梯度热处理快速优化Ti-6.8Mo-3.9Al-2.8Cr-2Nb-1.2V-1Zr-1Sn合金获得高强高韧性能的微观组织

时间：TIME\@"yyyy'年'M'月'd'日'"2022年3月29日学海无涯页码：第1-页共1页梯度热处理快速优化Ti-6.8Mo-3.9Al-2.8Cr-2Nb-1.2V-1Zr-1Sn合金获得高强高韧性能的微观组织1Introduction

Metastableβtitaniumalloyhasgreatpotentialapplicationinaerospaceandbiomedicalindustriesduetotheirhighspecificstrength,goodcombinationofstrengthandductility[1-2].Thedevelopmentofaerospaceindustryrequireshigherperformanceoftitaniumalloy,facilitatingthedesignanddevelopmentofnewβ-Tialloys[3-6],Ti-6.8Mo-3.9Al-2.8Cr-2Nb-1.2V-1Zr-1Snalloyisoneofthem.Asahighstrengthmetastableβ-Tialloy,thestrengthandductilityisverysensitivetomicrostructuremainlydependingonsolutionandageingtreatment.Itisakeywaytotailoringmechanicalpropertiesthroughestablishingtherelationshipbetweenheattreatmentandmicrostructures.

Previousstudiesfoundthatalthoughtheductilityofmetastableβtitaniumalloysignificantlydependsonthesizeofβ-grainandprimaryαphase,strengthconsiderablydependsonthemorphology,sizeandvolumefractionofαsphase[7-9].Therefore,controllingthesize,morphology,anddistributionofαphasethroughheattreatmentisoneofimportantwaysfortailoringmechanicalpropertiesofmetastableβ-Tialloys[4,10-12].Establishingtherelationshipbetweenheattreatmentparametersandmicrostructureofmetastableβ-Tialloyisarelativelycomplexprocess,includingannealing,solutionandageing.Afterdifferentsolutionandageingtreatments,themicrostructureofthealloycouldbetailoredtoequiaxed,lamellar,bimodalandevenmorehierarchicalfeatures[13-14].Besides,themorphologyandvolumefractionoftheprimaryαphase,sizeandspacingofthesecondaryphasearealsoverysensitivetosolutionandageing.InTi-5Al-4Zr-8Mo-7Valloy,αswith51nminwidthand85nminspacingresultedinultimatestrengthof1390MPawithelongationof10.3%afterthesolutiontreatmentat800℃andageingat570℃for8h[9].RENetal[15]achievedgoodcombinationofstrengthandductilityinTi5231alloy,ultimatestrengthof1238MPaandelongationof20%,owingtothemicrostructureconsistedof13vol%ofαpandαsof187nminspacingaftersolutiontreatmentat830℃andageingat620℃for6h.Throughchangingthesolutionandageingtemperature,thesizeandfractionofbothαpandαsphasescouldbetailoredforachievinggoodcombinationofstrengthandductility,suchasTi7333[16-17],Ti1023[18]andTi55531[11,19-20]alloys.

Inrecentyears,someeffortshavebeenmadetotakeadvantageofhigh-throughputtechnologiestoacquireamountsofmicrostructurefeaturesrapidlyinordertotailorandoptimizemicrostructureandmechanicalpropertiesoftitaniumalloy.AFONSOetal[21]obtaineddifferentcoolingratesforTi-20NballoybyJorminyquenchingtesttostudytherelationshipamongcoolingrates,differentmicrostructuresandmechanicalproperties.XUetal[22]accuratelydeterminedthepseudo-spinodaldecompositiontemperatureofTi5553alloythroughgradientheattreatmentandobtainedahighvolumefractionofsmallsizeαphasebypseudo-spinodaldecomposition,resultinginaveryhighstrengtheningeffect.Thecontinuouscomponentgradientcanbeachievedbydiffusionmultipleexperiment,andtheeffectofcomponentsonperformancecanbedeterminedconvenientlyandaccurately.Bythismethod,WUetal[23]studiedtheeffectofMoelementandVcontentonthemicrostructureofTi-Mo-Valloyingradientcompositionbyhigh-throughputmultiplesample,anddesignedTi-6Mo-3Valloywithultrafineαphase,whichhasyieldstrengthof1411MPaandelongationof6.5%.ZHANGetal[24]rapidlyestablishedtherelastionshipof“composition-microstructure-elasticmodulus”ofTi-Nb-Zrsystemandbulidtheelasticmodulusandhardnessdatabase.

