Precipitation microstructure and their strengthening effects of an Mg-2.8Nd-0.6Zn-0.4Zr alloy with a 0.2 wt. % Y addition

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Materials Science and Engineering A

j o u r n al hom ep ag e :w w w . e l s e v i e r . c o m / l o c a t e / m s e a

Precipitation microstructure and their strengthening effects of an Mg–2.8Nd–0.6Zn–0.4Zr alloy with a 0.2 wt.% Y addition

J.H. Li

, G. Sha

, W.Q. Jie

, S.P. Ringer

aAustralianCentreforMicroscopyandMicroanalysis,TheUniversityofSydney,MadsenBuildingF09,Sydney,NSW2006,Australia

bStateKeyLaboratoryofSolidificationProcessing,NorthwesternPolytechnicalUniversity,Xi’an710072,China

cARCCentreofExcellenceforDesigninLightMetals,TheUniversityofSydney,NSW2006,Australia

a r t i c l e i n f o

Articlehistory:

Received3October2011

Receivedinrevisedform10January2012 Accepted11January2012

Available online 20 January 2012

Keywords:

Magnesiumalloy Precipitation Mechanicalproperties TEM

Atomprobetomography

a b s t r a c t

Amicro-alloyadditionofY(0.2wt.%)producedasignificantimprovementinthetensileyieldstrengthof anMg–2.8Nd–0.6Zn–0.4Zr(wt.%)alloyat200^◦C,witha28%increasefrom143MPato183MPa.APT characterisationconfirmedthatsmallsoluteclustersinahighnumberdensitywerepresentinthe as-quenchedsample.TEMexaminationsrevealedthat␤-typeprecipitateshabitingonprism{1120}

planesweredominantinthesampleafteragedfor14hat200^◦C,andtheyco-existedwith␥-typepre- cipitatesonthebasalplaneofMgmatrix.Theprecipitationsequenceof␤-typeprecipitatesissolute clusters→␤→␤→␤1beforethepeakhardness.TherewassignificantclusteringofsolutesintheY- containingalloy,butYdidnotclearlypartitionintoclustersorprecipitatesandremainedintheMgmatrix.

␤,␤and␤1weremeasuredwithstoichiometricMg9(Nd,Zn),Mg4(Nd,Zn)andMg2(Nd,Zn),respectively.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Magnesiumalloyshaveimportantapplicationsintheautomo- tiveandaerospaceindustriesbecauseoftheirhighspecificstrength fortheweightreductionandbetterfueleconomy[1].However, themechanicalpropertiesofconventionalMgalloysareoftennot suitableforhigh-temperatureapplications.Thelowermechanical properties,particularlythelowertensileyieldstrengthatelevated temperatures,hinderthewiderapplicationofMgalloys[2].The developmentofMg alloys forthehigh-temperatureapplication remainstobeachallengingresearchtargetfortheinternational materialscommunity.

Rare-earth(RE) elementaddition hasbeenconsideredto be effectivetopromoteprecipitationhardeningandtoimprovethe high-temperatureperformanceofMgalloysduetotheformation ofthermallystablenano-sizedprecipitatesinahighnumberden- sity[3–18].Indeed,somecommercialhigh-strengthheat-resistant Mgalloys(i.e.WE54/43,QE22,etc.)containatleasttwotypesof REelements(oneismainandtheotherisminor).Frequently,RE

∗Correspondingauthorat:AustralianCentreforMicroscopyandMicroanalysis, TheUniversityofSydney,MadsenBuildingF09,Sydney,NSW2006,Australia.

Tel.:+61290369050;fax:+61293517682.

E-mailaddress:gang.sha@sydney.edu.au(G.Sha).

1 Presentaddress:ChairofCastingResearch,TheUniversityofLeoben,A-8700 Leoben,Austria.

elementsareselectedfromNd,Y,sometimesScandGd,etc.[2]for furtherimprovingthehigh-temperatureperformanceofMgalloys.

MgalloyswithanNdadditionexhibitastrongage-hardening response and improvement in mechanical properties. The pre- cipitationof␤,␤ and␤phasesandthemechanicalproperties ofsomeMg–Ndbasedalloyshavebeenextensivelyinvestigated [3–12].However,thehigh-temperaturemechanicalpropertiesof theseMg–Ndbasedalloysareoftennotidealforapplicationsabove 250^◦C.Yisanimportantalloyingelementtoeffectivelyimprove themechanicalpropertiesofMgalloysatelevatedtemperatures [13–18].SeveralimportantcommercialMgalloys,suchasWE54 andWE43,weredevelopedhavingacombinedhigh-leveladdition ofYandNd[13–15].TheprecipitationphasesintheseMgalloysare

␤,␤,␤1and␤,proposedwithstoichiometryofMg₃RE,Mg₅RE, Mg₃REandMg₃RE,respectively[14,16].Theyprovidestrengthen- ingeffectstothesehigh-Y-containingMgalloys[13–17].

