预处理对喷射沉积Al-Cu-Li合金中Al3Zr粒子析出及再结晶行为的影响

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页预处理对喷射沉积Al-Cu-Li合金中Al3Zr粒子析出及再结晶行为的影响1Introduction

Al-Cu-Lialloys,whichhaveexcellentmechanicalproperties,havebeenrapidlydevelopedinrecentyearsandareconsideredasideallightweightstructuralmaterials[1-3].However,Al-Cu-Lialloysarepronetorecrystallizeduringthehotdeformationprocesstoformcoarsegrainsandreducetheirmechanicalproperties[4].Zirconium(Zr)isaddedtoaluminumalloystoformAl3Zrdispersoids,whichcaninhibitthedegreeofrecrystallizationandimprovemechanicalpropertiesbecausethegrainboundaries,subgrainboundaries,anddislocationmotioncanbeeffectivelypinnedbyfineAl3Zrparticles[5-7].However,thelowsolubility,highlatticemisfit,andslowdiffusionrateofZratomsintheAlmatrixresultinseveremicrosegregationoftheZrsolute.ThedendritecoreisenrichedinZr,whileamuchlowerconcentrationispresenttowardthedendriteedge[8-9].Comparedwiththetraditionalcastingprocess,spray-depositedtechnologywithrapidcoolingandsolidificationavoidsseveresegregationduringcasting.Thespray-depositedalloyingotsarecomposedofequiaxialgrains,withalowerdegreeofmicrosegregationandamoreuniformmicrostructure[10-12].

Inthepastdecade,relevantresearchinthisfieldhasfocusedonoptimizingthehomogenizationprocesstoregulatetheprecipitationofAl3Zrdispersoidsandaffectthemicrostructureandpropertiesofthespray-depositedAl-Cu-Lialloys.LIUetal[13]showedtwo-stephomogenizationtofacilitatethehomogeneousnucleationofAl3ZrandtheeffectoftheAl3Zrprecipitateontherecrystallizationbehaviorandonthestrength,andtheductilityof2195alloys.WANGetal[14]showedthattwo-stageandrampheatinghomogenizationprocessescanpromotetheprecipitationofAl3ZrdispersoidsinspraydepositedAl-Lialloys.TheAl3ZrdispersoidstendtoprecipitatewhentheT1platesdissolve,whichcausesanonuniformdistributionanddecreasestherecrystallizationresistanceofthealloy.DUANetal[15]studiedtheeffectsofcoolingratesonprecipitatesinhomogenizedAl-Cu-Lialloysandfoundthatregardlessofthecoolingrate,theβ′phasewasformedathightemperatures.However,theprecipitationbehaviorofAl3Zrinspray-depositedAl-Cu-Lialloyduringhomogenizationanditsprecipitationmechanismhavenotbeenexplained.Therefore,itisnecessarytoinvestigatetheprecipitationbehaviorofAl3Zrdispersoidsduringhomogenizationspray-depositedAl-Cu-Lialloysanditseffectonrecrystallization.

Inthisstudy,theeffectsofdifferentpretreatmentsbeforehomogenizationontheprecipitationbehaviorofAl3Zrparticlesinspray-depositedAl-Cu-Lialloyandrecrystallizationofthealloywerestudiedandanalyzed.

