中英文翻譯-鉛元素對(duì)AZ91鎂合金形成過程的影響.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).鉛元素對(duì)AZ91鎂合金形成過程的影響摘要據(jù)調(diào)查，AZ91鎂合金在形成的過程中至少包含2%的微量元素。該結(jié)果顯示：鉛物質(zhì)能抑制化學(xué)過程的急劇進(jìn)行，從而降低化學(xué)反應(yīng)的速度，盡管在同一時(shí)間內(nèi)獲得兩種合金元素是非常難的。我們只能在400分鐘時(shí)間內(nèi)獲得鉛元素，900分鐘時(shí)間內(nèi)獲得基本合金。固體鉛的存在能抑制鉛鎂原子的擴(kuò)散，它的形成是有必要的并且可以抑制反應(yīng)的急劇形成。關(guān)鍵詞：AZ91鎂合金，鉛元素，化學(xué)反應(yīng)，微型機(jī)構(gòu)，數(shù)學(xué)分析1前言目前，鎂合金大量應(yīng)用于自動(dòng)控制工業(yè)中。多半原因是他們制定了具體的改進(jìn)方案，這種產(chǎn)品比鉛箔或鋼制品的質(zhì)量要輕(比鉛輕30%，比鋼輕75%)。在這許多普通商業(yè)的鎂合金中，AZ91被認(rèn)為是最好的化合物，因?yàn)樗休^強(qiáng)的機(jī)械性能及韌性。鉛箔的出現(xiàn)使機(jī)械和鑄造產(chǎn)