Deformation and dynamic recrystallization behavior of a high Nb containing TiAl alloy

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DeformationanddynamicrecrystallizationbehaviorofahighNbcontainingTiAlalloy

LiangCheng?,HuiChang,BinTang,HongchaoKou,JinshanLi

StateKeyLaboratoryofSolidi?cationProcessing,NorthwesternPolytechnicalUniversity,Xi’an,Shaanxi710072,China

articleinfoabstract

Thehotdeformationanddynamicrecrystalliztion(DRX)behaviorofahighNbcontainingTiAlalloywerestudiedusinghotuniaxialcompressiontests.Thetestswereconductedattemperaturesof1000–1150°Candstrainratesof0.001–0.5sÀ1.DuetohighNbadditions,thisalloypossessesawidehot-workingwin-dow.Thestress–straincurveexhibitspeakstressatlowstrainfollowedbydynamicsofteningandsteady-state?ow.Thedependenceofthepeakstressonthedeformationtemperatureandstrainratecanwellbeexpressedbyahyperbolic-sinetypeequation.Theactivationenergy,Q,wasmeasuredtobe427kJ/mol(4.3eV)andthestressexponentwasmeasuredas4.16.Basedontheconventionalstrainhardeningratecurves(dr/deversusr),thecharacteristicpointsincludingthecriticalstrainforDRXinitiation(ec)andthestrainforpeakstress(ep)wereidenti?edtoexpresstheevolutionofDRXandecis0.92timesep.InordertocharacterizetheevolutionofDRXvolumefraction,theDRXkineticswasstudiedbyAvramitypeequation.ThelowAvramiexponentsoftheproposedequationindicatealowerrecrystallizationratecomparedtoordinaryalloys.Besides,theroleofbphaseandthesofteningmechanismduringhotdefor-mationwerediscussedindetail.

Ó2012ElsevierB.V.Allrightsreserved.

Articlehistory:

Received8October2012

Receivedinrevisedform10November2012Accepted12November2012

Availableonline23November2012Keywords:

HighNbcontainingTiAlalloyHotdeformation

Dynamicrecrystallizationbphase

1.Introduction

Inrecentyears,TNB(abbreviationforhighNbcontainingTiAlalloyswithabaselinecompositionofTi–(42–45)Al–(5–10)Nb–X[1])alloyshavebeenpaidmoreandmoreattentionandregardedasanewgenerationTiAlalloy.DuetohighNbadditions,thetensilestrength,creepstrengthaswellasoxidationresistanceoftheal-loyshavebeensigni?cantlyimprovedcomparedwithlowNb-bearingTiAlalloys[2–5].Becauseoftheirattractiveproperties,TNBalloyshavebeenconsideredforhightemperatureapplicationsinaerospaceandautomotiveindustries.However,theplasticdeformabilityofTiAlalloysispoorowingtotheirnaturalbrittle-ness,andhighNbadditionscannotonlystrengthenbutalsoembrittlethecphaseandfurtherdecreasetheductilityoftheseal-loys[6,7].However,grainre?nementbyhotworkingisprovedtobeaneffectiveapproachtoenhancetheoverallperformanceofTNBalloys.Thus,itisofgreatimportancetostudythehotdefor-mationbehaviorofthesealloys.

Theoccurrenceofdynamicrecrystallization(DRX)duringhotdeformationbringsaboutgrainre?nementanddeformationresis-tancereduction[8].Accordingtothedescriptionof?owbehavior,thepredictionofDRXallowstheformingprocesscontrolinmulti-scale.ThegeneraldescriptivemodelforDRXisthatthenucleation

ofrecrystallizedgrainscaninitiateatacriticalstrainwhichisafunctionofinitialmicrostructureanddeformationconditions.Then,theevolutionofDRXmicrostructurecanproceedfurtherbyincreasingdeformation.Ithasbeenshownthatdeformationconditionssuchastemperatureandstrainrate,havepronouncedeffectsontheDRXstructureandDRXgrainsize[9–11].Consider-ableresearchonDRXofTiAlalloyshavefocusedonanalyzingimagesofthefrozenmicrostructurefordifferentdeformationcon-ditions[12–14],andmadesubstantialprogressinunderstandingtheDRXmechanism.However,onlyafewinvestigatorshavepaidattentiontoDRXkineticsofTNBalloysbyanalyzing?owcurvesobtainedbyhotcompressiontests.

ThepresentpaperaimsatstudyingthehotdeformationandDRXbehaviorofaTNBalloyindetail.Forthispurpose,thehot-workingwindowandempiricalmodelfordeformationweredevel-oped.Thestrainhardeninganddynamicsofteningofthe?owcurveswereanalyzedbasedon?owcurves.ThekineticmodelforDRXisdevelopedbyanindirectmethod.Furthermore,theroleofbphaseandthesofteningmechanismduringhotdeformationwereinvestigatedanddiscussed.

2.Experiment

TheingotofthehighNbcontainingTiAlalloywithanominalcompositionofTi–42Al–8Nb–0.2W–0.1YwaspreparedbyVARmelting,thentheingotwasHIPed(hotisostaticpressing)at1280°Cand140MPafor4h.Uniaxialhotcompressiontestswereperformedtoobtain?owcurves.Forthispurpose,cylindricalsampleswith8mmindiameterand12mminheightweremachinedfromtheingot.

Correspondingauthor.Tel.:+862988460568;fax:+862988460294.

E-mailaddress:chengliang525@http://wendang.chazidian.com(L.Cheng).

0925-8388/$-seefrontmatterÓ2012ElsevierB.V.Allrightsreserved.http://wendang.chazidian.com/10.1016/j.jallcom.2012.11.076

364L.Chengetal./JournalofAlloysandCompounds552(2013)363–369

CompressiontestswereconductedonGleeble-3500thermosimulationmachineunderargonatmosphereattemperaturesof1000,1050,1100,1150°Candstrainratesof10À3,10À2,10À1,0.5sÀ1.Tantalumfoilswereemployedaslubricanttomin-imizethefrictionduringthetest.Beforethecompression,eachspecimenwasheatedatarateof10°C/storeachthetargettemperature,andthenkeptitfor300s.Themaximumstrainobtainedinthetestswas60%.Toretainthedeformedmicrostructure,thespecimenswerequenchedtoroomtemperatureimmediatelyoncethehotcompression?nished.Thetruestresswascalculatedfromthecross-sectionalchangeundertheassumptionthathomogeneousdeformationoccurredthroughoutthewholevolume.Afterthatalltheobtained?owcurvesweremodi?edtoeliminatetheeffectoffrictionusingthemethodpublishedelsewhere[15].Metallographicobservationswereperformedinthecentersectionsofthedeformedsamples.

Ascomparison,as-castc-TiAlalloysampleswithnominalcompositionofTi–50Al–2Cr–2Nbwerecompressedunder1150°C/10À2sÀ1byusingthesamemethodmentionedabove.

3.Results

3.1.Deformationbehavior

ature(T)duringdeformation,theZener–Hollomonparameter(Z)isusedandtheirhyperbolicsinerelationshipcanberepresentedasfollows[17]:

Z¼e

_expðQ=RTÞ¼A½sinhðarÞ nð1Þ

Forthelowstressregime(ar<0:8),Eq.(1)canberepresentedby:

e

_¼Brn0

ð2Þ

Andforthehighstressregime(ar>1:2),Eq.(1)canberepresentedasfollows:

e

_¼B0expðbrÞð3Þ

whereQisactivationenergy,nandn0arestressexponents.a,A,B,B0arematerialconstants.Bylinearcurve?tting,parameterafortheproposedalloycanbeworkedout,andthevalueis3.6Â10À3.Therefore,thevaluesofarofthepresentalloyarebetween0.3and2.2,sotheactivationenergyshouldbecalculatedbyEq.(4):

