DOC

Mechanical alloying and characteristics of spark plasma sintering of Ti-A1 composite powders

By Gloria Shaw,2014-12-15 02:49
6 views 0
Mechanical alloying and characteristics of spark plasma sintering of Ti-A1 composite powders

    Mechanical alloying and characteristics of spark plasma sintering of Ti-A1 composite

    powders

    Science

    PressTrans.NonferrousMet.Soc.China17(2007)s608s613

    Transactionsof

    NonferrousMetais

    SocietyofChina

    WWW.CSH.edu.cn/ysxb/

    Mechanicalalloyingandcharacteristicsofsparkplasma

    sinteringofTiA1compositepowders

    CHENHua(陈华),XUETao(薛涛),MENHongzhi(f]洪志)

    SchoolofMaterialsScienceandEngineering,ChangchunUniversityofTechnology,Changchun130012,China

    Received15July2007;accepted10September2007

    Abstract:ThemechanicalalloyingprocessofTi

    Alcompositepowderswerecamedoutbyuseofhighenergyballmillingmachine.

    Structurevariationsofpowdermixturesduringmechanicalalloyingandcharacteristicofsparkplasmasinteringwereinvestigated.

    Theresultsshowthatduringmilling,TiA1.TiAlandTi2Alphaseintermetalliccompoundsareformed,simultaneouslywithpowder

    refinementforthe(TiH2-45Al0.2Si5Nb1and(TiH2-45Al0.2Si

    7Nb1mixtures.Theparticlesizesoftwopowdermixturesareless

    than300amaftermillingfor30h.Sinteringprocessofthemilledpowdercanbecompletedinashorttimebysparkplasmasintering.

    andthesinteringmicrostructureiscomposedoffineandhomogeneousTiAlandTi3AIphase.

Keywords:mechanicalalloying;TiAIintermetallics;sparkplasmasintering;microstructur

    e

    lIntrOductiOn

    TiAlbasedalloysaredesirableforhightemperature structuralpartsduetotheirlowdensity,highstrengthto massratioandexcellentoxidationresistance,andthat hasgreatadvantagesinthelightmassdesignofaerospace andautomotiveparts.However,singlephase

    TiAl(gammar)basedalloyshavehighbrittleness,and aredifficulttodeformatroomtemperature.Previouslya greatnumberofinvestigationsshowthat

    lowtemperaturedeformabilityofduplexphaseTiAl alloyswithTiAl(gammar)andTi3Al(alpha2)canbe considerablyimproved.Theobjectiveofthisstudy focusesonduplexphaseTiAlalloysinindustrial applications121.

    DuplexphaseTiAlalloyscanbeobtainedby

    powdermetallurgy(PM)technology.However,during traditionalPM.thediffusioncoefficientsofTiandAl atomsaredifferentandthereactivediffusiondistanceof atomsislong,leadingtoKirkendalleffect.Thus,agreat numberofdiffusionporesexistinsintering

    microstructure,anditisdifficult[3-41toobtainTiAl alloyswithhighperformance.Although,beforereaction sintering,Ti,Alandotherelementscanberefinedby largerdeformation,e.g.,extrusionorrollingprocess,and theultimateporosityofsinteredpartscanbedecreased, thiswayisdifticulttorealizeinactualindustry application[56.

Beforereactivesintering,coolweldingfracturing

    

    coldweldingprocessofmixturepowdercanbe repeatedlycarriedoutbyuseofhighenergymilling. Underhighenergystrikingcondition,finercompound particlescanbeformed.Thisisaneffectivemethod whichcanobtainfinepowdersandmicrostructure. Comparedwithtraditionalextrusionorrollingprocess, themillingprocessismoresimpleandpossible[78.

    Inthisworktheauthorsfocusedonthemechanical alloyingandcharacteristicsofsparkplasmasinteringof TiAlcompositepowders.