The addition of Y in high quantities, even for WE54 (Y:

4.75–5.5wt.%) and WE43 (Y:3.7–4.3wt.%)alloy, hasthedraw- backstoincreasealloydensityandproductioncost.Consequently, alowerleveladditionofYbecomesmoreattractiveifitcanpro- duceeffectiveimprovementsinthemechanicalpropertiesofMg alloys. Recently, a micro-alloy additionof only 1–2wt.% Yinto Mg–3Nd–0.5Zn–0.4Zr(wt.%)alloyhasbeenreportedtoremark- ably increase itstensile properties and creepresistance[18].It isgenerallybelievedthattheimprovementofthealloystrength is correlatedwithanenhancedprecipitationbytheaddition of Y. To date, there is a lack of detailed information about the 0921-5093/$–seefrontmatter© 2012 Elsevier B.V. All rights reserved.

doi:10.1016/j.msea.2012.01.043

evolutionofprecipitatesmicrostructure,and thepartitioningof solutesintoprecipitates during ageinglow-Y-containing alloys.

Better understanding precipitates microstructure formation is of importance for establishing the relationship between the microstructureandmechanicalpropertiesofthelow-Y-containing Mg–Ndbasedalloys.

Thispaperaimstoreportthemechanicalpropertiesandpre- cipitatesmicrostructureofanMg–2.8Nd–0.6Zn–0.4Zr(wt.%)alloy witha0.2wt.%Yaddition.Byunveilingcomprehensivestructural andchemicalinformationofsolute-richfeaturesformedinthealloy usingcarefulTEMcharacterisationsandquantitativeatomprobe dataanalyses,this investigation hasobjectivestoelucidate the precipitationsequence,toaddressthesolutepartitioningbehav- iorsduringageing,andtounderstandthestrengtheningeffectand mechanismsofthelow-Y-containingMg–Ndbasedalloy.

2. Experimentalmaterialandprocedures

Mg–2.8Nd–0.2Y–0.6Zn–0.4Zr alloy (wt.%) wasprepared with high purity Mg (99.9%), Zn (99.9%), Nd (99.9%), Mg–28Y and Mg–33Zrmasteralloysinanelectricresistancefurnaceunderthe protectionofananti-oxidizingflux(containing54–56%KCl,14–16%

BaCl₂,1.5–2.5%MgO,27–29%CaCl₂),andcastedintoasandmould.

Thechemicalcompositionsoftheexperimentalalloyweredeter- minedbyusinganinductively coupledplasma atomicemission spectrum(ICP-AES) apparatus.The solutiontreatmentof speci- menscutfromthealloyingotwasconductedat525^◦Cfor18hina saltbath.Theageingofwater-quenchedspecimenswasperformed inanoilbathforvariousageingtimeat200^◦C.TheVickershardness testingwasundertakenonLECOHardnessTester(LV700AT)with 1kgloadand10sdwellingtime.Eachdatapointreportedinthis paperrepresentedanaverageofatleast10measurements.Theten- siletestswereperformedusingstandardtensiletestingmachine (Instron1195)atroomtemperature(RT),200^◦C,250^◦C,300^◦Cand 350^◦C, respectively,witha crossheadspeedof5mm/minand a strainrateof2.0×10⁻³s⁻¹.A5minholdingwasappliedtoeach sampletobalanceitstemperaturebeforeeachhigh-temperature tensiletest.Eachdatapointreportedinthispaperwasanaverage ofatleast3testsamples.

TEMfoilspecimenswerepreparedbytwin-jetelectro-polishing inasolutionof25%HClO4 and75%methanolat−40^◦Cand20V, andthenusinglow-energyionbeamthinningforsurfacecleaning.

TheTEMobservationswereperformedinaPhilipsCM12operat- ingat120kVandahighresolutionTEM(JEOL-3000F)operating at300kV.Thesamplesforatomprobeanalysiswerepreparedby two-stageelectro-polishingofblankswithasizeofapproximately 0.5mm×0.5mm×15mm.Thefirststepwasconductedusingan electrolyteof25%perchloric acidinacetic acidat 15Vatroom temperatureandthesecondstepwasin2%perchloricacidin2- butoxyethanolat20V.Atomprobeanalyseswereperformedusing aImagoLEAP^TM3000SI,operatingataspecimentemperatureof 20K,20%voltagepulsefractionandunderultrahighvacuumcon- ditions(about1.8×10⁻¹¹Pa).