Q¼R

??@lne

_

@ln½sinhðÞ

Â

??@ln½sinhðarÞ T@ð1=TÞð4Þ

_e

Themeanslopeoflne

_–ln[sinh(ar)]andln[sinh(ar)]À1/Tcurvescanbeobtained,thusthevalueofactivationenergyQis427kJ/mol(4.3eV),whichissigni?cantlylowerthanthatofotherAl-lean(<44at.%)TiAlalloys(showninTable1),butmuchhigherthanactivationenergyforTiandAlself-diffusioninsinglec-TiAlalloys[23,24].SuchahighvalueapparentlyindicatesthatDRXwasinvolvedinthedeformation[16].Subsequently,asshowninFig.2,thereisalinearrelationshipbetweenlnZandln[sinh(ar)],thusnandAareequalto4.16and3.3Â1014,respectively.

Accordingtothesurfaceandinternalmorphologyofthecom-pressedsamples,hot-workingwindowforthecompressionofthepresentalloywasdeveloped.AsFig.3showsthat,soundsampleswithadeformationof60%canbeachievedatawiderangeoftemper-atureandstrainrate.Evenundertheformingconditionof1150°C/10À1sÀ1,specimenwithoutexternalandinternalcrackscanbeob-tained.IncomparisontoordinaryTiAlalloysorotherTNBalloy[25–28],thehot-workingwindowofthealloyisnoticeablywider,whichindicatesthepresentalloypossessesbetterhotworkabilitythanotherTiAlalloys.Thewiderhot-workingwindowismainlyas-cribedtothelargevolumefractionofbphase(Itsvolumefractionwasmeasuredtobeabout15.6%accordingtometallographicanaly-sis),andthereasonswillbediscussedlaterinSection4.1.3.2.Workhardening

AsshowninFig.1,theworkhardeningstageoftheproposedal-loyisextremelysteep(theelasticstagecannotbeidenti?edduetothelargeYoung’smodulus),the?owcurvesexhibitthepeakstresswithinaverylowstrain.Tostudythehardeningbehaviorindepth,curvesuptothepeaksareanalyzedusingthestrainhardeningrates,h=@r/@e,asafunctionofstrain.Ingeneral,theworkharden-ingcurveconsistsoftwostages.Inthe?rststage,workhardening

Table1

http://wendang.chazidian.composition

Initial

PreparationQ(kJ/microstructuremethodmol)Ti–42Al–8Nb–0.1Y

NLIM427Ti–42Al–6Nb–3Mn–0.2B[18]NLIM477Ti–43Al[19]FLIM528Ti–43.8Al[20]FLIM672Ti–43Al–9V[21]

FLIM577Ti–43Al–9V–0.3Y[21]FLIM451Ti–43Al–4Nb–1Mo[22]

NL

IM

578

ratedecreasesrapidlywithincreasingstrainduetodynamicrecov-ery,andthesofteningeffectgraduallydecreasesuptotheinitia-tionofasecondstage,whereDRXisconsideredtostartandwherearemarkablechangeoccursintheslopeofthecurves.Thisslopechangeisusedtoidentifyacriticalstress(rc)andstrain(ec)forinitiationofDRX.Withthefurtherincreaseofthedeformation,thehardeningratedeclinesfaster.Detailsoftheabovedescriptionhavebeenexplainedelsewhere[29,30].AsshowninFig.4,theworkhardeningrateofthepresentalloyseemsmuchhigherthanordinaryalloys.Whenthestrainreachestothecriticalvalue,thehardeningratedropssharplyduetotheinitiationofDRX.Afterthat,thepeakstressshowsupafteraextremelylowstrain(lessthan0.005)andthengraduallydecreases.ThisphenomenonmayindicatethattheDRX(ornucleation)explosivelyoccursoncethespecimensdeformedtothecriticalstrain,andthusgreatlyaffectstheshapeof?owcurve(e.g.muchsharperpeakthanthatofordin-aryalloyssuchassteels).