Inthisstudy,aconvenienthigh-throughputheattreatmentapproachwasdeveloped,whichcouldcreatetemperaturegradientforsolutionandageingtreatmentinonlyonesample.TheageinghardeningbehaviorandmicrostructuralevolutionofTi-6.8Mo-3.9Al-2.8Cr-2Nb-1.2V-1Zr-1Snalloywererapidlystudiedforoptimizingmicrostructureforimprovingstrengthandductility.

2Experimental

2.1Materials

Theas-receivedalloywasforgedrodsuppliedbytheBaotaiGroupCo.,Ltd.ThechemicalcompositionofthealloyislistedinTable1.TheinitialmicrostructureshowninFigure1consistsoffinebimodalα+βmicrostructurewithapproximately14vol%equiaxedαpphaseandfinedispersedlamellarαsphase,andtheaveragesizeofαpphaseisabout2-3μm.

Table1ThechemicalcompositionofTi-6.8Mo-3.9Al-2.8Cr-2Nb-1.2V-1Zr-1Snalloy(wt%)

AlVMoCrNbFeZrSnOTi

3.891.226.782.782.010.0481.051.000.107Bal.

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Figure1SEMimageofas-receivedalloy

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2.2Preparationofgradientsample

Around-rodsampleof10mmindiameterand92mminlengthwaswire-cutfromtheas-receivedalloy.Figure2showstheschematicofthegradientheattreatment.Thetubefurnacewasusedforgradientheattreatmentwithaprecisely-programmabletemperature-controlledzoneinthemiddlepart.Thetemperaturewassetat950℃,anddecreasedgraduallyfrom950℃to25℃inthestokehole.Thepositionofgradienttemperaturewastestedusingstandardsamplebythermocoupleaccurately,asshowninFigure2.Inordertoachieveaccurategradienttemperature,nineequallyspacedholeswerepunchedinthesamplebyelectricsparkdrillingwith10-mmspacing,andthenthethermocouplewireswereplacedintheholes.Topreventoxidationofthesampleduringgradientheattreatment,Cr2O3powderwasmixedwithwaterandthenevenlycoatedonthesamplesurfacebeforegradientsolutiontreatment.Afterwaterevaporated,thesamplewasputinaporcelainboatandplacedtothegradientpositioninthefurnace.

Figure2Schematicillustrationofgradientheattreatedsamplepreparation

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AscanbeseeninTable2,themeasuredtemperaturefromthermocouplewere746,770,795,819,844,865,885,900and909℃,respectively.Aftergradientsolutionfor2h,thesamplewasquenchedinwaterimmediately.Theβ-transustemperatureofthealloyisabout(845±5)℃.Thegradientsolutioninα+βandβphaseregionscouldbeobtainedinonlyonesample.Then,thequenchedgradientsolutionsamplewascutintofouridenticalsheets:oneofthemwasnotfurtheraged,andtheotherthreewereagedfor8hat450,550and600℃,respectively.Beforeageing,Cr2O3wasalsoevenlycoatedoneachsampleasananti-oxidationlayer.

Table2Gradienttemperaturemeasuredbythermocouplewires

PositionTemperature/℃

TC1746

TC2770

TC3795

TC4819

TC5844

TC6865

TC7885

TC8900

TC9909

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2.3Microstructureobservation

ThemicrostructureofgradientsolutiontreatedandagedsampleswereobservedusingMIRA2LMHscanningelectronmicroscope(SEM)andTecnaiG2transmissionelectronmicroscope(TEM)operatedat200kV.ForTEMobservation,thethinfoilswerepreparedbyatwin-jetelectropolishingtechniqueusingKroll’sreagent,whichcomposedof5%perchlorate,35%butylalcoholand60%methanol.Theaveragegrainsizeandvolumefractionofαpphase,thethicknessandlengthofαsphaseweremeasuredstatisticallybyImageJsoftware.

