ball to powder weight ratio was 15 : 1. By changingthe amount of molybdenum, 5 samples were prepared withthe composition of Ni

50Al50−𝑥

Mo𝑥(𝑥 = 0, 0.5, 1, 2.5, 5)

which were milled for 128 h.Structural changes of the samples during mechanical

alloying were studied using a Philips X’Pert diffractometerwith Cu-K𝛼 (𝜆 = 0.15405 nm) radiation over 20–100 2𝜃.Morphology and microstructure of powder particles werecharacterized by SEM in a Philips XL30. The mean powderparticle size was estimated from SEM images of powderparticles by image tool software. The average size of about 50particles was calculated and reported as mean powder parti-cle size. Crystallite size and lattice strain were also calculatedusing Williamson-Hall method [22]. In this method, peakwidth caused by lattice strain and grain size is considered[23, 24]. This relationship is expressed as follows:

𝛽 cos 𝜃 = 0.9𝜆𝐷

+ 2𝜀 sin 𝜃, (1)

where 𝛽 is the peak breadth in midheight, 𝜆 represents theX-ray wave length of the incident copper X-ray radiation, 𝐷is the average crystallite size, 𝜀 is the mean value of internalstrain, 𝜃 is the Bragg diffraction angle, and 𝐾 is a constantwith a value of 0.89. Accordingly, if 𝛽 cos 𝜃 is plotted in termsof sin 𝜃, a line with the slope of 2𝜀 and intercept of 0.9𝜆/𝐷willbe obtained. By extracting these data from the drawn lines,size of crystals and lattice strain can be determined and theireffect can be also separately specified. In order to determinewidth of peaks at half height, Sigma Plot 12.0 software, whichis advanced software in drawing and processing curves, wasused. Powders produced by milling were converted intotablet-like pieces with the diameter of 25mm and thicknessof 3mm during mechanical cold pressing process under thepressure of 849MPa in 20 sec. Samples were studied usingVickers microhardness method with the applied force of254.2mN for 10 sec using Struers Duramin hardness tester.The reported hardness value was the average of 6 times ofhardness test for each sample.

3. Results and Discussion

3.1. Morphology and XRD Analysis. Figures 1 and 2 showSEM images and average size of powder particles in terms ofchange in the amount of molybdenum after 128 h of milling,respectively. As shown in Figure 2, with increasing the atomicpercent of molybdenum, average particle size of the powderfollowed a decreasing trend. Minimum average particle sizeof the powders containing 5 at.%Mowas obtained as approx-imately 0.5 𝜇m. Higher magnification of powder particlesshowed that large particles in this step were in fact the resultof the accumulation of a large number of small particles[25]. Further, Figure 3 demonstrates X-ray diffraction patternof the mixture of initial powder and mechanical alloyingsamples with different molybdenum percentages after 128 hof milling. As seen in the XRD patterns of the samples beforemilling, only the peaks ofNi, Al, andMowere observed.Withincreasing Mo percentage, NiAl peaks intensity and widthwere reduced and increased, respectively.

After 128 h of milling, there was no trace of nickel andaluminum peaks in the XRD pattern (Figure 3). Reactionsfinished in the vial before milling completion and the finalproduct was 100% nickel aluminide. Since the atomic dif-fusion is time dependent, therefore sufficient milling timeis required to obtain the final products [26]. After theformation of (Ni, Al) solid solution and continuing theprocess of milling and topical heating, powder particles weregradually converted into NiAl. The main factors that affectthe mechanisms occurring in MA process are fracture andcold welding repetition of particles followed by an increasein their [27]. For reactivity in long time periods, the powdersbecame so homogeneous that rapidly got regulated. Powdersbecame finer and particles size distribution went uniformwith spherical and same-sized shapes. This issue showed theequality of cold welding rate and failure of powders due tocold working. With increasing the amount of molybdenum,more molybdenum was dissolved in NiAl intermetallic giventhe changes in lattice parameters. As a result of Mo dissolu-tion in NiAl intermetallic compound, stacking faults energywas reduced and effect of hard working became higher thanthat of the molybdenum-free sample, more dislocations andsubboundaries were formed, and consequently grain sizesbecame finer. Furthermore, by the dissolution ofMo atoms inthe Nimatrix, hardworking effects were increased; as a result,more powder particles were brittle andmore failure occurredin them which led to finer particles. In general, dissolutionof alloying elements in metal crystals and generation ofdistortion in them increase hard working in the cold workingprocess [28, 29]. In fact, Mo saturation in Ni reduces such aneffect.