2Materialsandmethods

ThematerialsusedinthisstudywerespraydepositedAl-Cu-LialloysanditschemicalcompositionisshowninTable1.Thealloycompositionwasdetectedbyinductivelycoupledplasma-atomicemissionspectroscopy(ICP-AES).Theingotswerepretreatedinthreedifferentways:annealingat573Kfor12h,compressingby10%at673K,andcompressingby30%at673K.Afterpretreatment,thethreegroupsofsamplesweretreatedwiththesamerampheatinghomogenization:beginningatroomtemperature,heatingfor10hto783K,maintaininganisothermalplateaufor12h.ThethreegroupsofhomogenizedsampleswerenamedsamplesPAS,PF1SandPF2S.Thehomogenizedsampleswerehotcompressedandthentreatedinsolutionat783K/h.Hotcompressionwascarriedoutfrom673Kwithatotalstrainof0.3.Themicrostructureswereobtainedwithopticalmetallographe(OM),electronbackscatterdiffractmeter(EBSD,ZEISSEVOM10),andtransmissionelectronmicroscope(TEM,TitanF20G2).AstatisticalanalysisofthenumberdensityandparticlesizeofAl3ZrwascarriedoutbyImageJsoftware.OMsampleswereobservedaftergrindingandpolishing.TheEBSDsampleswerefirstpolishedbymechanicalgrindingfollowedbyfabricpolishingandwerethenelectricallypolishedusingasolutionof10%perchloricacidand90%ethylalcoholatavoltageof20V.EBSDpatternswereobtainedusingaZEISSEVOM10scanningelectronmicroscopewithanOXFORDEBSDdetector.SamplesforTEMimaginganalysiswerepreparedbymechanicalgrindingtoathicknessof80mmandcutinto3mmradiusdisks.Then,electropolishingwasperformedusingaTenupol5machine(Struers)withasolutionof30%nitricacidand70%methanolat243to253Kand20V.TheprocessdiagramofthewholeheattreatmentandtestconditionsareshowninFigure1(a).Figure1(b)showsanOMmicrographofthespray-depositedalloy,whichrevealstypicalequiaxedgrainswithanaveragesizeof28.5μm.Somesmallporescanbefoundonthegrainboundaries.

Table1ChemicalcompositionofAl-Cu-Lialloy(wt%)

SiFeCuMnMg

0.030.044.10.040.28

ZrAgLiOtherAl

0.130.260.90.0794.15

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Figure1(a)Schematicshowingtheheattreatmentandtestconditions;(b)OMmicrographofthespraydepositedalloy

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3Results

3.1PrecipitationbehaviorofAl3Zrprecipitate

Figure2showstheSTEMimageofthethreehomogenizedsamples.InthePASsample,thedistributionofAl3Zrdispersoidswasrelativelyheterogeneous,andthereweremorelocalprecipitation-freezones(PFZs),asshownbytheredencloseddashedlineinFigure2(a).ComparedwiththePASsample,itcanbeobservedthatthedistributionuniformityofAl3ZrdispersoidsinthePF1Ssamplewasimproved,whilethelocalPFZsdecreased,asshowninFigure2(b).InthePF2Ssample,theAl3ZrdispersoidspresentedauniformlydensedistributionasshowninFigure2(c).ComparedwiththePASandPF1Ssamples,thedistributionuniformityofthedispersoidsfurtherincreased,andthelocalPFZsdisappeared.ThenumberdensityandsizedistributionoftheAl3Zrdispersoidswerestatisticallyanalyzedineachhomogenizedspecimen,asshowninFigure3.Amongthethreehomogenizedsamples,theAl3ZrdispersoidshadthehighestnumberdensityandthesmallestsizeinthePF2Ssample,andtheAl3ZrdispersoidshadthelowestnumberdensityandthelargestsizeinthePASsample.TheaverageradiioftheAl3Zrdispersoidsinthethreehomogenizedsamplesare14.9,13.6and11.0nm,respectively,andthedispersoidradiusgraduallydecreases.

Figure2STEMimagesofAl3Zrdistributionstakennearthe[100]Alzonesaxisinthreehomogenizedsamples:(a)PAS;(b)PF1S;(c)PF2S

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Figure3HistogramofAl3ZrparticlessizedistributionandnumberdensityobtainedfromSTEMimageinthreehomogenizedsamples:(a)PAS;(b)PF1S;(c)PF2S;(d)Numberdensity