Atom probe data sets werecarefully reconstructedusingan approachoutlinedrecentlybyMoodyetal.[19].Themaximum separationalgorithmwasemployedtoidentifysolute-richfeatures [20,21],inwhichNdandZnwereselectedasclusteringsolutes,a solute-separationdistanceof0.8nmandtheminimumsizeof15 soluteatomswereusedintheidentificationofsolute-richfeatures, suchasclustersandprecipitates.Solute-richfeaturescontaining

<15soluteswereneglectedintheanalysisbecausesuchsmall-size featureshighlyexistedinthevolumewithsolutesinrandomdistri- bution[11,12].Inordertoestimatetheprecisecompositionoflarge precipitates,selection-boxanalysiswasemployedtomeasurethe compositionfromthecoreregionofeachprecipitateinorderto

Fig. 1.Age-hardening responses of Mg–2.8Nd–0.2Y–0.6Zn–0.4Zr (wt.%) and Mg–2.8Nd–0.6Zn–0.4Zralloysagedat200^◦C.

reducetheeffectoftrajectoryoverlapbetweenprecipitateandthe Mgmatrixonthemeasurement.

3. Results

3.1. Agehardeningresponseandtensilepropertiesof Mg–Nd–Zn–Zrbasedalloys

Fig.1showstheage-hardeningresponseoftwoMg–Nd–Zn–Zr alloyswith/withouta0.2wt.%Yadditionduringageingat200^◦C.

TheMgalloywithanadditionof0.2wt.%Yexhibitedanenhanced strengtheningeffectincomparisonwiththeY-freealloy.Thehard- nessoftheas-quenchedsamplesincreasedfrom43.6±2.0HVto 58.0±2.0HV.Afterageingfor3hat200^◦C,thehardnessofboth alloysexhibitedstrongincreasesof∼18.8HVintheY-containing alloyand∼23HVintheY-freealloy.ThepeakhardnessoftheY- containingalloywas79.4±2.4HVafter14hageing.Furtherageing beyond14hledtoprogressivelydecreaseintheirhardness.

Fig.2showsthetensilepropertiesofaY-containingMgalloy andaY-freeMgalloywithdifferentthermalhistory,includingas- cast,as-quenchedafterasolutiontreatment,agedfor3hand14h at200^◦C.Thetensilestrengthsofthetwoalloys(Fig.2a)arecor- relatedwellwiththeagehardeningresponsesofthetwoalloys (Fig.1).Anadditionof0.2wt.%Yprovidedaclearimprovement intheyieldstrengths(YS)ofthealloyinallconditions.However, theirultimatetensilestrengths(UTS)andductilitybetweenthetwo alloysshowednosignificantdifference.Theyieldstrengthoftheas- quenchedsamplesexhibitedanimprovementfrom100±6.4MPa to133±7.2MPa(Fig.2a).Withtheincreaseinageingtimefrom 3hto14h,boththeirUTSandYSincreased,buttheirductilityval- uesdecreased,asshowninFig.2a.Importantly,theY-containing alloyinthepeak-agedcondition(T6,14h)exhibitedanimproved YSoverthatoftheY-freealloyinthetemperaturerangefromRTto 350^◦C(Fig.2b).Inparticular,theYSoftheY-containingMgalloy at200^◦Cis183±7MPa,28%higherthan143±4MPa,thatofthe Y-freealloy.Inordertogaininsightintomicrostructuresproducing theinterestingstrengtheningeffecttotheY-containingalloy,sam- plesindifferentagedconditionswereselectedtoperformfurther TEMexaminationandAPTanalysis.

3.2. TEMcharacterisationofprecipitatesinanMg–Nd–Zn–Zr–Y alloy

Fig. 3 shows a representative bright field (BF) [0001] TEM micrographandthecorrespondingselectedareadiffractionpat- terns(SADP)ofanas-quenchedsample.Noprecipitatesareevident inTEMimage,asshowninFig.3a.ThediffractionsintheSADPwere

Fig.2. TensilepropertiesofMg–2.8Nd–0.6Zn–0.4Zr(wt.%)alloyswithorwithout0.2wt.%Yaddition,(a)indifferentageingconditions,and(b)measuredatdifferent temperatures.

fromthe␣-Mgmatrix,asshowninFig.3b.TheTEMresultsindi- catedthatnoprecipitationtookplaceintheas-quenchedsample.