Thevaluesofpeakstrainandcriticalstrainatvariousdeforma-tionconditionsareshowninFig.5.Thetwoparameterscanbede-scribedasafunctionofZrespectively,andtheapproximaterelationshipbetweenecandepisidenti?edas:

ec¼0:92ep

ð5Þ

3.3.Dynamicsoftening

Comparedwithordinarymetalsandalloys,TiAlbasedalloyspossessmuchlowerstackingfaultenergy(i.e.thesuperlattice

intrinsicstackingfaultenergy(SISFE)).AndforTNBalloys,thestackingfaultenergyisfurtherreducedbyhighNbadditions[2].Thus,whenTNBalloydeformedtothecriticalstrain,DRXwilltakeplacedramatically(pointedinSection3.2).Thenthehighanglegrainboundaries(HAGBs)willmigratefastandannihilatelargeamountofdislocations.Thepeakstresswillshowupwhendy-namicsofteningdominates.

Manypapers[31–33]claimedthemainsofteningmechanismofTiAlalloyisDRX,aswellasthepresentstudy.However,itisdif?-culttostudytheDRXbehaviordirectlyduetothecomplexityofdeformedmicrostructure.BecausethesofteningofTNBalloysismainlycausedbyDRX,Xdcanthereforebeestimatedby[34]:

XrpÀrd¼

ð6Þ

pÀss

whererpandrssarepeakstressandsteadystate?owstress,respectively.AsshowninFig.6,theevolutionofrecrystallizedvol-umefractionofthepresentalloyisalsodifferentfromotheralloys.Forordinaryalloyswithlowstackingfaultenergy,theDRXcurveexhibitstheclassic‘‘s’’style,whichindicatesrecrystallizationrateislowatthebeginning,andincreasesdramaticallyuptoanin?ec-tionpoint,andthengraduallydecreasesuntiltherecrystallizationcompletes.However,undermostdeformationconditions,DRXbehaviorofthepresentalloyalmostdirectlyslowsdownafterthepeakstrainandtheDRXcurvedisplaysarelativelylowerDRXratethanordinaryalloys.

366L.ChengetImayev[12]havestudiedvariousTiAlalloys

lizedfractioncurveswereexperimentallythecompositionandmicrostructureofthepresentfromalloysproposedbyImayev,thetrendofDRXaccordancewiththatofTi–47Al–1.5Nb–1Cr–1Mn–0.2Si–0.5B(DRXratedown),whichcon?rmsthattheDRXcurvesentpaperarereasonable.Besides,itisrateofthepresentalloyseemsevenlower2Nballoyatthesamedeformationcondition(Fig.willbespeci?callydiscussedlater.

Inaddition,consideredasasolid-stateumefractionofDRXcanbeanalyzedbyAvramiX????eÀe??n

c??

d¼1ÀexpÀk

0:5

WhereXdistherecrystallizedvolumefraction,nisAvramiexponent.e0.5isthestrainfor50pctrecrystallization.AlthoughthecriticalstrainecforonsetofDRXispreferabletodescribethissofteningmechanism,mostauthorsassociatethecriticalstrainwithepduetoitseasydetermination.

Foralldeformationconditions,e0.5canbeobtainedfromtheDRXcurvesdirectly,andtherelationshipbetweenparameterZande0.5isshowninFig.7.Therefore:

e0:5¼4:6Â10À3Z0:13

ð8Þ

ParameterskandninEq.(7)canbecalculatedbyregressionanalysis.TheresultisshowninTable2.Itcanbeconcludedthatexceptthecrackedspecimens(redcolor),http://wendang.chazidian.comparisonofAvramiexponentsbe-tweenthepresentalloyandsomeotheralloysisshowninTable3.TherelativelylowerAvramiexponentofthealloycon?rmsalowerDRXrateasmentionedabove.

