(英文文献)Tensile properties of extruded ZK60-RE alloys

发布时间：2024-08-28

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MaterialsScienceandEngineeringA349(2003)207Á

212

http://www.77cn.com.cn/locate/msea

TensilepropertiesofextrudedZK60ÁREalloys

ChunjiangMa*,ManpingLiu,GuohuaWu,WenjiangDing,YanpingZhu

StateKeyLaboratoryofMetalMatrixComposites,ShanghaiJiaotongUniversity,Shanghai200030,People’sRepublicofChina

Received19February2002;receivedinrevisedform23September2002

Abstract

ZK60ÁREalloysweremadebymeltingZK60alloyandcerium-richrareearth(RE)metalinanelectricfurnace.ThecontentofREis0,0.5,1,1.5,2,3wt.%RE,respectively.TheinfluenceofREonmicrostructureandtensilemechanicalpropertiesofZK60magnesiumalloyswasstudied.Theresultsshowedthatcerium-richmischmetal(MM)hadanobviouseffectofreducingthegrainsizeoftheas-extrudedZK60alloys.TheultimateandyieldstrengthsofextrudedZK60ÁREalloyswereimprovedbytheadditionofRE.TheultimateandyieldstrengthincreasedwithincreasingREcontentintherangeinvestigated.Itisalsofoundthattheultimatestrengthandyieldstrengthdecreasewithincreasingextrusiontemperature.TheimprovementoftensilepropertiesofZK60ÁREalloyswasduetotheextrusiontextureandthefinemicrostructureinducedbyREadditions.#2002ElsevierScienceB.V.Allrightsreserved.

Keywords:ZK60alloy;Rareearths(RE);Extrusion;Microstructure;Tensileproperties

1.Introduction

Magnesiumalloyisthelighteststructuralmaterialapplicableinaerospaceandautomobileindustry,andithashighspecificstrengthandhighstiffnessandgooddampingcapacity.Furthermore,magnesiumresourcesareabundantandmagnesiumproductscanbeeasilyrecycled.Therefore,theuseofmagnesiumalloysisincreasing.However,duetoitshcpstructure,magne-siumalloyspresentpoorductility.Magnesiumalloysaremainlyusedforautomobilecomponents,whichareproducedbyadie-castingprocess[1].Inordertoachievemoresubstantialstructuralapplicationsofmagnesiumalloys,itisnecessarytodevelopwroughtmagnesiumproductssuchasrolledsheetorextrudedbarsandtubings.However,magnesiumalloyshavelowtensilestrengthandpoorductilitycomparedwithaluminumalloys.Recently,ithasbeenreportedthatrareearth(RE)additionscanimprovecastingcharac-teristics,corrosionresistance,hightemperaturetensilestrengthandambienttensileyieldstrengthofas-castmagnesiumalloys[2Á6].However,theeffectofREonthetensilepropertiesofextrudedmagnesiumalloyshas

*Correspondingauthor.Fax:'86-21-6293-2548.E-mailaddress:cjma@http://www.77cn.com.cn(C.Ma).

notbeenreported.Inordertoimprovethemechanicalproperties,especiallythetensileyieldstrength,ZK60magnesiumalloyscontainingREwerehotextrudedandthemicrostructureandmechanicalpropertiesarepre-sentedinthispaper.

2.Experimental

Inthepresentexperiments,ZK60waschosenasthemasteralloy.REadditionsweremadeintheformofcerium-richmischmetal(MM).Thechemicalcomposi-tionofMMisshowninTable1.TheZK60ÁREalloysweremadebymeltingZK60alloyinanelectricresistancefurnacebeingflushedcontinuouslywithSF6andCO2gasatabout7208C,thentheREof0.5,1,1.5,2,3wt.%wereadded,respectively.Themeltwasheldatthattemperatureforabout20mintoensurethattheREwascompletelydissolved.Afterthatthemeltwaspouredinametalmoldoff45mmandtheZK60ÁREalloybarsweremade.ThecompositionsofZK60ÁREalloysarelistedinTable2.TheingotZK60alloyswerethenhotextrudedintobarsoff12mmattemperatures300,340,380,and4208C,respectively,withanextrusionratioof10:1.Someoftheas-extrudedbarsweremachinedintotensilespecimenswithagauge

0921-5093/02/$-seefrontmatter#2002ElsevierScienceB.V.Allrightsreserved.PII:S0921-5093(02)00740-2

