Effect of repressing

8/7/2019 Effect of repressing

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P e r g a m o nActa mom-. Vol. 45, No. 10, pp. 42834296, 1997

0 1997 Acta Metallurgica Inc.Published by Elsevier Science Ltd. All rights reservedPrinted in Great BritainPII: s1359-6454(97)ooo73-6 1359-6454197 $17.00 + 0.00

EFFECT OF CYCLIC PRESSURE ON THE LOWTEMPERATURE CONSOLIDATION OF SEVERALCOMPOSITE POWDER SYSTEMS

C H I N G -Y AO H U A N G a n d G L E N N S . D A E H NDepartm ent of Materials Science and Engineering, T he Ohio State University, 204 1 College Road,Columbus, Ohio 43210, U.S.A.

(Received 2 Oct ober 1996; acrept ed 23 December 1996)

Abstract-The effect of cyclic pressure on the room temperature composite pow der consolidation has beenstudied. M ixed metal and ceramic powders w ere consolidated under static and cyclic pressure at roomtemperature in constrained uniaxial consolidation. In this paper, five composite powder systems,Pb/TiO z, Pb/SiC, Al/AbO,, Al/Sic, and Zn/AbO ,, have been studied at 40% ceramic by volume. Thiscomplements an extensive study of Pb/AhO j in static and cyclic consolidation that was reported earlier[l]. The experiments show that pressure cyc ling greatly enhances densification relative to static compactionat the same maxim um pressures. The proposed reason for improved densification in cyclic pressure is thatthe pressure changes (AP) may directly induce plastic deformation due to elastic mismatch strains thatdevelop between the metal and ceramic phases. These are induced owing to the compressibility difference(A/Y) etween the two ph ases. The six powder systems studied to date (at many pressure cycle amplitudes)show that all the data for density change nearly collapse to one trend line when plotted against thedimensionless strain ratio, ABAP/c,, wh ere the numerator represents the linear elastic misma tch strain andto is the matrix yield strain. This te rm mus t excee d a value near 3 in order to significantly improveconsolidation by pressure cycling. This effect is expected to have practical im portance as the pressuresrequired to improve con solidation are often accessible and the strengths of the compacts formed bypressure cycling are significantly greater than those produced in static consolidation. 0 199 7 ActaMetallurgica Inc.

I N T R O D U C T I O NPowde r metallurgy techniques hold much promisefor the creation of metal-ceramic composites, as hasbeen shown in many studies [2-h]. The ability to userelatively low consolidation tempe ratures offers theability to retain metastable phas es that might beformed by rapid solidification, for example. H ow-ever, when the ceramic fraction becomes large,it becomes difficult to densify the metal-ceramicaggregate [1, 7-l l] and often the compacts producedhave significant density g radients [12]. The mostcommon methods used to mitigate such problems arethe applications of higher compaction pressuresor temperatures. While this may be effective forconsolidation, higher pressu res require larger pro-cessing equipment and higher temperatures can causethe loss of metastable phases or produce undesirablematrix-reinforcement reactions, or both.

produced with each pressure cycle, incrementalconsolidation could take place over many cycles.Previous work from the Pb/ ALO 3 system supportedthis hypo thesis. In the experimen ts, simple con-strained die compac tion perform ed in static com-paction was compared to that done w ith a sinusoidalpressure-time variation. Figure 1 is reprod uced fromthat work . This shows that for lead reinforced withlarge amounts of alumina, compac tion is greatlyaided by pressure cycling. However for pure lead,compac tion is not aided by pressu re cycling. Theeffect is shown to be so strong that by the impositionof 100,000 l-Hz pressure cycles between 414 MPaand 0 MPa pressure, Pb-40% AlzOs will becomemore dense than unreinforced lead compacted at414 MPa for 100,000 s. Related work [13] alsodemonstrated that the density is more uniformlydistributed in these cyclically proce ssed com positecompacts.In a previous paper [l], we suggested that the The previous work considered several possibleapplication of cyclic, instead of static, pressure m ay mechanisms by which density might be improved andaid the densification of metal-ceramic powd er argued that elastic strain mis match is responsible.metallurgy compacts. We argued that upon a change Experiments show that there is generally a criticalin pressure, deviatoric stresses would be generated at pressure cycle amplitude that is required to initiatematrix-reinforcement interfaces and thos e stresses the enhancement in densification and beyond th is,in turn can produ ce p lastic deformation which can for a given number of cycles, the density increasesaid the filling of voids. As small strains may be almost linearly with increasing pressu re cycle

