REPORT ON:

DEPARTMENT OF MATERIALS ENGINEERING

REPORT ON:

FAILURE DURING DENSIFICATION

DONE BY: SARBAJIT MANNA

SR.NO.:12835

COURSE NAME:FRACTURE MECHANICS

COURSE ADVISOR:VIKRAM JAYARAM

DATE OF SUBMISSION:12835

No. OF PAGES:15

Sintering or densification is the process of compacting and forming a solid mass of material by heat and/or pressure without melting it to the point of liquefaction.Because the sintering temperature does not have to reach the melting point of the material, sintering is often chosen as the shaping process for materials with extremely high melting points such as tungsten and molybdenum. An example of sintering can be observed when ice cubes in a glass of water adhere to each other, which is driven by the temperature difference between the water and the ice.Examples of pressure-driven sintering are the compacting of snowfall to a glacier, or the forming of a hard snowball by pressing loose snow together.During the sintering with shrinkage, the total solid volume is maintained to be a constant value but the shape and size of each particle change with the formation of grain boundaries.

Sintering depends on the following parameters:

Temperature

pressure

Surrounding medium and atmosphere

Particle size

Material properties

Heating rate

In later it will be discussed in details that how sintering actually depends on these parameters and how failure during sintering influenced by these parameters.

How sintering of two different materials is determined?

The sintering of any two particles in contact is generally determined by a combined action of the following coupled mechanisms / phenomena:

1. Cohesion and adhesion described by the specific surface and interface surface

energies;

2. Boundary diffusion of atoms and voids induced by surface concentration

gradients;

3. Bulk diffusion of atoms and voids induced by volume concentration gradients;

4. Surface diffusion induced by surface curvature gradients;

5. Condensation and evaporation;

6. Elastic and creep deformations and stresses induced by loading and

temperature/concentration gradients;

7. Surface and volume material drift induced by gradients of electric field,

temperature, stresses, chemical field and other drift-driving forces.

Though,we are defining our topic and concentrating on failure occurs during sintering but,sintering is in a sense the opposite to fracturing-it causes the vanishing of the cavity between two materials.Understanding this process could also help us to heal cracks in materials and restore old structures.

GRAIN GROWTH DURING SINTERING:Microstructure coarsening is inherent to sintering. Coarsening happens because of energy differences across interfacesthere is a small energy difference between large and small microstructural features, such as grains or pores. The energy per unit volume varies with the inverse of the size, so small pores or small grains have a tendency to coalesce into larger pores or grains. The interface between the microstructural features might be a grain boundary, a liquid film, a second phase, or a pore. Typically the mean (or median) feature size is tracked during sintering. The greatest attention is devoted to grain growth with secondary attention being given to pore growth.the terminal condition in sintering is one large grain, but such a condition would require years of sintering.One mechanism for growth is direct absorption of one grain into a neighboring grain via coalescence. This preferentially occurs when two grains contact with a low degree of crystallographic misorientation.

A schematic of grain coalescence during sintering. Two grains come into contact during

sintering and grow a bond with a grain boundary at the point of contact. That grain boundary is eliminated by grain rotation, typically by the smaller grain, with melding together to form a single grain.

A Metallogrphy Fracture Investigation:Role of Sintering time-In general,strength of the material increases initially with increasing sintering time but then it starts decreasing gradually.The main reason of this is the excessive grain growth as was discussed earlier.Densification and grain growth compete; at short sintering times densification improves mechanical properties, but at long sintering times microstructural coarsening has the opposite effect.

Strength data for a sintered tungsten alloy showing optimized strength at a shorter sintering time because microstructure coarsening reduces properties