Afteragedfor14hat200^◦C,plate-likeprecipitateswereevidentin TEMimages,asshowninFig.4.Theplate-likeprecipitateshabited on{1120}␣,andabout16±4.3nminlengthand1.5±0.7nmin widthwithanaspectratio(lengthtowidth)ofapproximately10:1, asobserved in[0001]BF image.Theweakdiffraction spotsat 1/2{011 0}␣inthe[0001]SADP,asshowninFig.4b,werefrom

␤precipitates,withD0₁₉structure(a=b=0.64nmandc=0.52nm) [14,17].Thiswasfurtherconfirmedbyweakdiffractionsobserved at 1/2{0110}␣ and 1/2{2114}␣ in the {2110}␣ and [0110]_␣ SADPs,asshowninFig.4dandfrespectively.Weakdiffractionsat 1/2(2 ¯1 ¯1 2)_␣inthe{0110}_␣SADP,asmarkedwithawhitesolid arrow inFig.4f,suggested that␤ precipitateswerepresent in themicrostructureofthealloy.The ␤ phaseisknowntohave abase-centredorthorhombicunitcella=0.640nm,b=2.223nm, c=0.521nm[14,17],andhasalamella-likemorphologywithalon- gitudinalaxisparallelto[0001]_␣.

The{2110}_␣and[0110]_␣TEMBFmicrographs,asshownin Fig.4canderespectively,revealedthatthinprecipitates(marked withawhitesolidarrow)habitingonthebasalplaneoftheMg matrixwerepresentinthemicrostructure,andtheyareperpen- dicularto␤-typeprecipitates(markedwithablacksolidarrow).

No clearstreaks of thebasal precipitatesobserved in [2110]_␣ and[0110]_␣SADPs(Fig.4d andf)indicatedthatthebasalpre- cipitateswereprobablyatanextremelylowvolumefraction.The precipitatesonbasal planeof Mg alloyshave beenreported to

be␥-typephase(␥,MgZnREcontaining,hexagonal,a=0.55nm, c=0.52nm[4]).Similar␥-typeprecipitateswereobservedinan Mg–Nd–Gd–Zn–Zralloyagedat330^◦Cfor90min[9].

CarefulTEMexaminations,asshown inhigh-resolutionTEM imagesinFig.5,revealedthat␤1 phasestartedtoappearinthe microstructureofthealloyagedfor14h.The{002},{111}and {220}latticeplanesof aprecipitate, asshownin Fig.5b,were clearly resolvedin thehigh-quality latticefringeimage.The d- spacing of{002},{111}and{220}latticeplanes(Fig.5)were measuredtobe0.36, 0.416and 0.255nm, respectively,in good agreementwiththeidentificationas␤1phase,withanfccunitcell (a=0.72nm),whichisveryclosetoa=0.74nmreportedbyNieetal.

[14].The␤1precipitateswereprobablyataverylownumberden- sityinthemicrostructure,becausenoclearreflectionsof␤1were observedintheSADPsofthealloy,asshowninFig.4.

3.3. APTcharacterisationofprecipitatesinanMg–Nd–Zn–Zr–Y alloy

Fig. 6 shows the APT elemental maps of Nd, Y and Zn obtainedfromanMg–2.8Nd–0.2Y–0.6Zn–0.4Zr(wt.%)alloysam- plequenchedincoldwaterafterasolutiontreatment(for18hat 525^◦C).AstrongclusteringofNdwasobservedintheNdatom map, as shown in Fig.6b. Afterremoving solute atoms in the matrix,finesoluteclustersinahighnumberdensity,asshownin Fig.6a,wereevidentintheanalysedvolume.Nootherclearstruc- turalfeature(i.e.dislocation,precipitates,grainboundary[22])was

Fig.3. TEMbrightfieldimagein[0001]_␣zoneaxisandcorrespondingSADPofanMg–2.8Nd–0.2Y–0.6Zn–0.4Zr(wt.%)alloysamplequenchedafterasolutiontreatment.

Fig.4.TEMbrightfieldimagesandSADPsoftheMg–2.8Nd–0.2Y–0.6Zn–0.4Zr(wt.%)alloyagedat200^◦Cfor14h.(a,b)B//[0001]␣;(c),(d)B//[1120]_␣;(e,f)B//[0110]_␣.

observedintheanalysedvolume.After3hageing,plateletprecip- itatesenrichedwithNd,ZnandZrandhabitedon{2110},with anaveragelengthof10±3nmalong0110,wereevidentwithin theanalysedvolume,asshowninFig.7.Interestingly,thedistri- butionofYatomsremainedtobeuniformandwassimilartothat observedintheas-quenchedcondition.Afteragedat200^◦Cfor14h, theaveragelengthofprecipitatesalong0110directionswere measuredto be15±4nm,in goodagreementwith16±4.3nm observedbyTEMexaminationin Fig.4a,and Yremainedtobe uniformly distributed in the ␣-Mg matrix, as shown in Fig. 8.