4283


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HUANG and DAEHN : EFFECT OF CYCLI( PRESSURE

- Cyclic^___ Static

85 - -o------ _+_-"----___-_--- ___--*c ____----_G%

hhn ber of Cycles or Tim e (see)Fk . ComP arison of compact consolidation under steady pressure (open symbols) and cyc[ic compactionin Pb-AhQ powder compacts w ith O%, IO%, 20%, and 40% (by volume) alumina. In the static case,the horizontal axis corresponds to time and the applied pressure was held at 414 MP a. In the cyclic case,Pressure cycled between 414 and 0 MPa and 1 Hz cycles were used. Note that in pressure cycling largechanges in density take Place for the highly reinforced compacts, but there is little change for those dilutein reinforcement.

amplitude. The interpretation of this is that beneatha critical pressure change, the metal at the matrix-reinforcement interface is not loaded to its yield stressat the extrem e pressu res and densification is notaided. Following this argument, one would expectthat a goo d figure of merit in analyzing suchexperiments would be the ratio of the Eshelbymismatch strain [14-161 to the yield strain of thematrix. This can be expressed as A/I API&,, where AjIis the difference in compressibility between the matrixand reinforcement, AF represents the pressure changeimposed, and 6 0 represents the matrix yield strain.If this term is above som e value, the consolidation ofa given green composite could be aided by pressurecycling.

The present work seeks to find whether the effectof pressu re cycling aids consolidation in otherPowd er Metallurgy (P/M) composite systems. andwhether the term, A/3 API t , , is a good characterizingfigure of merit. Here we examine combinations ofthree different particulate matrix metals and threeceramic reinforcements under pressure cycling andstatic compaction at room temperature. In summary,

we find that all the systems appear to follow acommon trend that can be expressed in terms ofA/?AP/G,~nd similar to our previous work [l], themechanical properties of the compact improvedbeyond those seen in static compaction by a muchlarger factor than density, suggesting that inter-particle adhesion is also greatly improved. Theseresults can be applied to the design of processes thatmay improve the consolidation of composites frommixed metal and ceramic powders.

EXPERIMENTAL PROCEDUREThe experimental work foIlows that of our

previous studies [1, 171. Five composite powd ersystems based upon three metal powders (Pb, Al, Zn)and three ceramic reinforcements (A1203, TiOz, Sic)were studied. Several scanning electron images wereused to roughly estimate average particle size andassess shape. Representative images of the initialmetal and ceramic powders are shown in Figs 2and 3, respectively. Note that im ages o f Pb andAlz03 particles are available in Ref. [I]. The averag e


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HUANG and DAEHN: EFFECT OF CYCLIC PRESSURE 4285

Fig. 2. Scanning electron micrographs of the metal powders: (a) Al, (b) Zn

particle size of Pb powder is about 10 pm and theshape tends to be globular. The Al and Zn pow dershave av erage particle sizes of about 7 and 5 pm,respectively, and are nearly sp herical in shap e. Incontrast, the ceramic reinforcements tend to be moreirregular in shape. AlzO s is 6 pm in averag e particlediameter and rather plate-like in shap e. TiOz is0.2 pm in averag e pa rticle size and sph erical in shap e,and agglomerates rather strongly. The SIC has awhisker morphology 1 pm in diameter 20 pm in

length on average and also tends to agglomerate.The p roperties of the components [18, 191 andcombinations studied are summ arized in Tables 1and 2, respectively.

To compare with the previous study, we chose touse 40% (by volume) loading of ceramic as ourstandard condition, and the relationship betweenpressure cycle amplitude and density is of primaryinterest. T he five composite powd er sy stems weremixed in a twin shell mechanical dry blender for


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4286 HUAN G and DAEHN : EFFECT O F CYCLIC PRESSURE

Fig. 3. Scanning electron m icrographs of the ceramic reinforcements: (a) SIC whiskers, (b) TiO z powders

30 min to provide a uniform distribution. Howe ver,for Pb/TiO*, PbjSiC, and Al/SIC powder systems,it was difficult to obtain a uniform distributionbecause of agglomeration in the original c eramicpowders. But during powder mixing, significantde-agglomeration took place as was observed byexamining the mixed powders with the scanningelectron m icroscope (SEM ). Further detail on this isprovided in Ref. [17].

