中英文翻译-铅元素对AZ91镁合金形成过程的影响.doc

EffectofPbadditiononageingbehaviorofAZ91magnesiumalloyAbstractTheageingbehaviorofAZ91alloycontaining2wt.%leadisinvestigated.TheresultsshowthatadditionofPbsuppressesthediscontinuousprecipitationandtherebyslowsdowntheageingkinetics.Eventhoughthepeakhardnessobtainedforbothalloysisalmostsame,thepeakhardnessreachesafter2400minforPbaddedalloyagainst900minforthebasealloy.ThepresenceofPbinsolidsolutionmighthaveinhibitedthediffusionofAlandMgatoms,whichisessentialfortheformation,andgrowthofdiscontinuousprecipitationinthisalloy.Keywords:AZ91magnesiumalloy,Pbaddition,Ageingbehavior,Microstructure,DTA1.IntroductionMagnesium-basedalloysareincreasinglyusedintheauto-mobileindustries.Thereasonsforthislieprincipallyintheirimprovedspecicpropertiescomparedwithaluminumalloysorsteelsandlightweight(30%lessthanAl,and75%lessthansteels)1.Amongthemostcommoncommercialmagnesiumalloys,AZ91isendowedwithoneofthebestcombinationsofcastability,mechanicalstrengthandductility2.Presenceofaluminumprovidesmechanicalandcastingpropertiesaswellascorrosionresistance,whereaszinccontributestosolidsolutionstrengthening.Themaximumsolidsolubilityofalu-minuminMgis12.7wt.%at437Canddecreasingtoabout2wt.%atroomtemperature.Hencecastings(sandandperma-nentmouldcastings)ofthisalloyareheat-treatedtoobtainbettermechanicalpropertiesandstudiesareavailableinthelit-eraturetounderstandtheageingbehaviorofthisalloy36.Duringsolutiontreatmentat410,theMg17Al12phaseredissolvesintotheMgmatrixandformsahomogenizedsolidsolution.Onsubsequentageingatlowertemperature,aluminumprecipitatesoutintwoforms:i.e.discontinuousandcontin-uousprecipitates.Discontinuousprecipitationisthecellulargrowthofalternatinglayersofphase(Mg17Al12)andnearequilibriummagnesiummatrixathighanglegrainboundary2,7,8.Growthofthediscontinuousprecipitationregionsceasesrelativelyearlyintheprecipitationprocess.Continuouspre-cipitationformsintheremainingregionsofthematrixthatarenotalreadyoccupiedbydiscontinuousprecipitates7.Itisdemonstratedthatthecontinuousprecipitatesareresponsi-bleforagehardening,whereasthediscontinuousprecipitatesaredetrimentaltoagehardening6.Moreover,thediscon-tinuousprecipitatesformedduringhightemperatureexposure(creep)reducethecreeppropertiesconsiderably9.Dynamicdiscontinuousprecipitatesoccuratthegrainboundariesfromthesupersaturatedeutecticsolidsolutionleadtotheslidingofgrainboundariesandweakenthematerial9.Hencesup-pressingthisdynamicdiscontinuousprecipitationduringhightemperatureexposureisinevitabletoimproveitshightemper-atureproperties.Itisalsoreportedthatadditionofsomeofthetraceelementslikegoldandrareearthsuppressesthediscontin-uousprecipitationduringageing10,11.Buttheseelementsarecostlier.ThepreliminarystudyonthePbadditiontoMg7AlhasreportedthatadditionofPbhasthetendencytosuppressthediscontinuousprecipitation12.Besides,Pbischeapermate-rialcomparedtogoldandrareearth.So,inthisstudyadetailedinvestigationontheeffectofPbadditionontheageingbehaviorofgravity。