MaterialsScienceandEngineeringA452453(2007)8792EffectofPbadditiononageingbehaviorofAZ91magnesiumalloyA.Srinivasana,U.T.S.PillaiaMetalExtractionandFormingDivision,NationalMetallurbMetalsProcessingDivision,RegionalResearchLaborformAbstractprecipitationpeakreachesy.ofdiscontinuous2006ElsevierB.V.Allrightsreserved.KDT1.mobileimproorsteels)alloofofwellsolutionminuminMgis12.7wt.%at437Canddecreasingtoabout2wt.%atroomtemperature.Hencecastings(sandandperma-nentmouldcastings)ofthisalloyareheat-treatedtoobtainbettermechanicalpropertiesandstudiesareavailableinthelit-eratureDuringredissolvsolution.precipitatesuousgroequilibrium2,7,8relaticipitationareisblearetinuous(creep)discontinuoustheofpressingtemperatureexposureisinevitabletoimproveitshightemper-atureproperties.Itisalsoreportedthatadditionofsomeofthetraceelementslikegoldandrareearthsuppressesthediscontin-uousprecipitationduringageing10,11.Buttheseelementsare0921-5093/$doi:tounderstandtheageingbehaviorofthisalloy36.solutiontreatmentat410C,theMg17Al12H9252phaseesintotheMgmatrixandformsahomogenizedsolidOnsubsequentageingatlowertemperature,aluminumoutintwoforms:i.e.discontinuousandcontin-precipitates.DiscontinuousprecipitationisthecellularwthofalternatinglayersofH9252phase(Mg17Al12)andnearmagnesiummatrixathighanglegrainboundary.GrowthofthediscontinuousprecipitationregionsceasesCorrespondingauthor.Tel.:+914712515251；fax:+914712491712.E-mailaddress:(U.T.S.Pillai).costlier.ThepreliminarystudyonthePbadditiontoMg7AlhasreportedthatadditionofPbhasthetendencytosuppressthediscontinuousprecipitation12.Besides,Pbischeapermate-rialcomparedtogoldandrareearth.So,inthisstudyadetailedinvestigationontheeffectofPbadditionontheageingbehaviorofgravitypermanentmoldcastAZ91alloyhasbeencarriedout.Eventhough,thePbvapourishazardoustohealth；additionofsmallamountwithpropercarewillnotbeseriousone.TheagingbehaviorhasbeenexaminedusingBrinellhardnessanddifferen-tialthermalanalysis,combinedwithmicrostructureobservationdevelopedduringheattreatmentbyopticalmicroscopyandscan-ningelectronmicroscopy.seefrontmatter2006ElsevierB.V.Allrightsreserved.10.1016/j.msea.2006.10.119Received12September2005；receivedinrevisedTheageingbehaviorofAZ91alloycontaining2wt.%leadisinvestigated.andtherebyslowsdowntheageingkinetics.Eventhoughtheafter2400minforPbaddedalloyagainst900minforthebasealloAlandMgatoms,whichisessentialfortheformation,andgrowthofeywords:AZ91magnesiumalloy；Pbaddition；Ageingbehavior；Microstructure；IntroductionMagnesium-basedalloysareincreasinglyusedintheauto-industries.Thereasonsforthislieprincipallyintheirvedspecificpropertiescomparedwithaluminumalloyssteelsandlightweight(30%lessthanAl,and75%lessthan1.Amongthemostcommoncommercialmagnesiumys,AZ91isendowedwithoneofthebestcombinationscastability,mechanicalstrengthandductility2.Presencealuminumprovidesmechanicalandcastingpropertiesasascorrosionresistance,whereaszinccontributestosolidstrengthening.Themaximumsolidsolubilityofalu-b,B.C.PaibgicalLaboratory,Jamshedpur831007,Indiaatory,Thiruvananthapuram695019,India16October2006；accepted17October2006TheresultsshowthatadditionofPbsuppressesthediscontinuoushardnessobtainedforbothalloysisalmostsame,thepeakhardnessThepresenceofPbinsolidsolutionmighthaveinhibitedthediffusionprecipitationinthisalloy.Avelyearlyintheprecipitationprocess