The metallography fracture investigation done by Thomas F. Murphy, George B. Fillari, Gerard J. Golin gives an interesting result o fracture behavior during sintering.They did experiment on ferrous powder metallurgy materials with alloying mixed and bonded additives.They mainly investigated the fracture behavior with increasing sintering time at 1120 C.With increasing sintering time,three major factors are affected-Density-porosity,Chemical composition and alloying method and resultant microstructure.These all are affected directly o indirectly.Fracture surfaces from tested impact bars were examined in an SEM to find possible trends in the failure mechanism as the sintering time was extended.The kept the sintering time as follows:one-minute,five minutes,ten minutes,thirty minutes,fifty minutes.At the five-minute time, sintering has apparently started as evidenced by the small ductile regions at the sintered particle necks.At 10 minutes, the effects of increased sintering are clearly visible.Transgranular cleavage had started.Additionally, the progress of sintering, as indicated by the amount of material fracture,had increased considerably.At 30 minutes total furnace time, the greater amount of metallurgical bonding that had occurred results in more evidence of fractured surface,Ductile rupture, transgranular cleavage, and fracture through pearlite were all seen in significant proportions.The samples from the 50-minute total sintering time were also examined. An increase in the amount of transgranular cleavage fracture was apparent.Significant amounts of ductile rupture and fractured pearlite also were present.The fracture surface examinations appeared to show an increase in the amount of brittle,transgranular fracture as the sintering time was increased to a total of 50 minutes.This mainly due to homogenization of the copper distribution and consequent hardening of the ferrite.As sintering continues, the fracture surfaces exhibit progressively greater amounts of ductile rupture and brittle transgranular failure.

How fracture toughness increases with increasing yield strength:it has been shown that fracture toughness of sintered material increases with increasing yield strength.As it is known that with increasing densification or sintering pore density of that material decreases.Within the higher density levels,pores appeared to be smaller and more homogeneously distributed.Hence average distances between pores(neck size) increases with density.the relationship between yield strength and pore density is:

= 0exp(-nP)

Where n and sigma not is experimental constant and P is the volume fraction porosity.so with deceasing porosity volume,yield stress and tensile stress increases also in other words yield strength rises because the high effective load bearing section associated with increasing mean distance between

adjacent pores and the notch effect of pores.

Plastic deformation takes place at the inter pore regions and it can be regarded as a set of small circumferentially notched specimens and notched samples of ductile materials endure higher stresses as a consequence of the triaxiality of the stress state in front of the notch root which constraints the plastic deformation.From this perspective, it should be noted that fracture is observed to occur by appreciable plastic deformation of sintered necks, as reflected by the ductile nature of the fractographic features of these load bearing units, i.e.dimples resulting from microvoid coalescence and it was also seen that as density of sintered material increases, transgranular particle fracture was also found to take place,indicating the increasing role played by the intrinsic microstructure of sintered steels as porosity is reduced..From this experiment,a graph of load crack opening displacements was obtained with different density values and the evidence said that an extensive plastic deformation before fracture.It was also found that fracture in sintered steels is a successive rupture of sintering

necks, the thickness requirement can be neglected because the constraint on plastic flow is defined by the pore spacing rather than the specimen thickness.In this experiment, Fracture toughness-like values, KQ, raised as the density was increased and the relationship is linear given in the following figure:

Hence, it may be pointed out that, even under quite reduced porosity conditions, enhanced plastic deformation at sintering necks is still significant enough to continuously shield crack growth.

Typical load-crack opening displacement curves for three density levels provide extensive plastic deformation given in the following figure:

This experiments also gives another information about mechanical properties and that is The mechanical properties evaluated increased linearly with sintered density, regardless of the processing route followed for achieving a given density level.

HEAT TREATMENT ROLE ON FRACTURE OF SINTERED CERAMICS: With increasing temperature,porosity distribution gets changes in any ceramic materials like other materials.The behavior of crack propagation due to heat treatment or sintering can be given by the following experiment of bulk plasma sprayed alumina ,done by R J Damani.He experimented on bulk plasma sprayed alumina investigated under various annealed material condition.Toughness,as obvious,increases with increasing temperature during sintering.All material conditions exhibit pronounced R-curve behaviour. In the short-crack range broken splats act as bridges resulting in steeply rising toughness. In the longer-crack range larger features cause a less steep, but steady rise in toughness.The fracture behavior of plasma-sprayed alumina is the result of results the alignment

of microstructure and a splat-internal microcrack sub-structure which provides low energy crack paths. The alumina was sintered at various temperature and then the crack resistances of the samples was evaluated from the applied load and corresponding crack length according to ASTM standard E 399.The material exhibited pronounced R-curve behavior in all conditions where extended controlled crack advance was achieved.In materials before the reconstructive transformation fracture toughness rises sharply with initial crack advance (within about 30 to 40m of crack growth), and then continues in the long crack range to rise at a much lower rate.After the heat treatIl!ents at 900 DC and 1050 DC fracture toughness increases sharply.The main increase in toughening is in the early regime of crack growth.Both material conditions exhibit similar long-crack fracture toughness at between 1.5 to 2 times that of as sprayed material.In the two samples heat treated at 1550 C for 12 h the crack advanced immediately