镁合金资料

Table1

ChemicalcompositionofMMElement

Compositionwt.%

Ce50.2

La26.67

Nd15.28

Pr5.37

Fe0.65

Si0.01

Mg0.38

Mn0.11

Ca0.01

P0.003

Fig.1.TransversemicrostructureofZK60ÁREalloysextrudedat3808C.(a)ZK60;(b)ZK60Á1%RE;(c)ZK60Á2%RE;and(d)ZK60Á

3%RE.

lengthof25and5mmindiameter.MicrostructurewasobservedbyopticalmicroscopyandcompositionofphaseswasexaminedbyEDX.Thetensilefracture

surfaceswereobservedbyscanningelectronmicroscopy(PHILIPSSEM515).

3.Resultsanddiscussion3.1.Microstructure

Fig.1(aÁd)showsthetransversemicrostructureofZK60ÁREalloysafterextrusionat3808C.AsshowninFig.1(a),themicrostructureofas-extrudedZK60alloyconsistsofsmallparticlesidentifiedasMg2Zn3phaseembeddedinthematrix.Thegrainsizeisabout20mm.When1%REwasadded,somewhitedeformedRE-containingintermetallicsemergedinthematrix.TheRE-containingintermetallicswereidentifiedbyEDXasMgÁZnÁREcompounds(Fig.2).ThequantityofRE-containingintermetallicsincreasedwiththeREcontentasshowninFig.1(bÁd).ThegrainsizeofZK60ÁREalloysdecreasedcontinuouslyfrom20mmtoonly2Á3mmwhen2Á3%REwasadded.Obviously,REhasan

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Fig.3.MicrostructureofZK60ÁREalloysalongextrusiondirectionextrudedat3808C.(a)ZK60Á1.5RE;and(b)ZK60Á3RE.Extrusiondirectionis

horizontal.

Fig.4.OpticalmicrostructureofZK60Á2%REalloysextrudedatdifferenttemperatures.(a)3008C;(b)3408C;(c)3808C;and(d)4208C.

Table2

ChemicalcompositionofstudiedZK60ÁREalloyNominalalloy

Composition,wt.%Mg

ZK60

ZK60Á0.5REZK60Á1REZK60Á1.5REZK60Á2REZK60Á3RE

BulkBulkBulkBulkBulkBulk

Zn5.635.915.695.625.695.65

Zr0.430.490.460.450.410.49

Ce0.320.640.971.291.88

La0.140.270.420.560.81

Nd0.030.0610.0850.110.16

http://www.77cn.com.cnparingwiththeas-castZK60ÁREalloys[7],themicrostructuresofas-extrudedZK60ÁREalloysweregreatlyrefinedandthecoarsenet-likeRE-containingintermetallicswerebrokenanddispersedinthematrixuniformly.Itisobviouslyconfirmedthatdynamicrecrystallizationoccurredduringhotextrusionandthemicrostructurewasrefined.Themicrostructurerefine-mentcausedbyREadditionsmaybeintroducedbyfollowfactors.First,additionofREchangedthesolidusand,therefore,resultedinadecreaseinthesolidificationtime,whichrefinedthemicrostructure.Secondly,ther-mallystableRE-containingintermetallicswitharela-

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Fig.5.UltimatestrengthofextrudedZK60ÁRE

alloys.

Fig.6.YieldstrengthofextrudedZK60ÁRE

alloys.

tivelyhighmeltingpointpinnedthegrainboundariesandimpededgraingrowthduringextruding.Further-more,itwasconsideredthattheadditionofREchangedthevalenceelectronstructureofthealloysandthebond-energybetweenatomswasincreased,hencethestructurestabilityofthealloyswasimproved.Allofabovecanconfinetheformationandmovementoflargeanglegrainboundaries.Asaresult,thegrowthofrecrystal-lizedgrainsinRE-containingZK60alloyswasre-stricted.

Fig.3showsthemicrostructuresofZK60Á1.5%REandZK60Á3%REalloysalongtheextrusiondirectionextrudedat3808C.TheRE-containingintermetallicsaredistributeduniformlyintheZK60matrixalongthedirectionofextrusion.ThevolumefractionofRE-containingintermetallicsincreaseswithincreasingREcontent.

