Magnesiumalloyasthelighteststructuralmetalmaterials,higherthanthestrengthofaluminumandsteel,aluminumandsteelthanthestiffnessandquite.Currentlyautomotive,railtransportation,aviationindustryhasbecomeanimportantfieldofapplicationofmagnesiumalloysforautomotiveengine,gearboxhousing,cylinderblock,cylinderheads,intake/exhaustpipeandaircraftenginecasingandothercomponents.Butmagnesiumalloyattemperaturesabove120°C,itshigh-temperaturestrengthandcreepstrengthwilldecline,whichlimitsthemagnesiumalloyontheothertransportationandaerospaceapplications[1].Inthispaper,thebasiccomponentsofAZ91alloy,thealloymicrostructure,mechanicalpropertiesofYttrium(Y),corrosiveinfluenceofenergy,rareearthelementsinthealloyinvestigatedYformedbythephasecompositionandmorphology,inordertoimprovetheheatresistanceofmagnesiumalloysperformancetomeettheautomotiveandaerospaceindustryforheat-resistantmagnesiumalloyhighperformancerequirements.

ExperimentalDesign

ComponentDesign

Ysolubilityinmagnesiummaximumofupto12%,afteraddingbothusesolidsolutionstrengtheningofthegrainboundarystrengtheningtoimprovehightemperaturecreeppropertiesofmagnesiumalloysandofusingrareearthcompounds.ButpureYandoxygenaffinitylarge,itisdifficultdirectlyintoliquidmagnesiumalloy,sothetestintheformofMg-Yalloysjointhemiddle.Referringtotheliterature[2],theelementYisaddedtodeterminetherangeof0.5to1.5%,thealloycompositionofthedesignshowninTable1.

CastingProcess

Magnesiumalloysstrongactivityinthesmeltingprocessoftenseriousburning,needtotakeprotectivemeasures.Thistestusesacompositesolventsownresearch,puremagnesiumburningat3%orless,therateofactualcontrolofrareearthelementsreceivedmorethan90%.Firstaddedinthetestpitcruciblefurnaceabout5%ofthetotalfeedingamountoffluxaddedtotheingotandtheingotofpurefullfluxuponmelting,themoltenbasematerial,uniformlystirredandheatedtoatemperatureabovetheeutecticMgRE40Cwhenso,masteralloywasaddedtopreventoxidationofthealloyburningorsinkingintotheintermediatebottomofthecruciblewasaddedtothemasteralloywasaddedwithaspecificjig1/2ofthemelt,intermediatealloyconstantlymovinguntilcompletelymelted,thetemperatureisadjustedtothecastingthecasting.

solutionandagingtreatment

DifferentalloyscontainingrareearthelementdifferentialthermalanalysiscurveofDSCmeasurementofthelow-meltingcrystallinemeltingtransitiontemperatureofthecomplex.Thesolutiontemperaturewassetatbelowthemeltingtemperaturemeasured15C,byincreasingthesolutiontemperaturetoenhancetheefficiencyofdissolvedandundissolvedphasetoimprovetheeffectofsolidsolutionstrengthening.

Bymeasuringat170Cconditions,heat0to20hours,agingkineticsofthealloy,theoptimumagingtemperaturemeasurement,andtodeterminetheagingtreatment.

microstructureobservation

Aftermachiningthecastingtotakesamplesofthesamesampleposition,grinding,polishing,withpicricacid3g,acetate20ml,alcohol50ml,water50mlmixedetchantforetching.AfterthesamplewasdriedunderanopticalmicroscopeLeicaDMIRMorganizedtoobserveanduseadigitalcameratotakepicturesKODA.Theetchedmicrostructureofthesamplewasobservedunderascanningelectronmicroscope,LV6360,Japan,theUnitedKingdomIACAspectrometerwithenergyspectrumanalysistodeterminethechemicalcompositionofacompoundphase.

ThepreparedsampleswereanalyzedbyPhilipX-rayphasediffractometerusingCutargetvoltage45KVanalysis.

Determinationoftemperature,hightemperaturemechanicalpropertiesandcorrosionCastingthecastobtained,processedintoastandardtemperature,heatthesample,respectively,inCSS-4100ElectronicsInstronuniversaltestingmachineandamachineforthemechanicalpropertiesofthesampletestedbyimmersioncorrosionresistancetest,anetchingsolutionof3.5%NaClsolution.

Results

2.1-castmicrostructure

AlloymicrostructureanalysisofphotographsshowninFigure1.

Figure1shows,theelementYisnotaddedAZ91casttypicalcoarsedendritestructure,thematrixofa-Mgphase,6-Mg17Al12phaseiscontinuousorsemi-continuousdistributioninthematrix.6phaseisdivorcedformlamellareutecticphaseandtheexistenceofwhichdivorcedeutecticphaseunderthemicroscopewhitemeshdistributionpatterns,lamellar6-phaseprecipitatesinthealloysolidificationprocessfromthematrix,themicroscopeunderblack.AfteraddingtheYelements,castmicrostructureobviousrefinement,grainsanddendriticstructurearemoreuniformandfine,whichismainlygeneratedrareearthYphase,promptingalargenumberofheterogeneousnucleation,reducethesecondarydendritearmspacing,thinningdendriticstructure.