2.4Mechanicalpropertytesting

Hardnesstestingwasconductedon200HBVS-30Vickershardnesstesterwith9.8Nload.Eachgroupofdatahassevenhardnessvaluesandtheaveragevaluewasused.TensilepropertiesweretestedaccordingtoGB/T228—2022standard.Beforeheattreatment,sampleswerecutintotheround-rodshapefirstly,afterheattreatment,theround-rodsamplewasprocessedintoastandard25-mmlongtensilespecimen5mmindiameter.ThetensilespecimenisshowninFigure3.TensiletestswereconductedonMTSLandmarkatroomtemperaturewithstrainrateof10-3s-1,andastrainextensometerwasadoptedtoensuretheaccuracyofstress-straindatameasurement.

Figure3Thedimensionsoftensilespecimen(Unit:mm)

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3Results

3.1Microstructureofthesolutiontreatedalloy

Figure4showsthemicrostructureofgradientsolutiontreatedsample.Thevolumefractionofαpdecreaseswiththeincreaseofsolutiontemperature,from30%at746℃to3%at819℃.Whenthesolutiontemperaturewasabovetheβ-transustemperature,noαpphasewasobservedat844℃,whichmeansthattheαphasetransformedtoβbetween819and844℃.Whenthesamplewassolutiontreatedat746℃and770℃,theprimaryαphasehadtwodifferentmorphologies:globularαphase(αp)withdiameterof1-5µmandrod-shapedαphase(αr)withwidthof0.1-0.4µmandlengthof0.5-4µm.InFigures4(c)and(d),onlytheglobularαphasecouldbeobservedneartoβ-transustemperature.

Figure4SEMimagesofTi-6.8Mo-3.9Al-2.8Cr-2Nb-1.2V-1Zr-1Snalloysolutiontreatedatdifferenttemperatures:(a)746℃;(b)770℃;(c)795℃;(d)819℃;(e)844℃(Thenumberinthetoprightcornershowsthevolumefractionofαpintheβmatrix)

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Figure5showstheTEMimagesof900℃and746℃solutiontreatedsampleinordertoobservewhethertheathermalωphase(ωath)precipitatedduringquenching.Someperiodicstriationswereobservedinthebright-fieldTEMimages.Insertedselectedareaelectrondiffractionpatternsshowthat,otherthanthefaintdiffusescattering,therearenoreflectionsatthe1/3and2/3(112)βpositions,whichcouldindicatetheexistenceofωathphase.Thistypeofperiodicstriationwasalsoobservedintheothermetastabletitaniumalloys[25-27],showingthespinodaldecompositionfeature.Whenthetitaniumalloyhassufficientquantitiesofβ-stabilityelements,thestrongdrivingforceofphaseseparationduringquenchingleadstothedifferentiationofβphaseintoβ-leanandβ-richregion,andeventuallytoobviouslatticedistortion.Thelatticedistortionresultsinlocalisedatomicornano-scalestructuralmodulationintheβ-leanregion,i.e.,embryonicω,whichhasanintermediatestructurebetweenβandωphase[28].

Figure5Brightfieldandselectionelectrondiffractionpatternof(a)746℃STsample;(b)900℃STsample(STstandsforsolutiontreatment)

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3.2Gradientmicrostructuresduringsolutionandageing

3.2.1α+βsolutionfollowingageing

TheSEMimagesofthealloyafterα+βsolutiontreatmentat746,770,795and819℃for2h,andageingat450,550and600℃for8hareshowninFigure6.Theequiaxedαpphaseappearsatpriorβgrainboundary,whichindicatesthatαpphasecouldrestrainthegrowthofβgrain,andimprovetheductility[29].Theincreaseofsolutiontemperaturereducesthevolumefractionofαpandaffectsthemorphologyandsizeofαs[6,30].AsshowninTables3-5.Thewidthandphasespacingofαsdecreasewiththeincreaseofsolutiontemperature.Thestatisticalresultsshowthatwhentheageingtemperatureis600℃,thewidthandspacingofαsdecreasefrom57and142nmto47and65nmasthesolutiontemperatureincreasesfrom746℃to819℃,respectively,andthesameregularwasobservedwhentheageingtemperatureis450℃and550℃.