Theinfluenceofextrusiontemperatureonmicro-structureofZK60Á2%REalloysisshowninFig.4.ItcanbeseenfromFig.4(a,b)thatthealloysextrudedat300and3408Cretainthedeformedmicrostructure.Whentheextrusiontemperatureisincreasedto3808C(Fig.4(c)),themicrostructureshowslargequantityofsmallgrainsduetodynamicrecrystallization.Therecrystallizedgrainsizeisabout2Á3mm.Whentheextrusiontemperatureupto4208C,thegrainsizeshowninFig.4(d)growsto5Á6mm.Obviously,extrusiontemperaturecanchangethemicrostructure

Fig.7.ElongationofextrudedZK60ÁREalloys.

forthesamecontentalloy.Inallthealloysinvestigated,itwasobservedthatthealloysdonotrecrystallizewhentheextrusiontemperatureislowerthan3408C.How-ever,dynamicrecrystallizationoccurswhentheextru-siontemperatureincreasesto3808C.Therecrystallizedgrainsizeincreaseswithextrusiontemperature.3.2.Tensileproperties

Fig.5showstheultimatetensilestrengthofZK60ÁREalloysextrudedatdifferenttemperatures.TheresultsshowthatthetensilestrengthofZK60ÁREalloysisinfluencedbybothREcontentandextrusiontemperature.Forthealloysextrudedat300and3408C,thetensilestrengthincreaseswithincreasingRE.Forexample,forthealloysextrudedat3008C,thetensilestrengthincreasesfrom290MPaofZK60alloyto340MPawhen3%REwasadded.However,forthealloysextrudedathighertemperature,thetensilestrengthincreasestoahighvalueandthengoesdownwithincreasingRE.Furthermore,extrusiontemperaturehasanobviouseffectontensilestrengthespeciallyforthealloyscontaininghighpercentageofRE.Generally,thealloysextrudedathightemperaturepresentedalowertensilestrength.

Fig.6showsthevariationofyieldstrengthofalloysextrudedatdifferenttemperaturesasafunctionofRE.TheyieldstrengthisdependentonREcontentandincreaseswithREcontentintherangeinvestigated.Forexample,theZK60Á3%REalloyextrudedat3008Cshowedthehighyieldstrengthof324MPa,whiletheyieldstrengthoftheZK60alloywasonly228MPa,anincreaseofabout42%.Ontheotherhand,theyieldstrengthofZK60ÁREalloyswiththesameREcontentdecreaseswithincreasingextrusiontemperature.Appar-ently,theinfluenceofREonyieldstrengthisstrongerthanfortheultimatetensilestrength.

AsmentionedintheSection3.2,dynamicrecrystalli-zationoccurswhentheextrusiontemperatureincreasesto3808Cinallthealloys.ItcanbeseenfromFigs.5and6thatbothtensilestrengthandyield

strength

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Fig.8.TensilefractureofZK60ÁREalloysextrudedat3808C.(a)ZK60;(b)ZK60Á0.5RE;(c)ZK60Á1RE;(d)ZK60Á1.5RE;(e)ZK60Á2RE;and(f)ZK60Á3RE.

obviouslydecreasewithextrusiontemperatureastheextrusiontemperatureislargerthan3408C,especiallyforthealloyscontaininghighpercentageofRE.Theseresultsindicatethatthedynamicrecrystallizationoccursduringextrusionhasanegativeeffectontensilestrengthandyieldstrength.

TheinfluenceofREandextrusiontemperaturesonelongationisshowninFig.7.TheelongationdecreasesastheREcontentincreasesintherangeinvestigated.Inthecaseofalloysextrudedat3808C,theelongationdecreasedfrom17.8to5.1%whentheREcontentincreasedfrom0to3%.However,asshowninFig.7,theelongationofthealloysislittleaffectedbyextrusiontemperaturesorthedynamicrecrystallization.