    2Experimental

    Inthisexperiment.twokindsofcomponentTiAl

    mixturepowderswereinvestigated.Particlesizesand purityofmixturepowdersareshowninTable1.The powderparticlesweremixedaccordingtothe compositionsof(Till2-45A10.2Si5Nb1and(Till,-

    45A10.2Si7Nb)(molefraction,%).Thefinalproducts aftersinteringwereexpectedtobeTiAlandTi1Alphase. Ballmilloperatinginaninertatmospherewasused. Thepowderswereweighedinagloveboxfilledwith highpurityargon,balltopowdermassratioofabout10,

    Foundationitem:Project(20050513)supportedbytheScienceandTechnologyDevelopme

    ntProgramofJilinProvince,China

    Correspondingauthor:CHENHua;Tel:+8643I-857I6421;E

    mail:cherdaua63017@126.com

    CHENHua,etal/Trans.NonferrousMet.Soc.China17(2007) toavoidexcessiveincreaseofthelocaltemperature

    insidethevialandpowder,arestof60min/10hwas appliedduringmilling.Amillingspeedof360r/minwas choseninthisstudy.

    

    T

    a

    b

    le1Particlesizeandpurityoftestingpowders Powdermilledfor40hwassinteredatl200?for

    10minbyuseofsparkplasmasintering(SPS),rateof s609

    temperaturerisingwas5080?/min.andhotpressure

    was30-35MPa.

    Theasmilledpowderandsinteredmicrostructure wereexaminedandanalyzedbyXraydiffraction(XRD),

    scanningelectronmicroscopy(SEM),andtransmission electronmicroscopy(TEM).

    3Resultsanddiscussion

    3.1Morphologyandparticlesizeofmilledpowder Underhighenergymechanicalmillingcondition. (TiH2,A1,SiandNb)mixturepowdersundergogreat plasticdeformation.thusitiSclearlyseenthatthe particlesizegraduallydecreasesandmorehomogenizes withincreasingmillingtime.Fig.1showsSEMimages Fig.1SEMimagesof(TiH245A10.2Si

    5Nb)mixturemilledfordifferenttimes:Ca)5h;(b)15h;(c)25h;(d)30h;(e)35h;(D40h

    s610CHENHua,etal/Trans.NonferrousMet.Soc.China17(2007) oftheTill245A1.0.2Si.5Nb)mixturemilledfor5,15, 25,30,35and40h,respectively.Aftermillingfor5h

    (seeFig.1(a)),mostofpowderparticlesstillkeepinitial morphology;theaverageparticlesizeis4050"m.After

    millingfor15h,themixturepowderisobviouslyrefined (seeFig.1(b)),anaverageparticlesizeof810mcan

    beobtained.

    ~v'henmillingtimeincreasesto25and30h(see Figs.(1)and(d)),themostlyparticlessizearelessthan 0.5"m,andareeveninthenanometerscalerange.After

    millingfor35h,particlesizeincreasesalittleduetothe agglomeratingandweldingoffreepowder(seeFig.1(e)). Byfurthermilling,thepowderagglomeratedandwelded isbrokenup,particlesizeisreducedagain(seeFig.1(f)). Thetestresultsofthe(Till245A10.2Si7Nb)

    mixturepowderaresimilartooneofthe

    (Till245A10.2Si5Nb)mixturepowder.However,after millingfor40h,particlesizeofthe(Till245A1

    0.2Si7Nb,mixturepowderislessthanoneofthe

    (Till245A10.2Si5Nb)mixturepowder.Theparticle

    sizesobservedbySEMaremostlypowdergroupsdueto agglomeratingandwelding,especially,inthelateperiod ofmilling,

    Fig.2showsthevariationcurvesofparticlessize withincreasingmillingtime.Itcanbeseenthatthesize variationtrendofthe(Till245A10.2Si7Nb)mixtureis

    identicalwithoneofthe(Till245A10.2Si5Nb)mixture.