.Continuouspre-formsintheremainingregionsofthematrixthatnotalreadyoccupiedbydiscontinuousprecipitates7.Itdemonstratedthatthecontinuousprecipitatesareresponsi-foragehardening,whereasthediscontinuousprecipitatesdetrimentaltoagehardening6.Moreover,thediscon-precipitatesformedduringhightemperatureexposurereducethecreeppropertiesconsiderably9.Dynamicprecipitatesoccuratthegrainboundariesfromsupersaturatedeutecticsolidsolutionleadtotheslidinggrainboundariesandweakenthematerial9.Hencesup-thisdynamicdiscontinuousprecipitationduringhigh88A.Srinivasanetal./MaterialsScienceandEngineeringA452453(2007)87922.Experimental2.1.CastingTheAZ91magnesiumalloyusedforthisexperimentwaspreparedbymeltingtogethertherequiredquantitiesofcom-merciallyavailablepureMg,pureAl,pureZnandAl10MnmasterresistanceperatureThetheriedflux(250sionsduring2.2.heightPbinduring80%absolvsimpleimens.subsequentlyon2.3.treatedSETSYS16/18,asatmosphereheatof2.4.ferent0.25anpicricwereWallomicroscopetroscopeusingradiation.Fig.1.MicrostructureofAZ91alloy.3.Results3.1.Microstructure3.1.1.AscastmicrostructurecastAl,anddialsomicroanalysischemicalWobservthesionanalysisintensityFig.2.Microstructureof2%PbaddedAZ91alloy.alloy.Meltingwascarriedoutinasteelcrucibleinthefurnaceunderproperfluxcover.Whenthemelttem-was700C,2wt.%purePbwasaddedintothemelt.meltwasthenrefinedat720Candheldfor20minforoxideparticlestosettle.Therefiningofthemeltwascar-outbystirringthemoltenmeltafteraddingtherefiningintothemelt.ThemeltwaspouredintoapreheatedC)diecoatedrectangularmetallicmoldhavingdimen-240mm210mm40mm.Sulphurdustingwasdonepouringtoavoidburning.HeattreatmentCylindricalsamplesofsize15mmdiameterand15mmmachinedformthecastingsofthebaseAZ91alloyandaddedAZ91alloyweresolutiontreatedat410Cfor48hamufflefurnace.ToavoidoxidationofMgalloysamplessolutiontreatment,amixtureof20%cokemixedwithfoundrysandbedwaskeptinsidetheovenseparatelytoetheoxygen.Theexperimentalresultsshowedthatthismethodeffectivelyreducedtheoxidationofthespec-Thesesampleswerethenquenchedincoldwaterandagedat200Cupto50hforfurtherinvestigationstheageingkinetics.Differentialthermalanalysis(DTA)Piecesweighing20mgwerecutfromtheascastandsolutionsamplesandDTAmeasurementswereperformedusingSETARAMmachine.PureMgpiecewasusedareference.TheDTAexperimentwascarriedoutunderargonwithaheatingrateof10C/min.Temperatureversusflowcurvewasobtainedtounderstandtheageingbehaviorthesealloys.MicrostructuralcharacterizationThemachinedsampleswereinitiallygroundusingdif-gradesofpolishingpaper,andthenpolishedwithH9262mdiamondpaste.Polishedsampleswereetchedbyetchant(solutionof5mlaceticacid,10mlH2O,6gacidand100mlethanol).OpticalmicroscopicstudiescarriedoutusingLeicaDMRXMicroscopewithQ-inimageAnalysissoftware.ThedistributionofPbintheywasexaminedinaJEOL,JSE35cscanningelectron(SEM)attachedwithanenergydispersivespec-(EDS).X-raydiffractionstudieswerecarriedoutPhillipsPW1710powderdiffractometerwithCuKH9251Fig.1showsthetypicalmicrostructureofpermanentmouldAZ91alloy.ItconsistsofH9251-MgsolidsolutioncontainingZn,andeutecticcontainingmassiveMg17Al12H9252phasesupersaturatedMgsolidsolution.Theeutecticisseenasvorcedinnature.LamellarkindofsecondaryprecipitatesareseenneartothemassiveMg17Al12phase.TheresultsofbyEDSshowedthatbothparticleshadthesamecomposition.Fig.2exhibitsthemicrostructuresofAZ91containingPb.ithadditionof2%PbtotheAZ91alloynonewphaseisedinthemicrostructure.However,additionofPbreducesdiscontinuousprecipitatesatgrainboundaries.Thesuppres-ofMg17Al12formationiseasilyunderstandablewithXRDperformedonthesamples.Table1givestherelativevaluesofmajorrepresentativeBraggreflectionoftheA.Srinivasanetal./MaterialsScienceandEngineeringA452453(2007)879289Table1RelativeintensityvaluesofmajorrepresentativeBraggreflectionofMg17Al12SampledetailRelativeintensity(I/I0)of(hkl)(411)(%)(721)(%)(332)(%)(510)(%)AZ91ascast11.061.943.271.71AZ91+2Pbascast8.061.851.02AZ91aged7.781.420.801.11AZ91+2Pbaged5.450.960.400.68Mg17Al12precipitatesinbothascastandagedspecimenswithandwithoutPbaddition.FormthetableitcanbeseenthatthedecreaseinrelativeintensityvaluesinthePbaddedalloycom-paredtothebasealloyistheclearindicationofsuppressionofMg17Al12phase.3.1.2.Heat-treatedmicrostructureSolutiontreatmentofalloysamplesat410Cfor48hensuresthatalltheMg17Al12canberedissolvedintomatrix.Nomas-siveMg17Al12hasbeenseenwiththesolutiontreatedsamplesexceptforsomeAlMnparticles(identifiedbyEDX).However,grainboundaryofthebasealloyisclearlyseenwithsolutionizedcondition(Fig.3),whereasitisnotrevealedinPbcontainingalloys.200calandcantamountblackofamountsalloPbtheincreased,increasesFig.4.Opticalmicrostructureof120minagedsamples:(a)AZ91and(b)AZ91+2Pb.totheadjustedgrainsasevidentfromthemicrostructuresof240minagedsamples(seeFig.5).Nevertheless,thegrowthrateisalsofoundtobeslowwithPbaddedsample.FurtherfromTable1,itcanbeseenthattheagedPbaddedalloyshowslessrelativeintensityvaluesforMg17Al12precipitatesthantheagedbasealloy.3.2.AgeingbehaviorTheagehardeningbehaviorofAZ91withandwithoutPbadditionisshowninFig.6.AdditionofPbdoesnotchangetheascast,solutionizedandpeakhardnessmuch.However,thepresenceofPbisfeltwiththetimetoreachthepeakhardness.Thebasealloyreachesthepeakhardnessafter900min,whereasthePbaddedalloytakesmorethan2400min.3.3.DifferentialthermalanalysisThemicrostructuralchangesduringtheageingtreatmentatChavebeenobservedinallalloysamplesusingopti-microscopy.Fig.4showsthemicrostructureofAZ91Pbaddedalloysamplesagedfor120min.Thesignifi-microstructuraldifferencebetweenthesesamplesistheofdiscontinuousprecipitatesatthegrainboundary.Theappearanceatthegrainboundaryindicatestheinitiationdiscontinuousprecipitates.PbaddedstructureshowsfewerofdiscontinuousprecipitatescomparedtotheAZ91y.Infact,discontinuousprecipitatesarerarelyseenwithaddedalloy,whichindicatesthatPbeffectivelysuppressesnucleationofdiscontinuousprecipitation.AstheageingtimethenumberofnucleationsitesfordiscontinuousphaseandthealreadyformeddiscontinuousphasesgrowinFig.3.OpticalmicrostructureofsolutiontreatedAZ91.dissolutionAZ91Fig.7showstheDTAthermogramsintherangeofeutecticduringheatingoftheascastAZ91andPbaddedalloys.Theenthalpychangeandonsettemperaturefor90A.Srinivasanetal./MaterialsScienceandEngineeringA452453(2007)8792Fig.AZ91thethedissolutionPbcanFig.5.Opticalmicrostructureof240minagedsamples:(a)AZ91and(b)+2Pb.reactionaremarkedinthefigure.ItcanbeobservedfromfigurethatthedifferenceinenthalpychangeoftheeutecticindicatesthattheamountofMg17Al12presentintheaddedalloyisless.Fig.8presentstheDTAcurvesofthesolutiontreatedalloys.ItbeseenfromthefigurethatbothcurvesexhibitsimilartrendsFig.6.AgehardeningcurveforAZ91withandwithoutadditio