3.3.Tensilefracture

Fig.8showsSEMimagesofthetensilefracturesurfacesextrudedat3808C.TheSEMimageofZK60alloyinFig.8(a)showsaductilefracturesurface,inwhichalotofdeepdimplescanbeobserved.Therefore,ZK60alloypresentsgoodductility.Fig.8(b)showsthefractographofZK60Á0.5%http://www.77cn.com.cnparingthefractographwiththeopticalmicrostructureshowninFig.1,itcanbeconcludedthatthecrackedparticlesareRE-containingintermetallics.DuetothefragilecharacteristicofRE-containingcompounds,theparticleswerebrokenduringthe

tensile

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test(orduringextrusion)andbecomecracksource.Fig.8(c)isthefractographofZK60Á1%http://www.77cn.com.cnpar-ingtheimagewithFig.8(b),thefracturesurfaceiscomposedofmoreparticles.Someparticleswerebrokenandpulledout.ItisalsofoundthattheplasticdimplesinfracturesurfaceofZK60Á1%REalloydecreasedcomparingwithZK60alloy.Withcontinuouslyincreas-ingREcontent(showninFig.8dÁf),thefracturesurfacebecamemoreflatandmoreRE-containingparticlescanbeobserved.Asaresult,theductilityofZK60ÁREalloysdecreasedwithREcontent.3.4.Discussion

ItiswellknownthattheMgZnphaseisthemainstrengtheningphaseinZK60alloy.AccordingtothereportsofGaliyevandZhang[8,9],MgZn2andZn2Zr3phasemayalsobecontainedinthematrix.However,asillustratedabove,additionofREleadstotheformationofMgÁZnÁREintermetallics.Asaresult,thesolidsolubilityofZninmagnesiumisreduced.Furthermore,coarsenet-likeRE-containingintermetallicsmaketheas-castMgÁZnÁZrÁREalloyfragile[7].Therefore,cerium-richMMhadlittleeffectsontheambienttemperaturetensilepropertiesofmagnesiumalloysintheas-castcondition.However,additionofREindeeddevelopedthemicrostructureandimprovedthetensilestrength,especiallytheyieldstrengthoftheextrudedZK60ÁREalloys.TheexcellentmechanicalpropertiesofextrudedZK60ÁREalloysmaybeduetosomefactorsbelow.First,hotextrusionpromotedtheforma-tionofamicrostructureinwhichtheRE-containingintermetallicswasbrokenanddistributedparalleltotheextrusionaxis,responsibleforasubstantialstrengthen-ing.Anotherfactormaycontributetothedevelopedfinemicrostructure.Accordingtothewell-knownHallÁPetchrelation,theyieldstressdependsonthegrainsizeasfollows[10].DsGS0Kd(1=2

(1)

whereDsGSistheincreaseinyieldstressduetograinrefinement,Ktheaconstantanddisthegrainsize.TheKcoefficientinMgalloysappearsrathervariable,butitcanreachveryhighvaluesintherange200Á300MPamm(1/2[11,12].InthecaseofZK60alloy,thevalueof316MPamm(1/2hasbeenproposedandsuggestedtoberesponsibleforthehighyieldstressofrapidsolidifica-tionproductsofthisalloy[13].TheHallÁPetchequa-tioncanalsoaccountfortheyieldstressofZK60ÁREalloysstudied.Asdescribedabove,additionofREshowedanapparenteffectonreducinggrainsize.

Therefore,anincreaseofyieldstrengthduetograinrefinementmaybeanotherfactorwhichinfluencetheyieldstressofZK60ÁREalloys.Asaresult,theyieldstrengthincreasedwithREcontentforthealloysstudied.TheinfluenceofextrusiontemperatureonyieldstresscanalsobeaccountedbyEq.(1).ThegrainsizeofrecrystallizedZK60ÁREalloysincreasewithextrusiontemperature.Therefore,theyieldstressdecreasedwithincreasingextrusiontemperature.However,theelonga-tionofZK60ÁREalloysdecreasedinspiteoftherefinedmicrostructureduetoREadditions.ThismaybecontributedtothelargeamountoffragileRE-contain-ingintermetallicsinthematrix.

4.Conclusions

MechanicalpropertiesandmicrostructureofZK60ÁREalloysarestudiedinthispaper.Theresultsshowedthatcerium-richMMhasanobviouseffectonreducingthegrainsizeoftheas-extrudedZK60alloys.TensiletestsconfirmedthattheultimateandyieldstrengthofextrudedZK60ÁREalloyswasimprovedbyadditionofMM.TheultimateandyieldstrengthincreasewithMMcontentintherangeinvestigated.Itisalsofoundthattheultimatestrengthandyieldstrengthdecreasewithincreasingextrusiontemperature.TheimprovementoftensilepropertiesofZK60ÁREalloysisduetotheextrusiontextureandthefinemicrostructureinducedbyREadditions.

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