Figure6SEMimagesofTi-6.8Mo-3.9Al-2.8Cr-2Nb-1.2V-1Zr-1Snalloyaftergradientheattreatment:(a1)746℃,(b1)770℃,(c1)795℃and(d1)819℃solutiontreatmentfor2h;(a2-d2)450℃,(a3-d3)550℃and(a4-d4)600℃for8h

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ItcanbeseenfromTable3,theαpphaseslightlycoarsenswiththeincreaseofageingtemperature,whichindicateslowerstabilityduringageing.Thesizeofαsandthespacingbetweenαsareverysensitivetoageingtemperature.Thecomparisonof(a2)-(a4),(b2)-(b4),(c2)-(c4),and(d2)-(d4)inFigure6revealsthat,atthesamesolutiontemperature,αscoarsenswiththeincreaseofageingtemperature.Statisticalresultsonspacingandwidthofαsaftersolutiontreatedat746℃,795℃,819℃followedbyageingtemperaturesareshowninTables4and5.Itcanbeseenthatthewidthandphasespacingofαsincreaseby3-4timeswhentheageingtemperatureincreasesfrom450℃to600℃aftersolutiontreatedinthetemperaturerangeof746-819℃.Solutiontreatedat746℃,thewidthandphasespacingofαsincreasedfrom17nmand35nmageingat450℃to57nmand142nmageingat600℃,respectively.

Table3Statisticalresultsofdiameterofαp(dp),widthofαs(ws)andspacingofαs(λ)in746℃solutiontreatedsamplefollowedbyageing

Ageingtemperature/℃dp/μmws/nmλ/nm

4502.761735

5503.053978

6003.1757142

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Table4Statisticalresultsofwidthofαsinthesolutiontreatedsamplesat746℃,795℃and819℃followedbyageing

Ageingtemperature/℃ws/nm

746℃795℃819℃

450171514

550393429

600575147

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Table5Statisticalresultsofspacinglengthofαs(λ)inthesolutiontreatedsamplesat746℃,795℃and819℃followedbyageing

Ageingtemperature/℃λ/nm

746℃795℃819℃

450352522

550785832

60014210565

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Aftersolutionat746-770℃for2handfollowingageingat550℃,600℃for8h,thehierarchicalstructurecomposedofmicronscaleαpphase,submicronscaleαrphaseandnanometrescaleαsphasewascreated.Solutiontreatedat746℃for2hfollowingageingat600℃for8h,theαsphasebecomesthickerwiththeaveragethicknessof57nm.Theincreaseofthicknessofαsphasemakesthecrackpropagationpathbecomemoretortuousandrequiremoreenergytobypassαs,whichinturnincreasetheductility.Thehomogeneityofthestraingradientinthehierarchicalα-structureisbeneficialfortheductilityenhancementoftitaniumalloy[13].

3.2.2βsolutionfollowingageing

Figure7showsthegradientmicrostructurefeaturesofthealloysolutiontreatedat844-909℃for2hfollowingageingat450-600℃for8h.Whentheagingtemperatureis450℃,theαsphasedidnotappearinβmatrix.Whentheageingtemperatureis550℃,fineαsphaseprecipitatedinβmatrix.Whentheageingtemperatureis600℃,theαsphaseslightlycoarsens.Moreover,whensolutiontreatednearβ-transustemperature,αsismuchthicker.Atthetemperaturerangeof865-909℃,thesolutiontreatmenttemperaturehasnoobviouseffectonthemorphologyandsizeofαs.

Figure7SEMimagesofTi-6.8Mo-3.9Al-2.8Cr-2Nb-1.2V-1Zr-1Snalloyaftergradientheattreatment:(a1)844℃,(b1)865℃,(c1)885℃,(d1)900℃and(e1)909℃solutiontreatmentfor2h;Representageingat(a2-e2)450℃,(a3-e3)550℃,and(a4-e4)600℃for8h

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3.3Mechanicalpropertiesoftheagedalloy

TheagehardeningbehaviorofthealloyaftergradientsolutiontreatmentisshowninFigure8.ThehardnessvaluesofgradientsolutiontreatedsamplewerewithinHV283-295.Afterageing,thegradientsolutiontreatedsampleshowsobviousagehardening.Solutiontreatedsampleinβ-phaseregionhasmoresignificantagehardeningthanthatinsolutiontreatedsampleinα+βregion.Underthesameageingconditionat450℃,whenthesolutiontemperatureincreasesfrom746℃to819℃,themicrohardnessincreasesfromHV419.8toHV482.8;butwhenthesolutiontemperatureincreasesfrom844℃to909℃,themicrohardnessincreasesfromHV507.8toHV514.6,ageingat550℃and600℃thesamplesshowsimilarresults.Thisindicatesthattheagehardeningofsolutioninα/βphaseregionismoresensitivetosolutiontemperaturethanthatinβphaseregion.Inaddition,at450℃theagehardeningisstrongerthanthatat550℃and600℃.