3.2. Internal Strain. Figure 4 shows internal strain changesof NiAl lattice for different percentages of molybdenumafter 128 h of milling. As is determined, with increasing theamount of molybdenum, first, a strong increase occurredin the rate of received internal strain. Then, at 2.5 at.% Mo,the amount of internal strain stored in powder particlesincreased with less intensity. In addition to the internal straintransferred to the powder particles from the milling device,

Journal of Nanoparticles 3

(a) (b)

(c) (d)

(e)

Figure 1: SEM images of powders milled for 128 h in (a) 0, (b) 0.5, (c) 1, (d) 2.5, and (e) 5 at.% Mo.

with increasing molybdenum and dissolution of its atomsin Ni matrix, more defects, specifically dislocations, wereformed in the materials. As a result, strain of the materialincreased compared to molybdenum-free NiAl compound.Nevertheless, with increasing molybdenum content to morethan 2.5 at.%, increased accumulation of cold working effectled to matrix saturation of cold working and reduced strainincrease rate at high amounts of molybdenum. Alizadeh et al.

[30] reported increased internal strain in NiAl system withthe increased Cr atomic percent and showed that high levelsof chromium had a more impact than molybdenum on theinternal strain of NiAl.

3.3. Crystal Size. Figure 5 shows effect of the amount ofmolybdenum on changes in the size of NiAl crystals after128 h of milling. Crystal size in NiAl molybdenum-free


1 2 4 53 60

Mo-content (at.%)

0

0.5

1

1.5

2

2.5

3

3.5

4

Part

icle

size

(𝜇m

)

Figure 2: Average particle size of the powder for different amountsof molybdenum after 128 h of milling.

Inte

nsity

(a.u

.)

30 40 50 60 70 80 90

4000

0

NiAl

MoNiAl

1000

1000

0

0

1000

0

1000

0

800

0

Position 2𝜃 (∘)

(a)(b)(c)(d)(e)(f)

Figure 3: X-ray diffraction pattern of (a) nonmilled powdermixtureand 128 h milling of (b) Ni

50Al50, (c) NiAl

49.5Mo0.5, (d) NiAl

49Mo1,

(e) NiAl47.5

Mo2.5, and (f) NiAl

45Mo5.

42 31 5 60

Mo-content (at.%)

2.2

2.3

2.4

2.5

2.6

2.7

Inte

rnal

stra

in (%

)

2.8

Figure 4: Strain changes of NiAl lattice after 128 h of milling for the5 samples with 0, 0.5, 1, 2.5, and 5 at.% Mo.

samples after 128 h of milling was 26 nm and with increasingmolybdenum up to 5 at.%. This value was decreased to10 nm. With increasing molybdenum, crystallite size sud-denly decreased for the powders containing 0.5 at.% Mo,but this trend reached saturation with more increase ofmolybdenum, which can be due to the solubility saturation of

Mo-content (at.%)1 2 4 53 60

0

5

10

15

20

25

30

35

Crys

talli

te si

ze (n

m)

Figure 5: Size changes of NiAl crystal after 128 h of milling forsamples with 0, 0.5, 1, 2.5, and 5 at.% Mo.

1 2 4 53 60

Mo-content (at.%)

2.884

2.885

2.886

2.887

2.888

2.889

2.89

2.891

2.892

2.893

2.894

Latti

ce p

aram

eter

(Å)

Figure 6: Changes of NiAl lattice parameter after 128 h of millingfor the 5 samples with 0, 0.5, 1, 2.5, and 5 at.% Mo.