Examinationsofprecipitatesindifferentviewdirectionsconfirmed that most precipitates were elongated with their longitudinal axisparallel to[0001]_␣, which is in agreementwithour TEM

observationsof␤,␤ and␤1 precipitates.Suchprecipitatesare believedtobeeffectivetohamperthebasaldislocationmovement [23,24].

Thequantitativeanalysisresultsofatomprobedataareshown inFig.9.Thenumberdensityofthefinesoluteclustersaftersolu- tiontreatmentis5.0±0.2×10²³m⁻³(whichwasestimatedonthe basisofthetotalnumberofsoluteclustersidentifiedintheanalysed volume),asshowninFig.9a.Thenumber densityofsolute-rich featuresreachedthepeakof7.6±0.6×10²³m⁻³after1hageing, thendecreasedsignificantlyto3.4±0.6×10²³m⁻³(3h)andfur- therdownto2.4±0.3×10²³m⁻³(14h)withincreasinginageing time.ThesoluteconcentrationsofNd,ZnandZrinthe␣-Mgmatrix decreasegreatlyduringtheageingupto3h,inparticularthecase

Fig.5. HRTEMimages(a,b)oftheprecipitates(␤1)intheMgmatrixofanMg–2.8Nd–0.2Y–0.6Zn–0.4Zr(wt.%)alloyagedat200^◦Cfor14h.B//[0001]␣.

Fig.6.APTelementalmapsofNd,YandZnofanMg–2.8Nd–0.2Y–0.6Zn–0.4Zr(wt.%)alloysamplequenchedafterasolutiontreatment(T4).(a)AcombinedmapofNd (green),Y(blue)andZn(red)afterremovingsolutesinthematrix;(b)Nd(green)map;(c)Y(blue)map;(d)Zn(red)map.(Forinterpretationofthereferencestocolorin thisfigurelegend,thereaderisreferredtothewebversionofthearticle.)

Fig.7.APTelementalmapsofNd,Y,ZnandZrofanMg–2.8Nd–0.2Y–0.6Zn–0.4Zr(wt.%)alloysampleagedat200^◦Cfor3h.(a)Nd(green);(b)Y(lightblue);(c)Zn(red);(d) Zr(darkblue).(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthearticle.)

Fig.8. APTelementalmapsofNd,Y,ZnandZrofanMg–2.8Nd–0.2Y–0.6Zn–0.4Zr(wt.%)alloysampleagedat200^◦Cfor14h.(a)Nd(green);(b)Y(blue);(c)Zn(red);and (d)Zr(darkblue).(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthearticle.)

forNd,asindicatedinFig.9b.ThedecreaseoftheNd,ZnandZr soluteconcentrations ofthematrixisdue tothepartitioningof theseelementsinto theprecipitate, asshown in Fig.9c and d.

Theaveragesoluteconcentrationofprecipitatesincreasedwiththe increasingageingtime,asshowninFig.9c.Theeffectofageingtime onpartitioningratioofsolutes(calculatedbytheaveragesolute

concentrationofprecipitatesoverthatofthematrix)isshownin Fig.9d.Ndwasobservedhavingamuchstrongerpartitioningthan Znduringthe3hageing.

Fig.10showstypicalcompositionprofilesmeasuredfromthree different precipitates.The thin precipitates,widely observedin shortly agedsamples, containeda lowlevel of solutesandhad

Fig. 9.Quantitative APT analysis results about the evolution of structure and chemical compositions of the solute-rich features and the Mg matrix in Mg–2.8Nd–0.2Y–0.6Zn–0.4Zr(wt.%)alloyduringshortageingat200^◦Cupto14h.(a)numberdensity;(b)soluteconcentrationofthe␣-Mgmatrix;(c)soluteconcentration ofthesolute-richfeatures;and(d)thepartitioningratioofsolutesbetweensolute-richfeaturesandtheMgmatrix.