PermanentmoldcastAZ91alloyhasbeencarriedout.，Eventhough,thePbvaporishazardoustohealth；additionofsmallamountwithpropercarewillnotbeseriousone.TheagingbehaviorhasbeenexaminedusingBruneihardnessanddifferentialthermalanalysis,combinedwithmicrostructureobservationdevelopedduringheattreatmentbyopticalmicroscopyandscan-ningelectronmicroscopy.2.Experimental2.1.CastingTheAZ91magnesiumalloyusedforthisexperimentwaspreparedbymeltingtogethertherequiredquantitiesofcom-merciallyavailablepureMg,pureAl,pureZnandAl10Mnmasteralloy.Meltingwascarriedoutinasteelcrucibleintheresistancefurnaceunderproperuxcover.Whenthemelttem-peraturewas700C,2wt.%purePbwasaddedintothemelt.Themeltwasthenrenedat720Candheldfor20minfortheoxideparticlestosettle.Thereningofthemeltwascar-riedoutbystirringthemoltenmeltafteraddingthereninguxintothemelt.Themeltwaspouredintoapreheated(250)diecoatedrectangularmetallicmoldhavingdimen-sions240mm210mm40mm.Sulphurdustingwasdoneduringpouringtoavoidburning.2.2.HeattreatmentCylindricalsamplesofsize15mmdiameterand15mmheightmachinedformthecastingsofthebaseAZ91alloyandPbaddedAZ91alloyweresolutiontreatedat410for48hinamuffledfurnace.ToavoidoxidationofMgalloysamplesduringsolutiontreatment,amixtureof20%cokemixedwith80%foundrysandbedwaskeptinsidetheovenseparatelytoabsolvetheoxygen.Theexperimentalresultsshowedthatthissimplemethodeffectivelyreducedtheoxidationofthespec-miens.Thesesampleswerethenquenchedincoldwaterandsubsequentlyagedat200upto50hforfurtherinvestigationsontheageingkinetics.2.3.Differentialthermalanalysis(DTA)Piecesweighing20mgwerecutfromtheascastandsolutionthediscontinuousprecipitatesatgrainboundaries.Thesupper-treatedsamplesandDTAmeasurementswereperformedusingsignofMg17Al12formationiseasilyunderstandablewithXRDSETSYS16/18,SETARAMmachine.PureMgpiecewasusedanalysisperformedonthesamples.Table1givestherelativeasareference.TheDTAexperimentwascarriedoutunderargonintensityvaluesofmajorrepresentativeBraggreflectionoftheatmospherewithaheatingrateof10C/min.Temperatureversusheatflowcurvewasobtainedtounderstandtheageingbehaviorofthesealloys.2.4.MicrostructuralcharacterizationThemachinedsampleswereinitiallygroundusingdifferentgradesofpolishingpaper,andthenpolishedwith0.25mdiamondpaste.Polishedsampleswereetchedbyanetchant(solutionof5mlaceticacid,10mlH2O,6gpicricacidand100mlethanol).OpticalmicroscopicstudieswerecarriedoutusingLeicaDMRXMicroscopewithQ-WinimageAnalysissoftware.ThedistributionofPbinthealloywasexaminedinaJEOL,JSE35cscanningelectronmicroscope(SEM)attachedwithanenergydispersivespec-stereoscope(EDS).铅元素对AZ91镁合金形成过程的影响摘要据调查，AZ91镁合金在形成的过程中至少包含2%的微量元素。该结果显示：铅物质能抑制化学过程的急剧进行，从而降低化学反应的速度，尽管在同一时间内获得两种合金元素是非常难的。我们只能在400分钟时间内获得铅元素，900分钟时间内获得基本合金。固体铅的存在能抑制铅镁原子的扩散，它的形成是有必要的并且可以抑制反应的急剧形成。关键词：AZ91镁合金，铅元素，化学反应，微型机构，数学分析1前言目前，镁合金大量应用于自动控制工业中。多半原因是他们制定了具体的改进方案，这种产品比铅箔或钢制品的质量要轻(比铅轻30%，比钢轻75%)。在这许多普通商业的镁合金中，AZ91被认为是最好的化合物，因为它有较强的机械性能及韧性。铅箔的出现使机械和铸造产