Effect-of-Nb-particles-on-the-flow-behavior-of-TiA

EffectofNbparticlesontheflowbehaviorofTiAlalloyJianboLia,YongLiua,*,BinLiua,YanWangb,KunZhaoa,YuehuiHeaaStateKeyLaboratoryofPowderMetallurgy,CentralSouthUniversity,Changsha410083,PRChinabSchoolofAeronauticsandAstronautics,CentralSouthUniversity,Changsha410083,PRChinaarticleinfoArticlehistory:Received12August2013Receivedinrevisedform4October2013Accepted5October2013Availableonline9November2013Keywords:A.Titaniumaluminides,basedonTiAlB.DeformationmapD.MicrostructureabstractHightemperaturecompressivedeformationbehaviorsofPM-TiAlalloycontainingNbparticles(Tie45Ale5Nbe0.4W/2Nb(at.%))wereinvestigatedattemperaturesrangingfrom1050
Cto1200
C,andstrainratesfrom0.001s1to1s1.Theflowcurveswereemployedtodevelopconstitutiveequations,andtheapparentactivationenergyofdeformationQwasdeterminedas447.35kJ/mol.Arevisedprocessingmapwasconstructedonthebasisoftheflowstress,whichcanaccuratelydescribethedeformationbehaviorsandpredicttheoptimumhotforgingcondition.Theadditionof2%NbparticlesreducesthepeakstressandincreasestheactivationenergyofTiAl-basedintermetallic,however,itincreasestheinstabledomainintheprocessingmap.
2013ElsevierLtd.Allrightsreserved.1.IntroductionTiAl-basedalloysareofgrowingintereststotheaerospace,automotiveandenergyindustriessincetheyofferaremarkablecombinationoflowdensity,highspecificmodulusandstrength,goodcreepandoxidationresistance[1].However,lowductilityanddamagetoleranceatroomtemperatureaswellaspoorworkabilityatelevatedtemperaturesrestricttheapplicationsofTiAl-basedalloys[2,3].However,Ward-Closeetal.[4]proposedthatductileparticlescanimprovetheoveralldeformationabilityoftheinter-metallicmatrixandrestrictthegrowthofcracksduringthedeformation.Raoetal.[5]foundthatniobiumparticle-reinforcedTiAlalloyspossessquitepromisingimprovementsinbothmono-tonicfracturetoughnessandcyclicfatiguethresholdcomparedtomonolithicTiAl.Liuetal.[6]showedthatthepresenceofbphasecangreatlyimprovethedeformabilityathightemperatures,andthecontentofbphasecanbeincreasedbyintroducingNb.Mean-while,itisbelievedthatgrainrefinementbyhotworkingisaneffectiveapproachtoenhancetheoverallperformanceofTiAlal-loys[7].Thus,itisofgreatimportancetostudythehotdeformationbehaviorsofthesealloys.Inrecentyears,processingmapshavebeendevelopedbasedontheDynamicMaterialsModel(DMM),describingtheabilityofthematerialtodissipatepowerthroughmicrostructuralchangesduringdeformationprocessing.Themapshavebeenusedtoevaluatethethermaldeformationmechanismsofmagnesiumalloys,austeniticsteel,Ni-basedsuperalloys,titaniumalloysaswellasintermetallics.However,fewstudieshavefocusedontheapplicationofDMMforintermetallicscontainingductileparticles.Inthisstudy,arevisedprocessingmapwasconstructedbycombiningtheinstabilitymapandthepowerdissipationmapinordertoachieveanindepthunderstandingofthedeformationmechanisms.2.ExperimentalPowdermetallurgytechniquehasbeenexploredtosynthesizePM-TiAlalloybymixingalloypowderwithanominalcompositionofTie45Ale5Nbe0.4W(at.%)andNbelementalpowder(2%,at.%)followedbyhotisostaticpressing(HIP)at1250
Cfor5handapressureof170MPa.Thecompressionspecimenswithadiameterof8mmandaheightof12mmwerecutbyelectric-dischargemachining,andthecompressiontestswereconductedattemper-aturesrangingfrom1050
Cto1200
C,andstrainratesfrom0.001s1to1s1usingathermecmastor-Zthermalsimulationmachine.Thesampleswereheatedat5
C/s,andhomogenizedfor5minatthedeformationtemperature.Thesamplesweredeformedwithamaximumnominaldeformationof60%,finallythesampleswerequenchedtoroomtemperaturetopreservethedeformedmicrostructure.Inordertoreducethefriction,graphitepaperandglassprotectivelubricantwereused.Themicrostructureswerecharacterizedbyscanningelectronmicroscopy(SEM)inback-scatteredelectron(BSE)mode,trans-missionelectronmicroscopy(TEM)andX-raydiffractiontechnique(XRD).*Correspondingauthor.Tel.:þ8673188830406;fax:þ8673188710855.E-mailaddresses:[e-mail],[e-mail](Y.Liu).ContentslistsavailableatScienceDirectIntermetallicsjournalhomepage:
2013ElsevierLtd.Allrightsreserved.(2014)22e283.Results3.1.StartingmicrostructureAsshowninFig.1(a),thestartingmaterialismainlycomposedoftwoparts:matrixmaterial(darkregion)andwhitespheroidalpar-ticlesdistributinginmatrixrandomlywithasizeabout100mm.ItrevealsthreedifferentregionsofthestartingmaterialwhicharelabeledasA,B,Crespectively.CrystallographicstructureofthesampleswasdeterminedbyXRDmethodwithCuKaradiation,andthetubevoltageandcurrentwere45kVand40mArespectively.XRDanalysisshowsthatthealloyisprimarilycomposedofb(B2),g,a2phase,asshowninFig.1(b).CombiningwiththeTieAleNbsys-tem[8],itcanbedeterminedthatAregionisb(B2)phase.Intheb(B2)phase,theomegaphaseshowsgreycontrastandoccupiesmorethan10%(volumefraction)oftheb(B2)phase.Bregionrepresentsa2phase,andCregionrepresentsgphase.ThereisalittlepureNbparticleresiduesintheb(B2)phase,asshownbythearrow.3.2.FlowbehaviorsThetruestressestraincurvesobtainedfromisothermalcompressiontestsareshowninFig.2.Thecurvesexhib