Fig.10.TypicalsolutecompositionprofilesmeasuredacrossthecoreofthreedifferentprecipitatesinMg–2.8Nd–0.2Y–0.6Zn–0.4Zr(wt.%)alloyagedat200^◦Cfor14h.

a stoichiometry of Mg₉(Nd,Zn), as shown in Fig. 10a. The thin precipitateslikely are␤.The compositionprofiles ofa slightly thickerprecipitate,asshowninFig.10b,giveastoichiometryof Mg₄(Nd,Zn).Suchprecipitateswereoftenobservedinsampleaged formorethan3h.Theyarelikely␤.Athickprecipitatewasmea- suredhavingacorechemistryclosetoMg2(Nd,Zn),asshownin Fig.10c.Itisreasonabletobelievethattheprecipitateis␤1,given that␤1hasbeenpositivelyidentifiedbyTEMcharacterisationof a14-hageingsample(Fig.5).Thedifferentchemicalcompositions observedamongtheseindividualprecipitates(Fig.10)areconsis- tentwiththetrendobservedinFig.9c,inwhichtheaveragesolute concentrationofsolute-richfeaturesincreaseswithincreasingage- ingtime.Itisworthnotingthattheprecipitatechemistryunveiled bythisworkisdifferentfromMg₃RE,Mg₅RE(Mg₁₂NdY),Mg₃RE (Mg₁₄Nd₂Y)for␤,␤ and␤1respectivelyreportedinadifferent Mgalloy[14].Itisunclearifthedifferenceisduetotheinfluenceof thealloycomposition,ageingcondition,ordifferentanalysistech- niquesused.Thinbasal␥-typeprecipitateswerenotresolvedby theAPTanalysis.Thisisprobablyduetothehighlocalmagnifica- tioneffectoftheprecipitatesandtheAPTdetectionefficiencyof

∼55%.

4. Discussion

4.1. Soluteclusteringanditsstrengtheningeffectduringthe early-stageprecipitationinMg–Ndbasedalloys

Thesoluteclusteringoftenoccursduringtheearly-stageage- ing prior to the formation of precipitates (such as ␤ phase [11]), and can affect age-hardening response of various alloys [11,12,25,26]. Our APTcharacterisation unveiled that Ndwasa strong-clusteringalloyingelementandcontributedtheformation offinesoluteclusters(consistingofNdandZn)intheas-quenched Y-containing Mg alloy after a solution treatment,as shown in Fig.9c.TheclusternumberdensityintheY-containingalloywas 5.0±0.2×10²³m⁻³,muchhigherthan0.2±0.22×10²³m⁻³inan as-quenchedMg–Gd–Nd–Zn–Zralloy[11,12].Thisindicatesthat theYadditionhaspromotedtheclusteringofsolutesintheMg

alloy.Interestingly,noclearYclusteringwasdetectedduringthe earlyclusteringofsolutesasshowninFig.6c.Theprecisereason fortheenhancedNdclusteringintheMgalloywithaYadditionis unclear,whichisprobablycorrelatedwiththeeffectofYaddition onthesolubilityofNdinMgandtheenhanceddiffusionofNddue toitshighbindingenergywithvacancies(0.25eV)[27].Itisworth- notingthatalthoughthenumberdensityofsoluteclustersinthe MgalloyisclosertothatobservedinsomeAlalloys[22,25,26],it shouldnotbeasurprisethatthestrengthofthisMgalloyismuch lowerthanthoseoftheAlalloyssincethedeformationmechanisms ofMgalloysaregreatlydifferentfromthoseofAlalloys.Inaddition, notallsoluteclustersareabletoprovidethesamestrengthening effecteveninAlalloys[25].Thesignificantenhancedstrength(from 43HVto58HVinhardness,andfrom100MPato133MPainYTS) oftheY-containingalloyabovethatoftheY-freealloy,asshownin Figs.1and2,clearlyindicatedthatthesoluteclustersdidprovide effectivestrengtheningeffecttotheY-containingMgalloy.

4.2. Precipitatesandtheirstrengtheningeffectinthe Mg–Nd–Y–Zn–Zralloy

Theprecipitationsequenceof␤-typeprecipitatesinthelow- Y-containingMg–Ndbasedalloyis:soluteclusters→␤→␤→␤1

duringtheageingupto14haccordingtoourTEMandAPTchar- acterisations.␥-Typeprecipitateswereobservedinthealloy.The basal␥-typeprecipitatesareknowntobelesseffectivetohinder theslidingofdislocationsonbasalplanes,andarelesseffectiveto providestrengtheningeffectthanprime␤-typeprecipitatesinMg alloysatroomtemperature[23].Withincreasingageingtime,the sizeand chemicalcompositionof ␤-typeprecipitates,asshown in Figs.7–9,increased.Meanwhile, there wasfurtherenhanced partitioningofNdandZnintoprecipitateswiththeincreasingage- ingtimefrom1hto14h,asshowninFig.9candd,whichwere correlatedwellwiththeformationofmore␤and␤1precipitates containinghigherconcentrationsofNdandZn,asshowninFig.10.