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Al3Zrdispersoidswererandomlydistributednearthegrainboundaries,andtherewereclusterswithcertainshapesandoccasionallyseveralcoarseparticles,asshowninFigure4.ThenumberofAl3Zrdispersoidswassignificantlyreducedcomparedtothatoftheintragranulardispersoids,asshowninFigure4(d).Linearclustersconsistedofseveralparticlearrangements,asshowninFigure4(a)bytheredarrows.ComparedwiththatofPAS,thenumberofAl3ZrdispersoidsincreasednearthegrainboundariesofthePF1Ssample,asshownFigures4(b)and(d).IncontrasttothePASandPF1S,thenumberdensityofAl3ZrdispersoidsinthePF2Ssamplenearthegrainboundariesfurtherincreased(Figure4(d)),andthedistributionuniformitywassignificantlyimprovednearthegrainboundaries,asshowninFigure4(c).IncombinationwiththediffractionspotsinthelowerrightcornerofFigures4(a)and(b),thelinearclusterisparallelto100Al.TheselinearclusterphenomenaofAl3Zrdispersoidshavealsobeenobservedinmanystudies[16-17].TheprecipitationanddistributionofAl3Zrdispersoidsareaffectedbydifferentpretreatmentprocesses.TheAl3ZrdispersoidsintheintragranularandneargrainboundariesofthePASsampleareheterogeneouslydistributedandhavealocalPFZs,withthelowestnumberdensity,thelargestsize,andthehighestnonuniformity.ComparedwiththePAS,thelocalPFZsintheintragranularandneargrainboundariesofthePF1Ssampleswerereduced,thenumberdensityofAl3Zrdispersoidsincreased,thesizewasdecreased,andthenonuniformitywasimproved.TheAl3ZrdispersoidsofthePF2Ssamplewereuniformlydistributedintheintragranularandneargrainboundaries,andthelocalPFZswereinhibited,showingthehighestnumberdensity,thesmallestsize,andthehighestuniformity.CombinedwiththedistributioncharacteristicsofAl3Zrparticlesintheintragranularandneargrainboundaries,theprecipitationanddistributionofAl3Zrdispersoidsofthreedifferenthomogenizedstatesaresummarized,asshowninFigure5.

Figure4STEMimagesofAl3Zrdistributionsnearthegrainboundaryandnumberdensityofthreehomogenizedsamples:(a)PAS;(b)PF1S;(c)PF2S;(d)Numberdensity

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Figure5SchematicofprecipitationanddistributionofAl3Zrdispersoidsofthreedifferentpretreatmentsamples:(a)PAS;(b)PF1S;(c)PF2S

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3.2Recrystallizationbehavior

ToclarifytheeffectofthethreedifferentpretreatmentsontheprecipitationofAl3Zrdispersoidsandtherecrystallizationbehavior,thehotcompressionsampleswereannealedat783Kfor1h,followedbywaterquenchingtoambienttemperature.TheEBSDmapofthesamplesaftersolutiontreatmentbyhotcompressionisshowninFigure6.Boundarieswithmisorientationbetween2°and15°weredefinedaslow-anglegrainboundaries(LABGs)associatedwithsubgrains;misorientations15°weredefinedashigh-anglegrainboundaries(HABGs)associatedwithrecrystallizedgrains.ThewhiteandblacklinesrepresenttheLAGBsandHAGBs,respectively.Afterhotcompressionsolutiontreatmentofthesamples,manysubgrainsdevelopedintragranularly,whilemanyfinerecrystallizedgrainsformedatthetriangularboundaries.SomerecrystallizedgrainsatthetriangularboundaryofthePASsamplecoarsened,asshownbytheblackarrowinFigure6(a).ThegrainsizeandmisorientationintheEBSDmapofFigure6werecalculatedtoobtaintheaveragegrainsizeanddistributionmisorientation(Table2).Thegrainsizesofthethreehotcompressedspecimensareverysimilar.Inthethreehotcompressedsamples,thepercentageofHAGBsdecreasedfrom30.3%to17.3%.ThegradualdecreaseinthepercentageofHAGBsindicatesthatrecrystallizationresistanceisgraduallyimproved.Therefore,thenumberofrecrystallizedgrainsatthetrianglegrainboundaryisasfollows:PASPF1SPF2S.FromPAStoPF1SandPF2S,therecrystallizationdegreedecreasedgradually,andthedeformedgrainswerebetterretained,asshowninFigures6(a)-(c)andFigure7.

Figure6EBSDmapsofthreehotcompressionsamples:(a)PAS;(b)PF1S;(c)PF2S

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Table2Distributionmisorientation(2°)ofthreehotcompressionsamples

DifferenthotcompressionsampleLABGs/%HABGs%Averagegrainsize/μm

PAS32.830.326.1

PF1S42.121.626.0

PF2S55.517.326.4

LABS:low-anglegrainboundries;HABS:high-anglegrainboundries.