NoYalloygrainsizeofabout150~300^m,thesecondphaseparticleswithinthegrainsizeofabout10^m.WithincreasingYcontent,thealloysignificantlyrefinethedegreeoffragmentationof6-Mg17Al12secondphaseofincrease,whenYisaddedtothealloyreached1.25%ofthemaximumrefinement,reductionofgrainsizeto50~150^mso,thesecondphaseparticlesinthegrainboundaryandgraindistributionismoreuniform,reducingthesizeto4^morless.WhenYisaddedinanamountof1.5%,duetothehighcontentofrareearthelements,rareearthphasesegregationoccurs,resultinginararephasewithmorecoarseparticlesgeneratedagainbecomecoarsegrainstructure.

LewisplumonrareearthphasesegregationhasbeenfurtherconfirmedbySEManalysis.AscanbeseenbyFigure2,whentheAZ91+1.25%Yrareeartheitherbrokenofforasecondphaseinthematrixphaseandthegrainboundariesareuniformlydistributed,butwiththeadditionofYincreases,thephasesegregationoftherareearth,andhasbrokenoffthesecondphasebeginsagainbecomethick.

ByanalyzingtheXRDpatternofthealloy,thealloycanbeseenprimarilybyo-Mg,Mg17Al12,Al4MgYREphase.ByanalyzingthedifferentphasesYamountofalloyelementsareadded,thetestfoundthatthephaseisnotduetotheincreaseofrareearthYcontentischanged,norgeneratenewrare-earthphase.ThereasonrefinedrareearthelementYalloystructure[3],firstgenerationrare-earthphaseAl4MgYplayedtheroleofheterogeneousnucleation,thesecondisthesolidificationprocessbylimitingdiffusionkinetics,Yelementinthesolid-liquidinterfaceenrichment,increasingthealloycompositionistoocold,resultingincrystallizationwaytochange,whilefreshmanrarephaseinthegrainboundaries,hinderingthegraingrowthand6-Mg17Al12grewup,reducingthesecondarydendritearmspacing,therebyrefiningdendriticstructureofthealloytypicalmicrostructureofdendritesintofineequiaxed.

Mechanicalpropertiesofthealloy2.2

Tensiletestwascarriedoutaftercastingandhightemperaturesamplesprocessedintoastandardsample,theresultsareshowninTable2.

Fromthetable,arareearthelementYcansignificantlyimprovethehightemperaturepropertiesofthealloy,theaddedamountof1.25%atroomtemperatureandhightemperaturestrengthofthealloyreachesthemaximumvalue,respectively264.8MPaand164.8MPa.

2.3Corrosion

Using3.5%NaClsolutionerosiontestsundercorrosiveconditions.Figure4alloycorrosionmorphology.

AscanbeseenfromFigure4thecorrosionmorphology,Yinthealloyisnotaddedtogeneralcorrosionafterimmersion,largeareawasseverepittingcorrosion,somecorrosionpitdepthdevelopment;Afteradding0.5%Yreducecorrosionofthealloy,partofthepositiondisplayresistancetocorrosion,erosion,butoverallstillserious;whenYcontentincreases,graduallyreducethecorrosionofthealloy,added1.0%Yalloysurfacewasstartedlocallyuniformcorrosion,butcorrosiondepthshallower;Ydosage1.5%,onlyaslightsurfaceportionlocationpittingcorrosionofthealloyasawholeexhibitsauniformcharacteristics.

TheresultsofthecorrosionofalloysSEMmorphologyandEDSanalysisindicatedthatthetwophasesarenotcorrodedbytheMgandAl,canbeidentifiedthroughtheanalysisofthesetwophasesare6-Mg17Al12,oneisdivorcedeutectic6-phase,andtheotherlayerisasheetduringsolidificationfromthe6phaseprecipitatedinthematrix;canalsobeseenfromFIGthirdphasespresentlamellarlamellarstructureofthelamellarphaseanddifferentshaped6phase,byanalysisofthephasecontainingAl,Mg,MnelementshouldberarenascentphaseAl4MgYbuttheforminwhichthepresenceofMnisstillstudied.

Conclusion

1)YelementintheAZ91alloyphasegeneratedAl4MgYrare,heterogeneousnucleationplayaroleinreducingthesecondarydendritearmspacing,thinningdendriticstructure,grainsizefrom150~300^mreducedto50~150^m,intragranularsecondaryphaseparticlesizeof10^mreducedtoabout1-4^m.

)rareearthYcontentof1.25%,theroomtemperatureandhightemperaturemechanicalpropertiesofthebestwere264.8MPaand164.8Mpa,Yelementcontenttocontinuetoimprovesegregationoccurs,resultinginphasewithmorecoar