More complete de-agglomeration occurred inthe PbjSiC system than in the Al/Sic system. T he

average sizes of remaining Sic whisker ag glomeratesin the PbjSiC and Al/Sic systems a re about 150and 600 pm, respectively. This is probably due to thelarger particle size and much higher density of Pbrelative to Al, which provides a greater force to breakthe Sic whisker agglomerates in mixing. Henc e, weexpect somewh at greater matrix-reinforcement con-tact in the PbjSiC system than in the Al/Sic system.In the Pb/TiO l system, significant de-agglomerationalso occurred. The average final size of Ti0 2 powderagglomerates is about 4 0 pm. However, there are still


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HUANG and DAEHN : EFFECT OF CYCLIC PRESSURE 4287Table I. The values of compressibility and the average particle size inthe studied metal and

ceramic powdersProperties Pb Zn Al A1201 TiOz Sicb x 1O-6 (MPam) 22.37t 13.32t 13.147 4.17t 4.54t 3.85tAverage particle size (pm) 10 5 7 6 0.2 I:20 (d:l)tData from Ref. [18].fData calculated using b = 3(1 - 2v)iE from Ref. [19].

a few large agglomerates existing after mixing, whichare about 200 km in size. In this system, the TiOtparticles tend to essentially coat the Pb particles.

Compaction was carried out in a standard rightcylindrical die 12.7 mm in diameter. For eac h system ,the same mass of powd er was used to produce acompact about 9 mm in height. Static compactionwas carried out at 414 MPa for 10,000 s. Cycliccompaction was conducted by simply imposingsinusoidal force cycles to 100,000 cycles at 10 Hz.This provided the same time at pressure for bothstatic and cyclic compa ction. In all cases, except whenAP was 483 MPa, the maximum pressure in the cyclewas held at 414 MPa, and the minimum pressure w asvaried to control the cycle amplitude. In the casewhere A P was 483 MPa, the low pressure was setto zero. In all cases, loading was performed on aservo-hydraulic test frame in closed-loo p loadcontrolled mode. The accuracy and precision of theextreme loads are within 1% of their nominal values.

After compaction, the composite green density o fthe cylindrical sample s was determined by simplymeasuring the mass (to within $0.001 g) and thevolume (by measurem ents of the right cylinder to+O.O l mm precision). The fractional density is theratio of the bulk density of the specimen to thetheoretical density o f the powde r composite whichis determined by the simple rule of mixtures. In allcases, the compacts took the shape of the die cavityafter extraction and showed a diameter about 0 .3%greater than that of the die cavity owing to elasticspring back . To test the reproducibility of the densitymeasurem ents, nominally identical replication of 12compaction conditions of the Pb/A1203 systems wererun 2-3 times each . In all cases, the variance inmeasu red density between identical conditions was0.6% or less. The average standard deviation of theseexperiments is 0.17%.

In order to estimate the green tensile strengthof many composite systems that contain alead matrix, small rectangular parallelepiped bars12.7 mm x 3.2 mm x 2.0 mm were cut from the

center of the samples using a diamond saw andsubjected to three-point bend testing. In each case,the bars axis is normal to the compaction direction.The testing conditions were the same as the earlierstudy in the Pb/A1203 system [l]. No replicateexperiments were run to assess the reproducibility ofthis procedure. Howev er, from examination of thedeviation of similarly developed data (trend-line inFig. 10 of ref. [l]), a variance of 20% of the valueor less is expected. After testing, the fracture surfacesof composite powd er compacts were examined bySEM.

RESULTS

Effect of pressure cy cle amplitudePbIA1203, ZniALO ,, and Al/A lzO , syst ems. Th e

composite powd er compacts consisting of 40% AhO3ceramic powders and various metal powders wereconsolidated under static and cyclic pressu re con-ditions with the maximum pressure h eld at 414 MPaand variable low pressu res. Fractional density asa function of pressure cycle amplitude is show n inFig. 4. All three systems exhibit a region at lowpressure cycle amplitudes where density varieslittle with pressure am plitude, and beyond a criticalpressure amplitude there is another region wheredensity increases nearly linearly with pressureamplitude. The critical pres sure ch ange, APctit, is140 MPa for the Pb/A120 3 system, 210 MPa for theZn/AhO, system, and 32 0 MPa for the Al/Al,O,system.