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Figure7Histogramofrecrystallizedfractionofthreehotcompressionsamples

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4Discussion

SRINIVASANetal[18]foundthatAl3ZrhassmallvaluesofinteratomicspacingmisfitandinterplanarspacingmismatchwithrespecttoAl.ThesolubilitylimitofZrinAl(fcc)isverylow,andthemaximumsolubilityis0.23wt%duringtheperitecticreaction[19].ThediffusionrateofZrsolutesat470℃(7.07×10-19m2/s)[20]causesstrongmicrosegregationoftheZrsolutewithinadendriteduringcasting.TheformationoftheAl3ZrdispersoidstendstoproduceahomogeneousdistributioninthedendritecenterwhereZrsupersaturationishigh,leadingtoaheterogeneousdistributionininterdendriticareaswheresupersaturationissignificantlylower[21-22].However,thecharacteristicsofthelowdegreeofmicrosegregationofthespray-depositedalloywithequiaxialgrainswillhaveasignificantinfluenceontheprecipitationbehaviorofAl3Zrparticlesduringhomogenizationandrecrystallizationafterpretreatment.

Usually,dislocationsarefavorablelocationsforprecipitationandgrowthofprecipitates.Duringthehomogenizationprocess,ZratomsdiffusedtodislocationstomoderatetheirlargeatomicmisfitwiththeAlmatrix[23].WhenthesolubilityoftheZratomsnearthedislocationreachedtheprecipitationcondition,theAl3Zrdispersoidsbegantoprecipitate.Meanwhile,thedislocationclimbedunderthermalactivation.ThedislocationsclimbdragsZratomsoutofthesolidsolutionandAl3Zrformsbyfastpipediffusion.TheinteractionbetweentheZrsoluteanddislocationincreasestheprecipitationofAl3Zr,whichiscalled“repeatedprecipitationondislocations”[24].TEMimagesofAl3ZrparticlesbyrepeatedprecipitationondislocationsfromlinearclustersareshowninFigure8.

Figure8Al3Zrdispersoidsbyrepeatedprecipitationondislocations

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PASsampleswereannealedforalongpretreatmenttime,andthenumberdensityofdislocationswasreduced.Therefore,Al3ZrdispersoidscannotbeprecipitatedtoformPFZswhenZratomsolubilityisinsufficientinsomedislocation-freeregions,asshowninFigures2(a)and4(a).ThePF1SandPF2Ssampleswerepretreatedbyhotdeformation,andthenumberofdislocationsincreased,whichpromotedtheprecipitationofAl3ZrandthedecreaseinthelocalPFZs,asshowninFigures2(b),(c)and4(b),(c).ComparedwiththePF1Ssample,thePF2Ssampleshowedagreaterdeformationdegree,andthenumberdensityofdislocationswasthehighest.Therefore,theAl3Zrdispersoidnumberdensitywasthehighest,andthelocalPFZswaseliminated,asshowninFigures2(c)and4(c).Theincreasednumberdensityoftheprecipitatedphaseleadstoadecreaseinparticledistances,whichshortensthediffusiondistanceofthesolute,acceleratesthesoluteconsumption,reducesthesaturationofthesupersaturatedsolidsolutionfaster,andshortenstheaverageradiusofthedispersoids,asshowninFigure3.

ThedispersoidsretardedrecrystallizationcanbemeasuredbytheZenerpinningformula[25-26]:

Pz=3fvγGB2r

(1)

whereγGBistheinterfaceenergy(~0.3J/m2)[27];fvisthevolumefraction;andristheaverageradiusofthedispersoids.Thehighertheratiooffv/r,thestrongertheresistancetorecrystallization.AftersubstitutingtherelevantdatainTable3intoEq.(1),itcanbeseenthatthePzvalueofthePF2Ssamplehashighervalueof192.3kJ/m3comparedwiththePASandPF1Ssampleswith127.9and145.6kJ/m3,respectively.Amongthethreehomogenizedsamples,thePF2SsamplehasthehighestnumberdensityofAl3Zrdispersoids,thesmallestsize,andthebestuniformdistribution.Therefore,therecrystallizationresistanceofthePF2Ssamplesisthelargest,andmoredeformedgrainsareretained.ForthePASsamples,theAl3Zrdispersoidshadthelowes