    Atearlystageofmilling(millingfor510h),particles

    sizerapidlydecreaseswithincreasingmillingtime,after millingfor30h,theminimumparticlessizecanbe obtained.Afterfurthermilling,changeofparticlessize

jsjtte

    Millingtime&

    Fig.2Variationcurvesofparticlessizewithmillingtime Fromtheaboveanalysis.itisobviousthatthesize ofTiA1mixturepowderpossessesacriticalvalueduring milling,becausemechanicalalloyingoccurswhilethe particlesizesarerefined.Andthatmechanicalalloying providesbothchemicalcompositionandmicrostructure moditications.

    Atearlystageofmilling,theprocessofparticlesize refinementisdominantstatus,powdermixturesare brokenintogranuleorflakeundercontinuousimpacting, shearing,rubbingandcompressingcondition.Because thematerialswithbcccrystalstructurehavearelatively largenumberofslipsystems,thedeformationiseasy. Howevertheslipsystemsarefewformaterialswithhcp crystalstructure,thustheyaremoreeasilybrokeninto granulesbyappliedforce[9].Moreover,duringmilling, workhardeninghasgoodinfluenceonpowder

    refinement.

    However,aftermillinglongtime,thesurfaceenergy ofpowderparticlesisincreased,powderparticlescanbe easilygatheredandcombined.Inthiscase,mechanical alloyingprocessisthedominantstatus.

    Microstrainisreduced,andthevariationofparticle sizeislittle,thusthecoolweldingandfracturingprocess ofmixturepowderattainarelativemovebalance.

    3.2Structurevariationofmilledoowder

    Fig.3showstheTEMimageandselectedarea

    electrondiffractionpattemofthe(TiH2-45A1.0.2Si.5Nb,

    mixtm'emilledfor35hand40h,respectively.Itcanbe seenthatthelamellarstructureisformedaftermillingfor 35h(seeFig-3(a)arrow).Thisoccursbecausestrong extrusionandimpactleadtoweldingofmixturepowder andformingofthelamellarstructureof"sandwich".The resultofdiffractionshowsthatarrowregionisTi,Al phase.Ti2Alphaseismetastable.Thisresultprovedthat metastablephaseisformedduringmilling.BetweenTi andAlelementcanfornqvariousmetastablephases. however,inthisexperiment,othermetastablephases havenotappeared.

    ?Fig-3(b)showstheTEMmorphologyofpowder

    milledfor40h.Itcanbeseenthatparticlessizearevery fine.andareaboutl00nn1.TheresultofTEM

    diffractionanalysisshowsformingTiAlandTi1Alphases. obviously,millingprocesscanformTiAlintermetallic compounds.

    Fig.4showstheTEMimageandselectedarea

    electrondiffractionpattemsofthe(TiH2-45A1. 0.2Si.7Nb)mixturemilledfor35hand40h.respectively. Aftermillingfor35h.itcanbeseenthatalotoffine particlesexistonthepowderslice,fineparticlesizesare aboutl0nnl(seeFig.4(a)),thediffractionpattemoffine particleshowsthediscontinuouspolycrystallineringis diactionspotofTiAlandTiAlphase.Aftermillingfor 40h,powderparticlesizesare200-300nnl(see Fig.3(b)),andamorphousphasedoesnotappear. Fig.5showstheXRDpattemoftheunmilled

    powderandpowdermilledfor40h.Itisnotedthatthe

Xraypattemsofboth(Till245A10.2Si5Nb)and

    En/uNls?uI.

    CHENHua,etal/Trans.NonferrousMet.Soc.China17(2007) (Till2-45A1-0.2Si-7Nb)unmilledpowdersarealmost identical(seeFig.5(a))andrevealTill2,AI,SiandNb phase.However,aftermillingfor40h.thediffraction peaksofAl,NbandSidisappear,newphasesofTiAI. Ti3AI,Ti2AIandNbHareformed(seeFig.5(b)),andthe diffractionpeaksarebroadenedandtheirintensities slightlydecreased.Thisresultsfromparticlesize refinementandtheaccumulationofmechanical strains[1012.