after initiation perpendicular to the plane of the notch and the specimens failed by inter splat delamination.Thus only an initiation fracture toughness can be given at around 3 to 3.2 MPam1/2 This was up to 45 % higher than that of material heat treated at 1180 C, and almost 6 times the initial fracture toughness of as-sprayed material.Stable, straight crack propagation was achieved in all materials except that heat treated at 1550 C for 12 h. Crack deflection was more pronounced in heat treated materials. In all material: heat treated at less than 1550 C cracks tended to deviate eventually at about 30 to 35 to the plane of the notch and failure usually occurred by sudden delamination.Achieving straight crack growth became more difficult with increasing temperature of heat treatment and the amount of crack deflection and splitting also increased. Post-fracture examination of the fracture surfaces revealed a cog-tooth topography in the vicinity of the

notch tip, typical of neighboring splats which have broken across their thickness in different planes and have been pulled away from each other. In general, the main toughening mechanisms observed operating were crack flank bridging, crack hinging, crack deflection and crack splitting.Fracture toughness in the as-sprayed condition is substantially lower than that of a

conventionally sintered alumina.Further annealing leads to an increase in intersplat cohesion and to the recrystallisation of the splat-internal structure, thereby making crack propagation through splats increasingly difficult. The increasing integrity of the splats may be readily inferred from the polycrystalline nature of the fracture surface of a splat after heat treatment at 1550 C,Consequently, cracks are forced to greater deviation, in spite of the better cohesion of splats, and the toughness of the material improves and increases towards and above that expected of sintered, dense aluminas.On observing crack initiation and advance, it was noticed that a crack tended to appear at a notch tip and advance through the first few splats in its path with relatively little deflection.Thus there was clearly a transition between relatively straightforward crack advance in the short crack range, and the formation of large crack bridging ligaments in the long crack range. In the long crack range the crack also frequently underwent splitting and branching.Since samples typically finally failed suddenly by delamination along intersplat interfaces in the rest bands, these are evidently the most mechanically weak regions in the material. Their weakness may be explained if there is less bonding between splats at the start of a spray procedure, when the substrate is cool, than after it is well under progress and the substrate has warmed up: warm substrates promote better adhesion.toughness increases in the short crack range at a rate higher than

10 MPamv,/mm, whereas in the long crack range it increases at a rate of

(1to 4) x 10-3MPamv,/mm, which is four orders of magnitude lower. Thus, it is evident that different toughening mechanisms must dominate in each crack range.In the short crack range the crack advances by breaking through splats and the initial toughening is a consequence of the mechanical interlocking and pull out of broken splats segments from their sockets.The low toughness and toughening of the as sprayed material may be explained by the fact that cracks can easily advance through splats by using the many low energy paths provided by the interfaces between neighboring interplay columns and pre-existing transversal cracks due to residual thermal stresses.Thus, the segments pulled out are correspondingly small. It follows that, any reduction in the number of weak crack paths available, i.e., improvement in the integrity of individual splats, can result in a significant increase in toughness in this short crack range. This is what is observed in the material heat treated at 900 and 1050 C. Toughness continues to increase rapidly until an equilibrium condition is reached.So,in a brief it can be said that Heat treatment at'1180 C for 12 h results in replacement of the columnar substructure by a system of self-accommodating laths. The interfaces between laths still provide relatively weak, low energy preferential crack paths. The increase in integrity of the splat internal structure enhances toughening mechanisms and results in excellent toughness.After 12 h at 1550C the splats are internally completely recrystallised, but the macroscopic splat structure is largely retained. The intersplat cohesion is a little improved due to some sintering.Nevertheless,the material in this condition preferentially fails by delamination. This is mostly a consequence of the improved mechanical stability associated with recrystallisation ofthe splats, which makes crack advance through splats more difficult than crack deviation between splats.All the material conditions (with the exception of material heat treated at 1550 C for12 hours, for which no controlled crack extension was achieved) exhibit pronounced R-curve behaviour.The R-curves may be divided into two regions: very steep in the initialphase of crack propagation, and flatter, but persistent, in the long range. In the short crack range toughening is a consequence of the pull-out of splats; and in the long crack range, the toughening effect results from crack branching and hinging by larger microstructural features.