Figure8Agehardeningcurvesofgradientsolutiontreatedalloyagedat450,550and600℃for8h

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XRDpatternsinFigure9showthatonlyαandβphasepeaksappearedinthesamplesaftersolutiontreatedat746,795,819and900℃followingageingat550℃for8h,indicatingthatthereisonlyαphaseinβmatrixinthosesamples,whichisconsistentwiththeSEMmicrostructureshowninFigures6and7,indicatingthatthehardeningeffectofthealloyresultedfromαsphase.Theincreaseofageingtemperatureacceleratedthedecompositionofmetastablephaseandpromotedtheformationofequilibriumphase[31].Theincreaseofageingtemperaturefrom550℃to600℃acceleratedtheprecipitationofαsphase.Therefore,itcanbeinferredthatthereisalsoonlyαphaseinβmatrixageingat600℃for8h.

Figure9XRDpatternsofalloysolutiontreatedat746,795,819and900℃for2h,andthenagedat550℃for8h

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Basedontheagehardeningcurveandmicrostructurecharacterizationofgradientsample,theconditionspossiblycombinedhighstrengthandductilitywereselectedtoevaluatethetensileproperties,asshowninFigure10.Thealloyexhibitshighyieldstrength(YS)of1457MPabutarelativelylowelongationof2.1%aftersolutiontreatment(ST)at819℃followedbyageingat550℃.Whenagedat600℃,thestrengthisdecreased,buttheductilityislargelyimproved.Asuperiorcombinationofstrengthandductilityisachievedwhenageingat600℃afterSTat746℃,thealloyobtainedagoodcombinationofelongationof15%andyieldstrengthof1140MPa.Thestrengthdecreaseswithageingtemperatureincrease,whiletheductilityshowsanoppositetrend.Thisisattributedtothedecreaseofαsphaselengthandincreasedthicknesswiththeincreaseofageingtemperature.

Figure10Theengineeringstress-straincurvesofsolutiontreatedandagedalloysamples(“ST”standforsolutiontreament;“UTS”standsforultimatetensilestrength;“EL”standsforelongation;“A”standsforageingtreatment)

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Thesolutiontemperaturegreatlyinfluencesstrengthandductilityatthesameageingtemperature.Ageingat600℃for8h,whenthesolutiontemperaturewasincreasedfrom746℃to819℃,YSwasimprovedfrom1140to1383MPa,whiletheelongationdecreasedfrom15.0%to4.4%.Ageingat550℃for8hshowedthesametrend.Thisisduetothedecreaseofvolumefractionofαpphasesignificantly,andthethicknessandspacingofαsphasedecreaseaswell.

3.4Fractographyoftheagedalloy

Figure11showsthetensilefractographsofthespecimenaftersolutiontreatedat746℃for2hfollowingageingat600℃for8h.Itcanbeseenthattherearemanydimplesandsomesecondarycracksonthefracturesurface.Thefluctuationoffracturesurfaceissignificant,indicatingthatthecrackpathistortuous.Thefracturesurfacecanbeclearlydividedintotheshearingareaanddimpledregion,whichhasatypicalcupconeshapewithroughedges,indicatingaconsiderablemacroscopicplasticdeformationbeforethefinalfracture.Thefracturemorphologyshowsacompleteductilefracturewithasmallsecondarycrackneartheshearzone.ThedimplesinFigure11(b)indicatetheimprovedductility.

Figure11Fractographsofthealloyageingat600℃for8hwithasolutiontreatmentof746℃for2h:(a)Macro-fractography;(b)Dimplefractureregion

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4Discussion

4.1Influenceofsolutiontreatmentonmicrostructure

Thesolutiontreatmenttemperaturehasanimportanteffectonthemorphology,sizeandvolumefractionofprimaryαphase,whichaffectsthestrengthandductilityofthealloy.Atlowersolutiontemperature746-770℃,therearetwokindsofprimaryphasesinthealloy:oneistheglobularαpphase;theotheroneissubmicronαrphase.Nano-scaleαsphaseprecipitatedinβmatrixduringthesubsequenthighertemperatureageing.Thesethreekindsofαphaseconstitutethefeatureofhierarchicalstructure.Aftersolutionatrelativelyhighertemperature,αrphasedisappeared,andglobularαpphaseexisted.Afterageingathighertemperature,thealloyexhibitedbimodalmicrostructure.Whenthesolutiontemperatureexceededβtransitiontemperature,noαpphasewasobserved.Inthesubsequenthighertemperatureageing,αsnano-precipitatedispersedinβmatrix.