There is one small unusual feature in these data,i.e. for the Zn-AlzOj system , the density actuallydecrea ses slightly with increasing pressu re cycleamplitude in a small pressu re amplitude region.It appe ars that this is due to a significant timedependence of the zinc powd er at room temperatureand because as the pressure cycle amplitude increases,the time-averaged stress in the compact decreases.Thus, the consolidation rate decrea ses with increasingAP. Also, zinc is interesting from the point of view

Table 2. Experiments performed in pressure cycling and static consolidationReinforcement/Metal Pb Zn AlALO,

TiOzSicNone

Several Vr(see Ref. [l])

40%40%

Isotropic-no mismatch

40%

Not studiedNot studiedHexagonal-

produces mismatch

40%

Not studied40%

Not studied


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4288 HUAN G and DAEHN : EFFECT OF CYCLIC PRESSURE

Fig.an d 4. ffect of pressure cycle amplitude on the fractional density for P&40 % ALO ,, Zn40% A1 201,A14 0% AhO , pow der systems. All of the data tested in the cyclic pressure conditions were measuredafter 100,000 cycles at 10 Hz.

that it is an h.c.p. metal and the imposition ofpressure on an aggregate of h.c.p. grains will producedeviatoric stresse s that can, in principle, induceplasticity. This effect was also studied by consolidat-ing pure Zn powder in static and pressure cyclingconditions [17]. In the static pressure conditions, Znpowders were compacted at 414 MPa for 30, 1000,and 10,000 s. Also compaction was run for the sameamount of time with 1 and 10 Hz sinusoidal pressurecycles between 0 and 414 MPa. For zinc, the timedependence was much stronger than the pressurecycle depend ence. Increasing the dwell time duringstatic compaction from 30 to 10 ,000 s raised thedensity from 93% to 96%. Pressure cycling onlyadded about 1% to the density and the 1 Hz cyclesgave higher densities than the 10 Hz ones despite theimposition of fewer cycles. This show s that there isa complicated time dependence in the zinc system,but consolidation is not strongly enhanced by thelevel of pressure cycles that are imposed. These resultswere quite different from the Pb and Al caseswhich show fairly rate-insensitive but cycle-numberdependent behavior in these experiments.

Pb/AlzOs, PblTi02, and PbjSiC systems. Figure 5shows results from all three reinforcements with alead matrix. There is a clear AP,,, in the Pb/A120jand PbjSiC systems , but the transition between lowand high pressure cycle amplitude behavior is moreobscure in the Pb/TiOz system (but the slope of thelines does increase at large AP values, w hich we canuse to sugges t a AP,,,,). In this case, the increase indensification at low cycle amplitudes is likely dueto the break-up of significant Ti02 aggregates insomething like compression fatigue

The difference in compressibility between th eceramic and metal phas es is very similar in these threecases. Therefo re, the data in Fig. 5 give insight asto how changes in particle shape (and possiblyagglom eration) affect densification in pressu recycling. It is interesting to note that in each case, thevalues of APall are similar as are the slop es of the linesat large pressure cycle amplitudes. The differentreinforcements mostly affect the absolute values offractional density in a way tha t migh t be intuitivelyexpected.

AIIAlzO, and Al/Sic systems. Part of themotivation to study aluminum/alumina and alumi-num/silicon carbide comes about because bothsystems hav e been extensively stu died as potentialstructural materials. The results on densification as afunction of pressu re cycle amplitude are presented inthe usual way in Fig. 6. The critical p ressure cycleamplitude in the Al/SC system is significantly lowerthan that in the Al/A1203 system. To first order, weexpe ct that the similar compressibility misma tchbetween matrix and reinforcement should produce asimilar critical pressure change. The relative differ-ence between the AP,,, values might be attributedto the varied morphologies of the reinforcements.Whisker-like reinforcements may be expected to givereduced critical mismatch strains owing to greatergeom etric con straint [20], but a rigorous analysis th atwould directly apply to this case is difficult. Note thata very similar trend is seen in the lead system with theSIC and AllO reinforcements, althoug h the trend isnot as strong. Also, the SIC system tends toagglom erate much more significantly. T his may alsoaffect the absolute values of density. It might also be