Withtheincreaseinageingtime,thenumberdensityofsolute- reachfeaturesreachedthepeakvalueinthealloyat1hageingand decreasedprogressivelythereafter.Theincreaseinnumberdensity

ofsolute-rich features(including clustersand small␤precipi- tate)providedfurtherstrengtheningtothealloy,withahardness increaseof15HV,asshowninFig.1.Adecreaseofthenumber densityofsolute-rich features (Fig.9a), butanadditional hard- nessincreaseof7HV(Fig.1)inthealloyduringfurtherageingup to14h,indicatedthatvolumefractionofsolute-richfeaturesare importantforproducing strengtheningeffect.More information abouttheeffectofprecipitatevolumefractionontheprecipitation- strengtheningcanbefoundinRef.[28].

ManyMgalloyshavebeenobservedtoformprecipitatesina highnumberdensity,similartothatobservedinourY-containing Mgalloys,butdifferentage-hardeningresponse.For example,a hot-rolledMg–12Gd–1.9Y–0.69Zralloyagedat200^◦Cwasreported toproducesolute-richfeaturesintheorderof10²⁴m⁻³inanum- berdensity[16],higherthanthatobservedinourcastY-containing Mgalloy.Thewroughtalloywasreportedtohavethepeakhard- nessof130HV,muchhigherthanthatofourcastalloy.Thisis likelydue tothehighcontentof alloyingelementsandrefined grainsizeofthewroughtMgalloy.Incontrast,WE43alloywas reportedtohaveprecipitatenumberdensityof1.5×10²⁴m⁻³and thepeakhardnessof100HV[29],whichismuchclosetothatof ourY-containingMgalloy.Inliteratures,someMgalloys,suchas AZ91alloy[30],werereportedhavingsimilarhardeningresponse asourY-containingMgalloy,butmuchlowernumberdensityof precipitates(1.09×10¹⁹m⁻³)thanthat of ourY-containingMg alloy.Thelow-number-densitymeasurementcouldoftenfoundto berelatedwiththedifferentanalysistechniqueusedintheirmea- surement.OurTEMandAPTcharacterisationoftheas-quenched samples(Figs.3and6,respectively)clearlydemonstratedthatfine soluteclusterseasilyidentified by APTare hard tobe resolved byTEMexaminations.Asaresult,TEMexaminationscouldeas- ilyunderestimatethenumber densityof solute-richfeatures in themicrostructure.Inordertomakeasensiblecomparisonamong resultsobtainedbydifferentanalysistechniques,thelimitations ofeach analysistechnique havetobetaken intoconsideration.

This indeed creates some difficulties to correlate with results obtainedbydifferentanalysistechniques.Tobetterunderstanding age-hardeningresponseofAZ91alloy,somedetailedAPTcharac- terisationwillbeuseful.

The Y-containingalloyagedfor 14hat200^◦C wasobserved havingtheco-existence of␤-typeprismatic precipitatesand␥- typebasalprecipitatesinthemicrostructure,asshowninFig.4.

Singletypeprecipitates,either␤-typeor␥-type,werefrequently observedinmanyMgalloys.Forexample,␤-typeprecipitateswere observedintheMg–REalloys[9,14,17].The␥-typethinbasalpre- cipitateshavebeenobservedinMg–8Y–2Zn–0.6Zr(wt.%)alloy[31], Mg–1Gd–0.4Zn–0.2Zr (at.%) alloy [32], Mg–2.4Nd–0.4Zn–0.6Zr (wt.%)alloy[5]andMg–2.8Nd (wt.%)alloywiththeadditionof 1.3wt.%Zn[3,4].However,toourknowledge,thesimultaneousco- existenceof␤-typeprecipitatesand ␥-typeprecipitateshasnot beenreportedinpreviousstudiesofMg–Ndbasedalloys,although co-existenceofprecipitateslyingeitherparalleltothebasalplane oratanangletobasalplanehavebeenreportedinAZ91alloy[30].

