镁合金,LPSO

objectivesintheMg-2Dy-0.5Zn(at.%)alloy,inwhichboth18R-LPSOand14H-LPSOphasesareproducedindependentlybytuningtheheattreatmentprocessing.TheinfluenceofLPSOphasesoncorrosionpropertiesin0.9wt.%NaClhasbeeninvestigatedandthecorrosionmechanismshavebeenclarifiedindetail.Theseresultsprovidesomevaluablecluesfortheexplanationofdegradationbeha-viorsofdifferentLPSOphases,aswellasforthedeterminationofsuitablethermo-mechanicalprocessinMg-Zn-RE-basedalloyscon-tainingLPSOphasesinthefuture.

Figure1|DSCcurvesoftheas-castMg-2Dy-0.5Znalloyataheatrateof66

C/min.

Inaddition,thepresenceofdifferentLPSOstructuresiscloselyassociatedwithsolidificationprocessandheattreatmentcondition.ThecorrosionrateofMg97.25Zn0.75Y2alloycontainingtheLPSOstructurepreparedbyrapidcoolingissignificantlydependentonphasemorphology18.An18R-LPSOphasecanbedetectedinas-castordeformedsamples,whilsta14H-LPSOphasefrequentlyformsafterisothermalheattreatmentatelevatedtemperatures19.Both18Rand14HLPSOphasesareofparticularimportancetoimprovethemechanicalpropertiesfromtheviewpointofpractice,therefore,itisverynecessarytounderstandthedifferenceincorrosionprop-ertiesbetween18R-LPSOand14H-LPSOphases.

Basedontheaboveanalysis,twoquestionsareproposed:(i)whetherthecorrosionmechanismisthesameforboth18R-LPSOand14H-LPSOphases?(ii)whichphaseisbettertoimprovethecorrosionpropertiesalthoughbothofthemplayanessentialroleonimprovingthemechanicalproperties?Inordertoanswerthesequestions,itisbettertoobtainthedifferentLPSOphasesinthesamealloysystem,whichcaneliminatetheeffectofdifferentalloyingelementsoncorrosionproperties.Inthiswork,weachievethese

Results

Phasecharacterization.Twopeaksareobservedat539uCand633uCfromtheDSCheatingcurve(Fig.1)oftheas-castMg-2Dy-0.5Znalloy(MDZ-C),whereinthesecondarypeak(633uC)isverywide.TheDSCcoolingcurvealsorevealsthesimilartrend.Themerediscrepancyliesinthevariationofpeakpositions,whichismostlyaffectedbythermalhysteresis.AccordingtoMg-Dybinaryphasediagram20,thesecondarypeakcorrespondstomeltingprocess.Inordertoclarifythefirstpeak,thesampleissolid-solutiontreatedat545uCfor4h(MDZ-545).

ItcanbeseenfromFig.2thattheMDZ-Csampleismostlycomposedofa-Mgmatrix,anet-likeeutecticphaseandasand-wichedblock-shapedprecipitatewithanapproximatecompositionofMg87.88Zn4.91Dy7.21(Fig.2c),paratively,theMDZ-545samplemainlycontainsa-Mg,aeutecticphaseandafinelamel-lar-shapedphasewithanapproximatecompositionofMg89.34Zn4.42Dy6.24(Fig.2f).

Basedonthebright-fieldTEMmicrographoftheMDZ-Calloy(Fig.3a),wecanseethatatypicalsandwichedstructureisdistributedingrainboundaries.Inthecorrespondingselectedareaelectrondiffraction(SAED)patternofgrayphase(Fig.3b)withtheincidenceofelectronbeamdirectionof[1000]Mg(Fig.3c),someextraweakerspotsaredetectedatpositionsn/6(wherenisaninteger)betweendirectspotand(0002)Mgdiffraction,indicatingthattheLPSOphaseis18R-typestructure13,22.Therectangleprecipitateisdistributedalonggrainboundaries(Fig.3d),whichisascribedtoMg24(Zn,Dy)5eutecticphasewithfcccrystalstructure.Thecalculatedavalueis1.023nm(Fig.3e),whichisslightlylessthanthatofMg24Dy

5

Figure2|TypicalSEMgraphsofMDZalloysunderdifferentstates;(a)and(b)theMDZ-Calloy;(c)representativeelementalcompositionof18R-LPSOphase;(d)and(e)theMDZ-545sample;(f)representativeelementalcompositionsof14H-LPSOphase.