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HUAN G and DAEHN : EFFECT OF CYCLIC PRESSURE 4289

0 loo 200 300 400 500AP UtdPa)

Fig. 5. Effect of pressure cycle amplitude on the fractional density for Pb40% AhO jpr Ph40 %T iO+and P&IO% Sic, powder systems. All of the data tested in the cyclic pressure conditions were measuredafter 100,000 cycles at 10 Hz.

argued that the lower AP , ,, ,is affected by the break-upof Sic aggregates.Green str ength in bending

The mechanica l properties of the three compositesystems with a lead matrix were examined bymeasuring the bend strength of small machined bars.The results are shown in Fig. 7. In each case studied,cyclic compaction greatly improves the strength o fthe green compo site and strength is generally

enhanced with increasing numbers of applied cyclesand pressure cycle amplitude. This is in accord withthe fractional density results. Howe ver, th e relativestrength changes are much greater than the relativechanges in density. For example, while density mayincrease lo% , strength can increase by a factorof 4-6. This shows that interparticle bonding is alsostrongly im proved by pressu re cycling. Also, therelative strengths of the three systems seem to showa rather consistent pattern where the AlzOi and

Fig. 6. Effect of pressure cycle amplitude on the fractional density for AlMO% A1~01~ and A140% Sic,powder systems. All of the data were measured after 100,000 cycles at 10 Hz.


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4 2 9 0 HUAN G and DAEHN : EFFECT O F CYCLIC PRESSURE

El Pb/AW,p1 2 0 ?? PblSiCw

?? Pb /Ti O ,p

!l!t r t i C Cyclic Cyclic

P.414 M Pa AP414 MPa AP-414 MPa10,000 set 10,000 c y c l e s 100 , 0 00 cycles

CyclicAP=483 MPa100,000 cycles

Fig. 7. Compa rison of green strength between static and cyclic compaction for Pkeram ics compositesystems. Cyclic data were obtained at 10 Hz. In the case of AP = 4 83 MPa, the peak pressure was also483 MPa. In th e other cases, the peak pressure was 414 MPa.

SIC reinforced systems show similar strengths afterthe same processing route, while the TiOl systemis significantly weaker than the A&Or or SICreinforced composites after identical processing. W eexpect that because the TiOl particles are so muchsmaller than the lead particles, they tend to coat theinitial particle surfaces and this inhibits interparticlebonding.M icr os t r uc tu r a l obs e r va t ions

Comparisons of the fracture surfaces of PbjSiCand Pb/TiOz composite powder compacts obtainedunder static and cyclic pressu re conditions are show nin Figs 8 and 9, respectively. All the images presentedhere were taken near the center o f all specimens frommachined bars that were broken in bending. In thePbjSiC system, the images show that a larger numberof elliptically shap ed cavities ex ist in the static casethan in the cyclic case. The size of these cavitiesis similar to that of the Sic whisker agglomeratesafter mixing. The cavities presumably develo p theirelliptical shape owing to axial compaction. Theexistence of the fewer and smaller cavities in the cycliccase suggests that Pb particles have become heavilydeformed and appear to be somew hat able topenetrate the Sic whisker agglomerates and mightpossibly bond with whiskers. Also note that in thehigh magnification images, particles appea r to bediscrete after static comp action, but the materialis much more homogeneous and continuous aftercyclic compaction.In the Pb/Ti02 system , the image in the static caseshows little evidence of either gross plastic defor-mation of the lead or particle-particle bonding. Incontrast, the surface of the cyclically compacted

sample is much m ore homogeneous and less granular,suggesting much better interparticle bonding.

The fracture surfaces of Al/SIC and Al/A120rcomposite powder compacts obtained under staticand cyclic pressu re conditions are show n in Figs 10and 11, respectively. Pressure cycling producessignificantly improv ed den sification relative to thestatic condition as well as more extensive particledeformation and apparen t interparticle bonding.Many of the Al powde rs in the static case retain muchof their original particle shap e even thou gh plasticdeformation occurs on the Al particle surface. Incontrast, the original particle shape of Al powders inthe cyclic case can no longer be detected, with theexception of a few of the smaller Al powders.