Structural Failure due to non uniform heating or sintering:Sintering furnace temperature gradients influence final dimensions. Gradients encountered during heating are especially problematic.a photograph of two versions of the same electronic package after sintering at the same temperature is given below.Curvature occurred in one case due to non-uniform heating.

CRACK DUE TO DIFFRENTIAL SHRINKAGE DURING SINTERING:

Differentiak shrinkage due to non-uniformity can cause stress development that may lead to cracking.The problem is that the more a part has to shrink to become fully dense ,the less tolerance there is for non-uniformity.Once a portion of of the green compact shrinking away from adjacent material introduces flaw,it will continue to grow upon further shrinkage.If it does not grow to the point of failure in the green compact,it can still grow to the failure in the sintering process.Of worse,the sintered specimen could fail permanently during application.This failure due to differential shrinkage can be prevented upto a certain degree ,the unifomity of the green specimens results in isotropic densification that allows a precise and predictable final shape with unprecedented tolerance that conventional techniques obtain only after expensive treatments or polishing.The key to the success of this process is materials processing that results in uniformity of green structure to produce uniform shrinkage during the sintering process.However, differences in the shrinkage of the powder mixtures often result in large stresses developing during the sintering process that can compromise the structural integrity of the component before the fully-sintered strength is achieved.the induced level of shrinkage stress depends directly on the level of differential shrinkage.The shrinkage is due to the decrease in pore volume and porosity with increasing time and temperature.For example,In pressureless sintering of metal-matrix ceramic composite,done by Yasser M. Shabana 1, Hugh A. Bruck *, Michael L. Pines, Jonathan G. Kruft the metal particles undergo elastoplastic deformations, while the ceramic particles deform elastically.Porosity is represented by the void volume fraction, with the change in porosity being directly related to the shrinkage.The porosity content in the green (before sintering) compacted structure is high, and exists in-between matrix particles and between the matrix particles and the reinforcement particles.During the sintering process, diffusion between particles forms solid bonds that reduce the surface energy by reducing the interfacial area. With extended heating, the pore volume and porosity are further reduced as the matrix particles sinter between themselves and around the reinforcement particles, leading to isotropic shrinkage.During the sintering process, microstresses evolve due to the shrinkage of the matrix that can cause plastic deformation of ductile reinforcement particles or of a ductile matrix that further reduces porosity.

Fig:Nature of porosity and densification behavior of matrix particles around reinforcement particles during pressureless sintering.

The cracks arises during this pressureless sintering due to only the differential shrinkage.Thermomechanical model can predict the crack initiation.

Crack arises during densification because of the low fracture toughness.Sintering cracks are caused by gravity, thermal gradients, polymer burnout,or vibration. Because of the low starting fracture toughness, small flaws grow, even defects as small as 10 m enlarge during heating.The photograph in below is of a crack that was not evident prior to sintering. It formed early in the heating cycle due to differential densification between thick and thin regions.

Crack formed during sintering of an alumina surgical spoon. The long crack indicates it formed early prior to significant strengthening, and the location corresponds to a junction of thick and thin sections.

Creep and failure by creep during sintering:

Creep occurs at high temperatures, usually leading to distortion but not cracking.If the sintering rate increases,creep resistance decreases.It can be stated as follows that during sintering,grain growth occurs and number of grain boundary area decreases.Now ,during creep,materials becomes ductile due to the fact of grain boundary sliding but during sintering,number of grains decrease and sliding of grains becomes very difficult and this is one of the main reason of failure by creep during sintering.Sintering stress may increase the creep rupture lifetime markedly.It is necessary to consider the growth by thermal fluctuations slightly beyond the critical sizebefore deterministic growth can take over.