Inthiswork,thehierarchicalstructurehasasuperiorcombinationofstrengthandplasticity,mainlyduetothefollowingreasons:1)Theαs/βinterfacecouldblockthemovementofdislocation[31],whichisthemaincontributionforhighstrength;2)Thehierarchicalstructurehashighvolumefractionsofαpandαrphase,whichproducestrainhardeningcompatiblewithtransformedβmatrixtomaintainuniformdeformation;3)Thesoftαpphaseandfineαsphaseendowthealloyhighstrengthandgoodductility.However,themicrostructurecomposedofdifferentsizesandmorphologiesareplasticallynon-homogeneoustosomeextent[32].Forthebimodaltitaniumalloy,plasticdeformationisinitiatedinsoftαpphase,resultinginhigherplasticstrainthanglobaltensilestrain[33].Duringfurtherdeformation,strainincompatibilitybetweenαpandtransformedβincreases,whichinturndecreasestheplasticity.Thehierarchaldistributionofαphasecausesmorehomogeneousstrainpartitioningandimprovestheplasticity[14].

Thesolutiontemperaturecouldalsoaffectthewidthandspacingofαsphase.Thesolutiontemperatureaffectsthevolumefractionofprimaryαphase,thedistributionofelementsandthestabilityofβphase[34-36].Differentelementstendtobeconcentratedindifferentphase,forexample,βphaseisrichwithβ-stabilisingelements,suchasV,MoandCr,whileαphaseisrichwithα-stabiliserAl[35].Whenthealloyissolutiontreatedinα+βphaseregion,alloyingelementsareessentialforphaseformationandmicrostructureformationinβ-Tialloy,andtheirdiffusiondeterminesthecompositionandstabilityofαandβphaseinthealloy[36-37].Thismeansthatwiththeincreaseofsolutiontemperature,thesoluteconcentrationofβstabilizersretainedinmatrixdecreases,andsodoesthephasestabilityoftheresidualβphaseasaresult.Thedifferenceofβ-stabilityexertsaremarkableinfluenceontheprecipitationofαs,which,inturn,resultsinanincrementofdrivingforceforαphasenucleationduringageing[31,38-39].Thisisthereasonwhyatthesameageingtemperature,αsphasebecomesfinerandthespacingbecomessmallerwiththeincreaseofsolutiontemperature.

Sincetheαs/βinterfacestrengtheningistheprimarystrengtheningmechanisminmetastableβtitaniumalloy,thespacingofαs(λ)determinesthedistancethatdislocationcouldslidefreely,whichinturndeterminesthedislocationaccumulationatαs/βinterface.Thisstrengtheningmechanismissimilartofinegrainstrengthening[12,33].Therefore,thinnerαsphaseandsmallerαsphasespacingmeanshorterdistanceatwhichthedislocationcouldslipfreely,whichincreasesstrengthbutdecreasesductilitywiththeincreaseofsolutiontemperatureatthesameageingconditions.

4.2Influenceofageingonmicrostructure

Ageingprocesscouldtailorthesizeandspacingofαsphasewhichaffectsstrengthandductilityofβ-Tialloy[30,40].Theprimaryαphasewithdifferentmorphologiesandvolumefractionswasobtainedbydifferentsolutiontreatmentsfollowedageingat450-600℃.Thesizeofαsandthegrainboundarybecomecoarserwithincreasingageingtemperature,aspresentedinFigure7.

Previousstudiesshowedthatthetransitiondrivingforceofαsphasefromβphaseisinsufficientatlowerageingtemperature[41].Theisothermalωphase(ωiso)couldbepossiblytransitionphaseduringageingatlowertemperature.Theappearanceofnanometreωisowouldsignificantlyincreasestrengthbutdecreaseductility[42].Ageingat450℃,themicrohardnessofalloyisconsiderablyhigherthanthatatotherageingconditions,whichmeansthatωisophasemightprecipitateinthealloyduringlowertemperatureageing.