DISCUSSION AND ANALYSISThese results show that the five composite systems

with relatively different combinations of materialcharacteristics and morphology behave very similarlyin pressure cycling. Repeated pressure changesconsistently significantly improve compact density,green strength, and appear to produce morehomogeneous microstructure with improved inter-particle bonding. We have proposed that this effect isdue to compressibility mismatch between the matrixand reinforcing phases inducing plastic deformation.A significant amount of suppo rt for this ideawas presented in a previous publication [l]. Forexample, the observation that pressure cycling doesnot significantly improve the densification ofsingle-phase powders of cubic crystals is in support ofthis idea. Also, other possible mechanisms forenhanced densification were considered [l]. Possibly,


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HUAN G and DAEHN : EFFECT OF CYCLIC PRESSURE 4291the most plausible other explanation is that pressurecycling produces sample heating owing to plasticwork , or punch-die friction (elastic deformatio n canalso produce a temperature change, but these heatflows are reversible). Indeed, pressu re cycling doescause moderate temperature rises. At 10 Hz at steadystate (i.e. over 2 h of cycling), a tempe rature rise of5C is measured on the die (upon removing thecompact, we also note the temperature rise of thesample and die are quite similar). How ever, at 1 Hz,there is no detectable temperature rise on the die. Asthe change in measured density is the same in bothcases, the effect of temperature rise can be discounted.Furthermore, similar temperature rises take placein cyclic comp action of pure metals, and cycliccompaction at 10 Hz only slightly increased thedensity of pure lead and zinc com pacts relative tostatic compaction.

The main purpose of this paper is to examine ifthere is a first-order trend th at runs through severaldata sets. By the logic presented briefly in theintroduction and a little more thoroughly elsew here[l, 13, 17,211, we believe that if mismatch strains areresponsible for improv ed densification, the ratioAfiAP/c, should represent a rough, but predictivefigure of merit fo r pressu re cycling compaction ,so long as data sets are compared at the samereinforcement fraction and number of cycles. Ofcourse, chang es in relative sizes of the metal andreinforcement, differences in particle morpholo gy,differences in initial packing density, and compli-cations like agglom eration are fully expe cted tomodify this first-order model.

In order to assess this figure of merit, the yieldstrain (or yield stress and Youngs modulus) of thematrix powd er materials must be known. This was

Fig. 8(a,b). Caption overleaf.


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4292 HUAN G and DAEHN : EFFECT O F CYCLIC PRESSURE

Fig. 8. Fracture surfaces of Pb-40% Sic, green composite powder com pacts broken in three-pointbending; (a) and (b) show the material tha t w as consolidated at 414 MPa static pressure for 10 ,000 s (c)and (d) show m aterial subjected to 1 00,000 cycles at IO Hz to a peak pressure of 414 MPa. These imageswere taken near the center of specimens and the compaction direction is vertical.

estimated by simple uniaxial com pression tests onunreinforced metal compacts. Pure metal powderswere first compacted at 414 MPa for 10 ,000 s toa final L/D ratio of 2.0 and the fractionaldensity fortuitously turned out to be 93% for each ofthe three metal com pacts. The results of thecompression tests on each of these compacts areshown in Fig. 12. The exact yield point is difficult toprecisely identify in each case, but estimates that arereasonable for this first-order analysis a re available.We estimate the yield stresses of the Pb, Zn, andAl systems to be approximately 30 MPa, 90 MPa,and 7 0 MP a, respectively. In the case of the Alsystem, the compact began to peel apart long beforepeak load, suggesting the actual yield strength ofthe material was not reached. As a result wh ile

70 MPa may appear to overestimate the aluminumflow stress based on the data shown in Fig. 12, thismay still underestimate the actual material flowstress.

The data shown in Figs 4-6, as well as data fromthe Pb-A 1201 system published earlier [1], is replottedin terms of the change in fractional density relative tostatic compaction vs the normalized mismatch strain,A jAP/c, in Fig. 13. The results show that despitedifferences in morphologies, relative sizes, andproperties, all six data sets nearly collapse toone relatively narrow band. Specifically, density isincreased by pressure cycling in each system when themismatch strain becomes grea ter than approximatelythree times the matrix yield strain, and verysignificant changes in density a re observed after


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Fig. 9. Fracture surfaces of P&4 0% Ti02 , green composite powder com pacts broken in three-pointbending; (a) shows the material that w as consolidated at 414 MP a static pressure for 10 ,000 s; (b) showsmaterial subjected to 100,000 cycles at 10 Hz to a peak pressure of 414 MPa.

