镁合金,LPSO

Figure3|(a)typicalTEMgraphoftheMDZ-Csample;(b)and(c)SAEDpatternsof18R-LPSOphasealong[1000]directionandMgmatrixin(a),respectively;(d)eutecticphaseintheMDZ-Calloy;(e)SAEDpatternofeutecticphasealong[114]directionin(d);(f)TEMgraphoftheMDZ-545sample;(g)SAEDpatternof14H-LPSOphasealong[1000]directionin(f).

phaseduetothesubstitutionofDybyZn.Thesimilarfinelaminar-shapedconfigurationisalsoobservedintheMDZ-545alloy(Fig.3f).However,thewidthisreducedandtheaspectratioisincreased.AccordingtotheSAEDoffinelayer-shapedprecipitatealong[1000]Mgdirection(Fig.3g),itisfoundthatsmallperiodicdiffractionspotsattheintervalof1/14ofdistancebetweendirectspotand(0002)Mgreflection.Thespotsof(00014)and(0002)Mgarecoincid-entinthesameposition.Basedonthediffractionpeakappearancealongc-directionandthepreviousresults7,23,itcanbeconfirmedthatthefinelaminar-shapedLPSOphaseis14H-typestructure.Opencircuitpotentialandimmersiontest.Fig.4depictstheevolu-tionofcorrosionpotential(vs.SCE)ofimmersiontest.TheOCPoftheMDZ-Calloyisincreasedfromapproximately21750mVto21579mVin20min.Incontrast,theMDZ-545alloymerelytook3mintoreacharelativelystablevalueof21639mV.Additionally,notethattheactivestatedissolutionpotential(Ecorr)oftheMDZ-C

alloyfluctuatesgreatlyincontrasttotheMDZ-545alloy,whichismainlyascribedtotheruptureofoxidationfilm24.

Theeffectoftheimmersiontimeonmicrostructuresandsurfacemorphologiesofcorrosionproductsarestudiedtounderstandtheformationprocessoftheoxidationfilm.FortheMDZ-Csample,thecoarseprecipitatesingrainboundariesarestillidentifiedclearlyafterimmersingfor10minor30min(Fig.5a-1and5b-1),however,theconfigurationbecomesunclearafterimmersingfor60minand120min(Fig.5c-1and5d-1).Inaddition,itwashardlyobservedcracksonthesurfacefilmofcorrosionproductsafterimmersingfor10and30min.Nevertheless,somecrackscanbeseenbetweensec-ondaryphaseandthematrixafterimmersingfor60min(Fig.5c-2).Underhighmagnifications(Fig5a-3,5b-3and5c-3),ahoneycombcorrosionfilmwithcorrosionproductsisdetected.Moreover,acor-rosionfilmfirstlyformsonthesurfaceofMgmatrix(Fig.5b-3).Withincreasingimmersiontime,thecorrosionfilmgrowstowardsanoblesecondaryphase.Somecracksof,1mminwidthareobservedintheareasbetween18R-LPSOphaseandmatrix(whitearrows,Fig.5c-2).

Nevertheless,itisworthytonotethatthecracksarerecoveredafterimmersingfor120min(Fig.5d-2and5d-3),suggestingthattheremediationofoxidationfilmoccurs25.AnotherelucidationisthattheMDZ-Calloytakesabout120mintoformacompactoxidationfilm26.

AsimilarcorrosionprocessisalsoobservedintheMDZ-545sample.Theprecipitatesbecomevaguewhentheimmersiontimeis60min(Fig.6).Underhighermagnifications(Fig.6a-3,6b-3and6c-3),ahoneycombconfigurationfilmisobservedontheentiresurface.Astheimmersiontimeisextended,theoxidationfilmbecomesmoreandmorecompact.UnlikethediscontinuousoxidefilmonsurfaceoftheMDZ-Calloy,thecorrosionfilmformsonthesurfaceofbothMgmatrixandthe14H-LPSOphaseconcurrently(Fig.6a-3).

Figure4|OCPcurvesofMDZalloysunderdifferentstatesin0.9wt.%NaClsolution.