WahabiandCabrera[42]thoughtthatifAvramiexponentisrel-ativelyhigh(thevalueofnisabout2),nucleationwouldtakeplacesongrainandtwinedges,andDRXiscontrolledbynucle-ationduetotherelativelysmallamountofnucleationsites.Ifthecoef?cientislow(thevalueofnisabout1),nucleiwouldformintheinterfacialsurfaceofgrainandtwinboundaries,DRXiscon-pleformationsoftwinsanddislocationcells,andseemsagreewiththeanalysisinSection3.3.Thus,DRXofpresentalloymaybedom-inatedbygraingrowth.4.Discussion4.1.Hotworkability

Micro-crackinitiation,whichsigni?cantlyaffectsthehotdefor-mabilityofTiAlalloys,issensitivetoprocessingparametersandphasecomposition.However,itisdifferentfromotheras-castTNBalloys(whichusuallyconsiststhreephases:c,asmallvolumefractionofbanda2),asshowninFig.8,theproposedalloyonlyconsistsoftwophases,candarelativelylargervolumefractionofbphase(X-raymeasurementsqualitativelycon?rmedthatthequantityofthebphaseismuchmorethanordinaryTNBalloys[45,28]).Theroleofbphaseduringhotworkinghasbeeninvesti-gatedbymanystudies.Ingeneral,bphaseisconsideredsofterthanthea2orcphaseduetoitsbccstructure.Hencethebphasecanactuallyenhancesgrainboundarycohesionandmayactlikealu-bricantlayerduringdeformation,thusitcanpromotegrainbound-aryslidingeffectivelytoreleasestressconcentration[18,46–49],andhighstraincanbeachievedwithoutproducingmicro-cracks.Inthisstudy,however,bphasedisplaysadistinctivemorphology.AsFig.8(a)shows,bphasecannotonlydistributeinthetriple-junctionsoflamellacoloniesbutalsopresentasribbonsbetweenclaths.Thismorphologyseemsabletorelaxthestressconcentra-tionandhencedelayscavitationsandfractureprocessingmoreeffectively.Thusthepresentalloyshowsagoodhotworkability.Itisgenerallyacceptedthatactivationenergycanre?ecttheworkabilityofmetalsoralloys.ForTiAlbasedalloys,activationen-ergyisquitesensitivetothecomposition.Accordingtothecom-parisonofvariousTiAlbasedalloys,Kim[10]foundthattheactivationenergyisrelatedtotheatomicfractionratiobetweenTiandAl:

Q¼À460þ800ðTi=AlÞð9Þ

Forthepresentalloy,thecalculatedvalueofactivationenergyisapproximate639kJ/molusingEq.(9),whichismuchhigherthanexperimentalresult.ThedisagreementofthetworesultsismainlyattributedtohighNbadditions,andtheresultingbphase.Becauseofitsbccstructure,bphaseprovidesmoreslipsystemsthancphaseduringdeformationandconsequentlydecreasestheactiva-tionenergy.

However,Appel[50]suggestedthatthedeformationincompat-ibilitybetweenbphaseandmatrixwillresultinhighinternalstressesandthusinducemicro-cracksintheirinterfacesandre-ducethedeformabilityofthealloy,especiallyunderintensive

L.Chengetal./JournalofAlloysandCompounds552(2013)363–369

367

hotworkingsuchascombiningtorsionandcompression.Thisdeformationbehaviorofthebphaseoftenovershadowsthelowworkabilitynaturewhensmalltestpiecesarecompressedinordertodeterminehot-workingwindows.However,thedeformationmodeofthecurrenttestissimilarwithupsetting,thushot-work-ingwindowdevelopedinSection3.1stillappliestoindustrialupsetting.

strainrate,asshowninFig.9(a),themicrostructureisnearlyequi-axialduetoDRXandlamellaefragmentation-spheroidization.Ononehand,theonsetoflamellaebreakuptakesplaceatthetriple-junctionoflamellaecolonieswhereinternalstresscanbeeasilyconcentrated.Ontheotherhand,therecrystallizedcgrainswillre-sultinthelamellaebeingbrokenintofragments[51].Therefore,recrystallizedgrainsandspheroidizingfragmentsareindistin-guishableindeformedsamples.