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英文翻译-铅元素对AZ91镁合金形成过程的影响 原版.pdf

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英文翻译-铅元素对AZ91镁合金形成过程的影响 原版.pdf

MaterialsScienceandEngineeringA452–453200787–92EffectofPbadditiononageingbehaviorofAZ91magnesiumalloyA.Srinivasana,U.T.S.PillaiaMetalExtractionandFormingDivision,NationalMetallurbMetalsProcessingDivision,RegionalResearchLaborformAbstractprecipitationpeakreachesy.ofdiscontinuous©2006ElsevierB.V.Allrightsreserved.KDT1.mobileimproorsteelsalloofofwellsolutionminuminMgis12.7wt.at437Canddecreasingtoabout2wt.atroomtemperature.HencecastingssandandpermanentmouldcastingsofthisalloyareheattreatedtoobtainbettermechanicalpropertiesandstudiesareavailableintheliteratureDuringredissolvsolution.precipitatesuousgroequilibrium2,7,8relaticipitationareisblearetinuouscreepdiscontinuoustheofpressingtemperatureexposureisinevitabletoimproveitshightemperatureproperties.Itisalsoreportedthatadditionofsomeofthetraceelementslikegoldandrareearthsuppressesthediscontinuousprecipitationduringageing10,11.Buttheseelementsare09215093/doitounderstandtheageingbehaviorofthisalloy3–6.solutiontreatmentat410◦C,theMg17Al12H9252phaseesintotheMgmatrixandformsahomogenizedsolidOnsubsequentageingatlowertemperature,aluminumoutintwoformsi.e.discontinuousandcontinprecipitates.DiscontinuousprecipitationisthecellularwthofalternatinglayersofH9252phaseMg17Al12andnearmagnesiummatrixathighanglegrainboundary.Growthofthediscontinuousprecipitationregionsceases∗Correspondingauthor.Tel.914712515251fax914712491712.Emailaddressutspillairediffmail.comU.T.S.Pillai.costlier.ThepreliminarystudyonthePbadditiontoMg–7AlhasreportedthatadditionofPbhasthetendencytosuppressthediscontinuousprecipitation12.Besides,Pbischeapermaterialcomparedtogoldandrareearth.So,inthisstudyadetailedinvestigationontheeffectofPbadditionontheageingbehaviorofgravitypermanentmoldcastAZ91alloyhasbeencarriedout.Eventhough,thePbvapourishazardoustohealthadditionofsmallamountwithpropercarewillnotbeseriousone.TheagingbehaviorhasbeenexaminedusingBrinellhardnessanddifferentialthermalanalysis,combinedwithmicrostructureobservationdevelopedduringheattreatmentbyopticalmicroscopyandscanningelectronmicroscopy.–seefrontmatter©2006ElsevierB.V.Allrightsreserved.10.1016/j.msea.2006.10.119Received12September2005receivedinrevisedTheageingbehaviorofAZ91alloycontaining2wt.leadisinvestigated.andtherebyslowsdowntheageingkinetics.Eventhoughtheafter2400minforPbaddedalloyagainst900minforthebasealloAlandMgatoms,whichisessentialfortheformation,andgrowthofeywordsAZ91magnesiumalloyPbadditionAgeingbehaviorMicrostructureIntroductionMagnesiumbasedalloysareincreasinglyusedintheautoindustries.Thereasonsforthislieprincipallyintheirvedspecificpropertiescomparedwithaluminumalloyssteelsandlightweight30lessthanAl,and75lessthan1.Amongthemostcommoncommercialmagnesiumys,AZ91isendowedwithoneofthebestcombinationscastability,mechanicalstrengthandductility2.Presencealuminumprovidesmechanicalandcastingpropertiesasascorrosionresistance,whereaszinccontributestosolidstrengthening.Themaximumsolidsolubilityofalu◦b,∗,B.C.PaibgicalLaboratory,Jamshedpur831007,Indiaatory,Thiruvananthapuram695019,India16October2006accepted17October2006TheresultsshowthatadditionofPbsuppressesthediscontinuoushardnessobtainedforbothalloysisalmostsame,thepeakhardnessThepresenceofPbinsolidsolutionmighthaveinhibitedthediffusionprecipitationinthisalloy.Avelyearlyintheprecipitationprocess.Continuouspreformsintheremainingregionsofthematrixthatnotalreadyoccupiedbydiscontinuousprecipitates7.Itdemonstratedthatthecontinuousprecipitatesareresponsiforagehardening,whereasthediscontinuousprecipitatesdetrimentaltoagehardening6.Moreover,thedisconprecipitatesformedduringhightemperatureexposurereducethecreeppropertiesconsiderably9.Dynamicprecipitatesoccuratthegrainboundariesfromsupersaturatedeutecticsolidsolutionleadtotheslidinggrainboundariesandweakenthematerial9.Hencesupthisdynamicdiscontinuousprecipitationduringhigh88A.Srinivasanetal./MaterialsScienceandEngineeringA452–453200787–922.Experimental2.1.CastingTheAZ91magnesiumalloyusedforthisexperimentwaspreparedbymeltingtogethertherequiredquantitiesofcommerciallyavailablepureMg,pureAl,pureZnandAl–10MnmasterresistanceperatureThetheriedflux250sionsduring2.2.heightPbinduring80absolvsimpleimens.subsequentlyon2.3.treatedSETSYS16/18,asatmosphereheatof2.4.ferent0.25anpicricwereWallomicroscopetroscopeusingradiation.Fig.1.MicrostructureofAZ91alloy.3.Results3.1.Microstructure3.1.1.AscastmicrostructurecastAl,anddialsomicroanalysischemicalWobservthesionanalysisintensityFig.2.Microstructureof2PbaddedAZ91alloy.alloy.Meltingwascarriedoutinasteelcrucibleinthefurnaceunderproperfluxcover.Whenthemelttemwas700◦C,2wt.purePbwasaddedintothemelt.meltwasthenrefinedat720◦Candheldfor20minforoxideparticlestosettle.Therefiningofthemeltwascaroutbystirringthemoltenmeltafteraddingtherefiningintothemelt.Themeltwaspouredintoapreheated◦Cdiecoatedrectangularmetallicmoldhavingdimen240mm210mm40mm.Sulphurdustingwasdonepouringtoavoidburning.HeattreatmentCylindricalsamplesofsize15mmdiameterand15mmmachinedformthecastingsofthebaseAZ91alloyandaddedAZ91alloyweresolutiontreatedat410◦Cfor48hamufflefurnace.ToavoidoxidationofMgalloysamplessolutiontreatment,amixtureof20cokemixedwithfoundrysandbedwaskeptinsidetheovenseparatelytoetheoxygen.TheexperimentalresultsshowedthatthismethodeffectivelyreducedtheoxidationofthespecThesesampleswerethenquenchedincoldwaterandagedat200◦Cupto50hforfurtherinvestigationstheageingkinetics.DifferentialthermalanalysisDTAPiecesweighing20mgwerecutfromtheascastandsolutionsamplesandDTAmeasurementswereperformedusingSETARAMmachine.PureMgpiecewasusedareference.TheDTAexperimentwascarriedoutunderargonwithaheatingrateof10◦C/min.Temperatureversusflowcurvewasobtainedtounderstandtheageingbehaviorthesealloys.MicrostructuralcharacterizationThemachinedsampleswereinitiallygroundusingdifgradesofpolishingpaper,andthenpolishedwithH9262mdiamondpaste.Polishedsampleswereetchedbyetchantsolutionof5mlaceticacid,10mlH2O,6gacidand100mlethanol.OpticalmicroscopicstudiescarriedoutusingLeicaDMRXMicroscopewithQinimageAnalysissoftware.ThedistributionofPbintheywasexaminedinaJEOL,JSE35cscanningelectronSEMattachedwithanenergydispersivespecEDS.XraydiffractionstudieswerecarriedoutPhillipsPW1710powderdiffractometerwithCuKH9251Fig.1showsthetypicalmicrostructureofpermanentmouldAZ91alloy.ItconsistsofH9251MgsolidsolutioncontainingZn,andeutecticcontainingmassiveMg17Al12H9252phasesupersaturatedMgsolidsolution.Theeutecticisseenasvorcedinnature.LamellarkindofsecondaryprecipitatesareseenneartothemassiveMg17Al12phase.TheresultsofbyEDSshowedthatbothparticleshadthesamecomposition.Fig.2exhibitsthemicrostructuresofAZ91containingPb.ithadditionof2PbtotheAZ91alloynonewphaseisedinthemicrostructure.However,additionofPbreducesdiscontinuousprecipitatesatgrainboundaries.ThesuppresofMg17Al12formationiseasilyunderstandablewithXRDperformedonthesamples.Table1givestherelativevaluesofmajorrepresentativeBraggreflectionoftheA.Srinivasanetal./MaterialsScienceandEngineeringA452–453200787–9289Table1RelativeintensityvaluesofmajorrepresentativeBraggreflectionofMg17Al12SampledetailRelativeintensityI/I0ofhkl411721332510AZ91ascast11.061.943.271.71AZ912Pbascast8.06–1.851.02AZ91aged7.781.420.801.11AZ912Pbaged5.450.960.400.68Mg17Al12precipitatesinbothascastandagedspecimenswithandwithoutPbaddition.FormthetableitcanbeseenthatthedecreaseinrelativeintensityvaluesinthePbaddedalloycomparedtothebasealloyistheclearindicationofsuppressionofMg17Al12phase.3.1.2.HeattreatedmicrostructureSolutiontreatmentofalloysamplesat410◦Cfor48hensuresthatalltheMg17Al12canberedissolvedintomatrix.NomassiveMg17Al12hasbeenseenwiththesolutiontreatedsamplesexceptforsomeAl–MnparticlesidentifiedbyEDX.However,grainboundaryofthebasealloyisclearlyseenwithsolutionizedconditionFig.3,whereasitisnotrevealedinPbcontainingalloys.200calandcantamountblackofamountsalloPbtheincreased,increasesFig.4.Opticalmicrostructureof120minagedsamplesaAZ91andbAZ912Pb.totheadjustedgrainsasevidentfromthemicrostructuresof240minagedsamplesseeFig.5.Nevertheless,thegrowthrateisalsofoundtobeslowwithPbaddedsample.FurtherfromTable1,itcanbeseenthattheagedPbaddedalloyshowslessrelativeintensityvaluesforMg17Al12precipitatesthantheagedbasealloy.3.2.AgeingbehaviorTheagehardeningbehaviorofAZ91withandwithoutPbadditionisshowninFig.6.AdditionofPbdoesnotchangetheascast,solutionizedandpeakhardnessmuch.However,thepresenceofPbisfeltwiththetimetoreachthepeakhardness.Thebasealloyreachesthepeakhardnessafter900min,whereasthePbaddedalloytakesmorethan2400min.3.3.DifferentialthermalanalysisThemicrostructuralchangesduringtheageingtreatmentat◦Chavebeenobservedinallalloysamplesusingoptimicroscopy.Fig.4showsthemicrostructureofAZ91Pbaddedalloysamplesagedfor120min.Thesignifimicrostructuraldifferencebetweenthesesamplesistheofdiscontinuousprecipitatesatthegrainboundary.Theappearanceatthegrainboundaryindicatestheinitiationdiscontinuousprecipitates.PbaddedstructureshowsfewerofdiscontinuousprecipitatescomparedtotheAZ91y.Infact,discontinuousprecipitatesarerarelyseenwithaddedalloy,whichindicatesthatPbeffectivelysuppressesnucleationofdiscontinuousprecipitation.AstheageingtimethenumberofnucleationsitesfordiscontinuousphaseandthealreadyformeddiscontinuousphasesgrowinFig.3.OpticalmicrostructureofsolutiontreatedAZ91.dissolutionAZ91Fig.7showstheDTAthermogramsintherangeofeutecticduringheatingoftheascastAZ91andPbaddedalloys.Theenthalpychangeandonsettemperaturefor90A.Srinivasanetal./MaterialsScienceandEngineeringA452–453200787–92Fig.AZ91thethedissolutionPbcanFig.5.Opticalmicrostructureof240minagedsamplesaAZ91andb2Pb.reactionaremarkedinthefigure.ItcanbeobservedfromfigurethatthedifferenceinenthalpychangeoftheeutecticindicatesthattheamountofMg17Al12presentintheaddedalloyisless.Fig.8presentstheDTAcurvesofthesolutiontreatedalloys.ItbeseenfromthefigurethatbothcurvesexhibitsimilartrendsFig.6.AgehardeningcurveforAZ91withandwithoutadditionofPb.Fig.tionwithmationpeakThisreportedadditionformation.indicateenceaddition,phase4.eithersolubilitygoneysesconfirmagreementeutectic7.DTAthermogramsfortheascastAZ91withandwithoutadditionofPb.8.DTAthermogramsforthesolutiontreatedAZ91withandwithoutaddiofPb.twopeaks.ThefirstexothermicpeakcorrespondstotheforofMg17Al12precipitatesandthesecondendothermiccorrespondstothedissolutionofMg17Al12precipitates.isinlinewiththeresultsofageingbehaviorofAZ91alloybyZhengetal.13.ThelessenthalpychangewithPbconfirmsthesuppressionoftheamountofMg17Al12TheshiftintheonsetpointandthepeaktemperaturethattheformationMg17Al12isdelayedwiththepresofPb.TheshiftinthedissolutiontemperatureincaseofPbalsosuggesttheincreaseinthestabilityofMg17Al12tosomeextent.DiscussionAsexpected,additionofPbhasnotformedanycompoundwithMgorAlintheAZ91alloy.Since,PbhasverygoodinMg45at195◦CalltheaddedPbmighthaveintothesolidsolution14.MicrostructuralandXRDanaloftheascastAZ91aswellasthePbaddedAZ91alloysthatthesealloyscontainonlytwophaseswhichisinwiththeearlierwork15,16.ThedivorcedkindofobservedwithAZ91alloyisduetothepresenceofzinc,

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