Role of pores in influencing fracture during sintering:Pores play one of the major important role for accelerating fracture during sintering.Increasing sintered density resulted in lower pore fraction, smaller average pore size, and more spherical pore shape. Increase pore size was directly correlated with an increase in the irregularity of pore shape.A large amount of strain localization takes place in the sintered regions between pores.For example in powder metallurgy steel(P/M), networks of pores are quite effective in localizing the strains in the steel ligaments between the pores.Thus, a very small section of the microstructure is actually being plastically deformed, so that a large portion of the materials is largely undeformed.The modeling results are confirmed by experimental observations that porosity causes deformation to be localized and inhomogeneous. The strain intensification in the sintered ligaments between pores, likely serve as areas for crack initiation.Once the onset of crack initiation takes place, the large pores will be linked, and the effect load-bearing area of the materials locally will decrease very quickly, resulting in fracture of the material.An increase in porosity decreases the overall sintered ligament fraction and spacing between pores, thus accelerating the intensification of strain in the matrix material.Here sintering time plays its role,if it is optimised very accurately enough then fraction of pores will reduce significantly without affecting creep rupture.Comparing damage mechanisms between round and angular pores in materials with identical pore fraction it was observed that highly localized slip bands formed at the sharp tips of angular pores, producing uneven distribution of strain around angular pores.This resulted in highly localized and inhomogeneous plastic deformation compared to the deformation around round pores which was much more homogeneous.Distributions while the strength of the material is controlled by the fraction of pores, macroscopic ductility is also influenced by the size distribution and degree of clustering of the pores, for example in P/M steels, sintered ligaments of the steel control fracture of the material. An equally important result is that, even in the highest density material, a large amount of strain intensification takes place at as ingle pore cluster in the microstructure.Thus,even when the overall amount of porosity is relatively low (4-5%), strain intensification may take place around pore clusters. It follows that the homogeneity and distribution of the porosity is as important as the fraction of porosity in controlling the evolution of plastic strain, and thus, the onset of crack initiation.Fractography of tensile fracture surfaces provided further insight into the role of porosity in fracture of these materials. At the lowest density, fracture took place primarily by localized void nucleation and growth in sintered necks of the material.Perhaps the most significant influence of porosity was in fatigue behavior. Stress versus cycle curves revealed that the 7.0 g/cm3 P/M steel alloy had significantly lower fatigue endurance limit than the other two alloys. It is well known that single large pores or clusters of pores act as stress concentration sites for fatigue crack initiation.

Ultimate failure during high temperature sintering typically involves cavity growth at grain boundary triple junctions and linking along grain boundaries to form a crack that ultimately propagates catastrophically.Conclusions: It was discussed more than briefly that how fracture occurs during densification or sintering.There are many many factors which motivate for fracture during sintering like,particle size,temperature,pressure,surrounding medium,time of sintering,incorporated materials and also the role of differential shrinkage,density,volume of pores and sufficient basic idea has been given to clear the concept of crack initiation,propagation during sintering and change of this rate of propagation which ultimately leads fracture,can be controlled by controlling the above parameters.

References:

FRACTURE TOUGHNESS OF HIGH-DENSITY SINTERED STEELS,J. Bris1, F. Bentez2, A. Mateo1, J. Calero2, M. Anglada1, L. Llanes,Anales de Mecnica de la Fractura Vol. II (2006)

SINTERING FROM EMPIRICAL OBSERVATIONS TO SCIENTIFIC PRINCIPLES by Randall M german

EFFECT OF GRAIN SIZE ON CRACK GROWTH IN ALUMINA,M.E. Ebrahimi, J. Chevalier, M. Saadaoui and G. Fantozzi

4.A Metallographic Investigation Into the Effect of Sintering on an FC-0205 Premix,Thomas F. Murphy, George B. Fillari, Gerard J. Golin

5.Fabrication of Dense Shoulder Components through Traveling Zone Sintering Assisted by a Multi-Way Loading System,Shuji Tada, Zheng Ming Sun, Hitoshi Hashimoto and Toshihiko Abe,Materials Transactions, Vol. 45, No. 2 (2004) pp. 319 to 322 #2004 The Japan Institute of Metals.

6.impact Fracture Toughness of Porous Iron and HighStrength Steels,GIOVANNI STRAFFELINI

7.Modeling the evolution of stress due to differential shrinkage

in powder-processed functionally gradedmetalceramic composites during pressureless sintering,Yasser M. Shabana 1, Hugh A. Bruck *, Michael L. Pines, Jonathan G. Kruft.International Journal of Solids and Structures 43 (2006) 78527868

Fr.acture at high temperature ,by Hermann Riedel.

EFFECT OF DENSITY ON THE MICROSTRUCTURE AND MECHANICAL BEHAVIOR OF POWDER METALLURGY FE-MO-NI STEELS ;N chawla

THE END