100, 000 pressure cycles when the mismatch strainbecomes about six times the matrix yield strain.

These da ta can be compared with those on powderconsolidation of white cast iron powders subjected topressure through 10 cycles of ferrite-austenite phasetransformations induced by temperature cyclingperformed by Ruan o .et al. [22]. In this case, thecalculated normalized mismatch strain is about 140 ,but full densification was obtained in 10 phasetransition cycles. In isothermal deformation at

the same pressure, densities no higher than 98 % oftheoretical could be obtained. Thus, the proposedfigure of merit of the normalized mismatch strainseems to give insight. Many cycles with smallmismatch strain can give similar effects to a few cycleswith very large mismatch strains.

Of course, the explanation presented here falls farshort of a careful first-principles model. Factors suchas relative particle size, agglomeration, particle shape,and possibly the frictional or sticking conditions


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4294 HUANG and DAEHN : EFFECT OF CYCLIC PRESSURE

Fig. 10. Fracture surfaces of A14 0% Sic, green composite powder com pacts broken in three-pointbending: (a) shows the material that w as consolidated at 414 MP a static pressure for 10 .000 s. (b) showsmaterial subjected to 10 0,00 0 cycles at IO Hr to a peak pressure of 414 MPa.

at the matrix-reinforcement interfaces are fullyexpected to influence the results. Thus, one would no tnecessarily expect the data shown in Fig. 13 tocollapse to a single line, and more variation than isshown may well be expected even within the confinesof our explanation. While it would be desirable to model this process from first principles, the authorsbelieve this would presently be a formidable task.Regardless of this, it appears that this mechanism cansimply be used to improve the consolidation of

powder composites as many hot or cold compacdevices can simply be equipped to apply cyclic 1at rapid load ing rates.

CONCLUDING REMARKSSix composite powder systems w ith 40% reinfc Irce-

ment have been studied both in static compaction andin pressure cycling compaction under val :iouspressure a mplitudes. A very consistent trend ran

Zionoads


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Fig. 11.Fracture surfaces of AlAO % A120 jp green composite powder compacts broken in three-pointbending; (a) shows the aterial tha t w as consolidated at 414 MP a s tatic pressure for 1 0,00 0 s, (b) showsmaterial subjected to 100,000 cycles at 10 Hz to a peak pressure of 414 MPa.

through all these data. At small pressure cycleamplitudes , cycling did not improve co nsolidation;how ever beyond a critical amplitude, densityincreases in a nearly linear m anner with increasingpressure cycle amplitude. We have proposed that thiseffect takes place becaus e the elastic compressibilitymismatch strain (APAP) can become large relative tothe yield strain of the matrix (co). When the elasticmismatch becomes much greater than the matrixyield strain, the impo sed ch ange in hydro static stress

can induce plastic deformatio n. As the entireexperimen t takes place in compres sion, this plasticstrain can aid in closing porosity [23]. This is similarto the manner in whic h the plastic deformatio n inrolling or extrusion can close porosity in materials,except that in this case plastic deformation accumu-lates over small plastic strain increments in eachcycle.

When all six data sets were compared in terms ofchange in density vs A/SAP/c,, we find that despite


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4296 HUAN G and DAEHN : EFFECT OF CYCLIC PRESSURE

-0.~00 0'01 0.02 0.03 0.04 0.05

Fig. 12. Norm alized load-displacement curves taken incompression for compacts of pure Pb, Zn, and Al powdersconsolidated to 93% of theoretical density. The range of u0inferred is 2244 MPa for Pb, 655100 MPa for Zn, and

4658 MPa for Al.

significant differences in the morphologies andproperties of the systems, all the data for change indensity vs AjAP/t, collapse to one trend whereinwhen the normalized strain mismatch is less than 3,there is little or no effect of pressure cycling, andbeyond that, significant increases in densificationwith increasing pressure cycle amplitude are shown.Although we present this explanation withoutdetailed analysis, we believe the experimentalevidence to be quite convincing. Our first paper onthis [l] also gives much support for the concept.Specifically there, it was shown that the effectvanishes if powder systems consisting of all isotropicmetal are compacted, and that to a very goodapproximation, only the number of pressure cyclesmatters and the pressure cycling rate is relativelyunimportant. The present work empirically shows the

-2-l I 8 I0 2 4 6 3 10

Normalized Mismatch Strain ( APAp/G)Fig. 13. Difference in fractional density between the staticand cyclic compaction as a function of the normalizedmismatch strain in six composite systems. All cyclic datawere obtained after 100,000 pressure cycles. The yieldstresses used to calculate the normalized mismatch strain foreach metal powder are assumed 30 MPa for Pb, 90 MPa for

Zn. and 70 MPa for Al.

effect to be quite ge neral and the effect can bcextended to new systems through consideration otA/?AP /t, as a normalizing term. Also, even with onlythis empirical understanding of this effect. it mightbe used profitably in practical powder compactionas cyclic loads a re easy to apply rapidly and thematrix yield strain can simply be reduced by heatingthe powder compacts. This makes the effect useful ina wide num ber of systems.

1.2.

3.4.5.

6.

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10.11.12.

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20.21.

22.23.

REFERENCESHuang, C. Y. and Daehn, G. S.. Acta metall., 1996, 44,1035.Ramakrishnan, P., in Adv ances in Powder Metallurgyand Particulate Mat erials, Vol. 9, ed. J. M. Capus andR. M. German. Metal Powder Industries Federation.Princeton, NJ, 1992, p. 237.Hunt, W. H. Jr and Rodjom, T. J., Ibid. p, 21.Sagar, R., Madan, P. K., Kumar, M. and Sachdeva, S..Ibid. p. 45.Mileiko, S. T.. in Fabricati on of Comp osit es, Vol. 4,of Handbook of Composites, ed. A. Kelly and S. T.Mileiko. Elsevier Science Pub., New York, 1983. p. 221.Rack, H. J.. in Metal Matrh Composit es: Processingand Interfaces, ed. R. K. Everett and R. J. Arsenault.Academic Press, San Diego, CA, 1991, p. 85.Singh, B. N., Pow der Metall ., 1971, 14, 277.Lange, F. F., Atteraas, L., Zok, F. and Porter. J. R.,Act a metal l., 1991, 39, 209.Li, S., Khosrovabadi, P. B. and Kolster, B. H..in Adv ances in Pow der Metallurgy and ParticulateMaterials, Vol. 2, ed. J. M. Capus and R. M. German.Metal Powder Industries Federation, Princeton, NJ1992, p. 185.Besson, J. and Evens, A. G., Act a met all., 1992. 40,2247.Li, E. K. H. and Funkenbusch, P. D.. Metall. Trans.,1993, 24A, 1345.Lenel, F. V., Pow der Metallurgy Principles andApplications. Metal Powder Industries Federation.Princeton, NJ, 1980.Huang, C. Y. and Daehn, G. S.. in Net Shape Processingof Powder Mat erials, Vol. 216, ed. S. Krishnaswami.R. M. McMeeking and J. R. L. Trasorras. ASME,New York. 1995, p. 69.Ashby. M. F., Gelles, S. H. and Tanner, L. E..Phil. Mag., 1969, 18 , 757.Eshelby, J. D., Proc. R. Sot . Lond., 1957. 241A, 376.Clyne, T. W. and Withers, P. J. (eds), in An Introductionto Metal Mat rix Composit es. Cambridge UniversityPress, New York, 1993.Huang, C. Y.. Application of pressure cycling onmetal-matrix composite processing. Ph. D. thesis, TheOhio State University. Columbus, OH, 1996.Simmons. G. and Wang, H., Single Crystal Elastic,Const ant s and Calculat ed Aggregate Properties: AHandbook. MIT Press, Cambridge. MA, 1971.Lackey, W. J., Stinton, D. P.. Cerny, G. A.,Fehrenbacher. L. L. and Schaflhauser, A. C., in CeramicComponentsfor Engines, ed. S. Somiya, E. Kanai and K.I. Ando. Elsevier Science Pub., New York, 1986, p. 770.Daehn, G. S.. Anderson. P. M. and Zhang, H., Scriptamet all .. 1991, 25, 2219.Hwdng , C. Y. and Daehn, G. S.. m Superplasticityand Superplastic Forming, ed. A. K. Ghosh and T. R.Bieler. TMS, Warrendale, PA. 1995.Ruano, 0. A., Wadsworth. J. and Sherby, 0. D.,Metall. Trans., 1982. 13A, 355.McClintock, F. A., ASME. J. appl. Mech., 1968, 35,363.

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