    Fig.3TEMimagesandselectedareaelectrondiffraction patternsof(Till245A10.2Si5Nb)powdermilledfor35h(a) and40h(b)

    Aftermillingforlongtime.itiSnotedthatTi.A1. A1.NborNb.Siintermetalliccompoundshavenot formed.SiandNbelementsaredissolvedintoTiA1. TiAlorTiH,phases.HELLWIGetal131have

    thoughtthatcontentsofNbinTiAlandTi3Alphaseare equa1.andthemaximumsolidsolutioncontentofNbin TiAlphasecanreach9%(molefraction).thecontents ofSiinTiAlandTi1Alphasearelessthan2%.Nb contentiS7%andSicontentiS0.2%,hence,theyare fullydissolvedintoTiAl'Ti1AlorTiH,phases. 300nm

    s6l1

    Fig.4TEMimagesandselectedareaelectrondiffraction

    pa~ernsof(TiH245A10.2Si-7Nb)mixturemilledfor35h(a); 40h(b)

    20/(.)

    20/(.)

    Fig.5XRDpa~ernsformixturepowders:(a)Unmilled; (b1Milledfor40h

    s612CHENHua,etal/Trans.NonferrousMet.Soc.China17(2007) 3.3Microstructurecharacteristicsofsparkplasma sintering

    Thesparkplasmasinteringprocesshasthree

    characteristics.First,themixturepowderscanberapidly heatedbysparkdischargebetweentheparticles.Because ofreducingdiffusedfreeenergyofatoms,diffusion processesofatomsareaccelerated,andrapidsintering processcanbeobtained.

    Second,duringsintering,highenergyparticles strikeinterfaceofpowderparticles,leadingtosurface evaporationofpowderparticles,therebySPSprocesscan affectthedeputationandactivation.

    Thud,byuseofSPS,powderscanbecompressedat acertainpressureandthushighdensievenfuUdensity TiAlalloycanbeobrained14-161.

    Becausepowdersarestruckduringmilling,lattice defectsandinstressofpowdersincrease.andtheactivity ofmilledpowderisimproved.Themuchfinerthe particleare,themorehighertheactivityis.Inaddition, finepowdersincreasediffusionsurface,anddiffusion distanceofpowdersareshort,thusthefu11sinteringtime issignificantlyshortened.Ordinaryvacuumsintering

    processoftenneeds2hormoretimeforTialloy, however,sinteringprocesscanbecompletedin10minat 1200?bySPS.

    Fig.6showsthemicrographsofthe(Till245A1

    0.2Si5Nb1andfTill2-45A10.2Si7Nb1mixturesintered

    atl200?for10min.Itcanbeseenthatfine,dense

    andhomogeneousmicrostructureareformedbySPS. Fig.6MicrostructuresofsinteredsamplesbySPS: (a)Till245A10.2Si5Nb;(b)Till245A10.2Si7Nb

    SinteringmicrostructureofthefTiH,45Al0.2Si7Nb1

    mixturehashigherdensification,andgrainsarefiner. MostofthegrainsofthefTiH2-45A10.2Si5Wo)are

    smallerthan2gm,theotherparticlesare2-4uJn(see Fig.6(a)).Thegrainsofthe(Till245A10.2Si7Nb)

    mixturearesmallerthan2gmandveryhomogeneous (seeFig.60011'

    Grainssizesofsinteringmaterialsdependonbotll powdercharacteristicandsinteringmanners.Before sintering.powdersmilledfor40hareveryfine.andfine intermetalliccompoundsareformed.Itisknownthat finepowderhaslargergrowingtrend.However,interface bondoccursamongpowderparticleswithSPSatshort time,i.e.,diffusionandmigrationofatomsiustoccurin boundariesbetweenparticles.Inthisstate, recrystallizationprocesshasnotyetbeencarriedout, thusfineglobulargrainsareobtained.

    XRDdiffractionanalyrsesofthef1'iH2-45A1

    0.2Si5Nb1andthefTiH2-45A10.2Si7Nb1milled

    powdersshowthatTiAlandTi3Alphasesareformed

Report this document

For any questions or suggestions please email
cust-service@docsford.com