Mo atoms in nickel lattice. As a result, hardworking effect wasreduced and consequently dislocations and subboundarieswere formed to some extent and then the intensity ofcrystallite size reduction was decreased. Intense grain sizereduction is one of the methods for improving ductilityso that nanometric dimensions of grains led to furtherimprovement in ductility and creeping resistance comparedto micron grains [31, 32]. Above DBTT temperature, therewas strong dependence between tensile ductility and grainsize [33]. Room temperature yield strength of Ni

50AL is

independent of grain size; in contrast, even in alloys withsmall deviation from stoichiometry, it strongly depends ongrain size and increased with the reduction in grain size [34].

3.4. Lattice Parameter. Figure 6 demonstrates changes in thelattice parameter of NiAl compound after 128 h of milling fordifferent atomic percentages of molybdenum. According tothis figure, lattice parameter had one increase for the powderscontaining 1 at.% Mo, but this trend reached a stable statewith more Mo increase, which could be due to solubilitysaturation of Mo atoms in Ni lattice; consequently, intensityof increasing lattice parameter was reduced. Albiter et al.

Journal of Nanoparticles 5

1 2 4 53 60

Mo-content (at.%)

580

600

620

640

660

680

700

Mic

roha

rdne

ss (H

v)

Figure 7: Microhardness changes after 128 h of milling for the 5samples with 0, 0.5, 1, 2.5, and 5 at.% Mo.

[19] addedmolybdenum toNi56Al44compound and reported

similar results. They also reported that adding Fe to thesame compound had a more effect on lattice parameter thanMo. The reason for more increase of lattice parameter withincreasing molybdenum could be attributed to increasedcrystal defects and then dislocations as a result of severeplastic deformation of particles.Thus, subgrainswere formed;subgrains and dislocations made vaster pathways for themovement of molybdenum atoms and were displaced bymore plastic deformation of particles; however, molybdenumatoms were not displaced. By repetition, there would be anincreased possibility for the penetration of Mo atoms.

3.5. Microhardness Measurements. Figure 7 shows micro-hardness changes in NiAl lattice after 128 h of milling fordifferent percentages of molybdenum after cold pressingoperations. As can be seen, with increasing molybdenum,microhardness changes first increased and then decreased.Microhardness changes inNiAl samplewithoutmolybdenumshowed the number of 660Hv after 128 h of milling. Withincreasing the amount of Mo to 1%, a sharp increase up to690Hv was observed in microhardness rate. Then, at 2.5 at.%Mo, this value was reduced to 603Hv and, afterward, itremained almost constant.

This complexity can be considered as the interactionof two processes: one is cold working process that occursbefore and after the formation of NiAl compound and theother is the formation of NiAl intermetallic compound thatleads to heat release. With increasing atomic percent ofmolybdenum, more cold working effects were stored in(Ni, Al) solid solution and thus more energy was storedthere. As a result, during the random conversion of (Ni,Al) solid solution into a regular intermetallic compound,more heat was released. More heat release would completelyeliminate the remaining cold working effects generated inthe (Ni, Al) solid solution. This factor could cause morehardness drop at high molybdenum amounts. Therefore,several factors are effective in this regard and it needs tobe further investigated since: this subject and cold workingelements duringmechanical alloying and before and after the

formation of NiAl intermetallic compound are complex; afterthe formation of intermetallic compound, heat is releasedand both material and phase change and at the same time,in addition to phase change, the structure itself can changeunder the influence of the two above-mentioned elements.

4. Conclusions

Nanocrystalline Ni50Al50−𝑥

Mo𝑥(𝑥 = 0, 0.5, 1, 2.5, 5) inter-

metallic compoundwas successfully produced bymechanicalalloying of different amounts of molybdenum. Molybdenumincrease led to increased defects in the material, especiallydislocations. As a result, strain rate of the material increasedcompared to NiAl compound. Increasing Mo not onlyreduced the final crystallite size but also had an importanteffect on modifying microstructure. Variations in crystallitesize were very severe at first and decreased later when reach-ing saturation. Crystal size in the NiAl molybdenum-freesample was 26 nm and increasing Mo to 5 at.% reduced it to10 nm. Mo increase to 1% intensely increased microhardnessrate to 690Hv. Broadening peaks of X-ray diffraction patterndue to molybdenum increase were mainly due to decreasedcrystallite size and increased lattice strain. In the presence ofmolybdenum, the produced alloy’s lattice parameter showedhigher values.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

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