Itisworthnotingthatthetwotypesofprecipitatescanproduce differentstrengtheningeffects.Thebasal␥-typeprecipitatesare effectivetoserveasobstaclesfornon-basalslippingofdislocations, andhenceimprovethehigh-temperaturemechanicalproperties of Mg alloys [33]. The formation of ␥-type precipitates in the Mg–2.8Nd–0.2Y–0.6Zn–0.4Zr(wt.%)alloy(Fig.4)wasconsistent withtheimprovedyieldstrengthofthealloyatelevatedtemper- atures(Fig.2b).Theprismatic␤-typeprecipitatesinthealloyare believedtoplayanimportantroletoenhancealloy’smechanical propertiesatroomtemperature.TEMobservation(Figs.3–5)and atomprobetomography(Figs.7and8)revealedahighnumber densityofplate-shape␤seriesprecipitatesformedonprismatic planesofthe␣-Mgmatrixduringageingtreatment.Todate,itis

unclearwhytwotypesofprecipitatesformedsimultaneouslyin thealloy.Morefundamentalresearchisnecessarytounveilfactors controllingtheformationoftwotypesofprecipitates.Thiswillbe importantfordesignanddevelopingnewadvancedMgalloyswith excellentperformanceatroomtemperatureandelevatedtemper- atures.

5. Summary

1.AnewMg–2.8Nd–0.6Zn–0.4Zr(wt.%)alloywith0.2wt.%Yaddi- tionshowedasignificantimprovementonhardnessandtensile propertiesduringageingat200^◦Cforupto14h.Thealloy,in particular,exhibitedenhancedtensileyieldstrengthinthetem- peraturerangeof200–350^◦C.

2.The low-level Y addition has promoted the significant clus- tering of Nd and Zn, and subsequently the formation of high-number-density␤-typeprecipitateswithNd,Znandslight Zrenrichments.

3.ThepartitioningofNdinto␤-typeprecipitatesinthealloywas muchstrongerthanotheralloyingelements.Interestingly,the distributionofYatomsremainedinthealloywithincreasingin ageingtime,withoutexhibitinganysignificantpartitioninginto precipitatesinthealloyduringageingat200^◦C.

4.Theimprovementofmechanicalpropertiesatelevatedtemper- atureswascorrelatedwellwithanenhancedprecipitationof

␤-typeprecipitates,including␤,␤and␤1,andtheco-existence withbasal␥-typeprecipitatesinthemicrostructureofthealloy.

5.The stoichoimetry of ␤, ␤ and ␤1 was measured to be Mg₉(Nd,Zn),Mg₄(Nd,Zn),andMg₂(Nd,Zn)respectivelyintheMg alloy.

Acknowledgments

Theauthorsaregratefulforscientificandtechnicalinputand supportfromtheAustralianMicroscopy&MicroanalysisResearch Facility (AMMRF) node at the University of Sydney. Jiehua Li alsowishestothanktheChinaScholarshipCouncilfor financial support.ThisworkispartlysupportedbytheDoctorateFounda- tion ofNorthwestern PolytechnicalUniversity under Grand No.

(CX200705).

References

[1] I.J.Polmear,Mater.Sci.Technol.10(1994)1.

[2]B.L.Mordike,Mater.Sci.Eng.A324(2002)103–112.

[3]T.J.Pike,B.Noble,J.LessCommonMet.30(1973)63.

[4] P.A.Nuttall,T.J.Pike,B.Noble,Metallography13(1980)3–20.

[5]D.H.Ping,K.Hono,J.F.Nie,ScriptaMater.48(2003)1017–1022.

[6]R.Wilson,C.J.Bettles,B.C.Muddle,J.F.Nie,Mater.Sci.Forum.41(267)(2003) 9–422.

[7]L.R.Gill,G.W.Lorimer,P.Lyon,Adv.Eng.Mater.9(2007)784.

[8]P.J.Apps,H.Karimzadeh,J.F.King,G.W.Lorimer,ScriptaMater.48(2003) 1023–1028.

[9]G.Sha,J.H.Li,W.Xu,K.Xia,W.Q.Jie,S.P.Ringer,Mater.Sci.Eng.A527(2010) 5092–5099.

[10]J.H.Li,W.Q.Jie,G.Y.Yang,Trans.NonferrousMet.Soc.China18(2008)s27–s32.

[11]G.Sha,J.H.Li,W.Q.Jie,S.P.Ringer,in:K.U.Kainer(Ed.),Proceedingsof8th InternationalConferenceonMagnesiumAlloysandtheirApplications,Wiley- VCHVerlagGmbH&Co.KGaA,Weinheim,2009,pp.40–45.

[12]J.H.Li,G.Sha,T.-Y.Wang,W.Q.Jie,S.P.Ringer,Mater.Sci.Eng.A(2011), doi:10.1016/j.msea.2011.10.092.

[13]J.F.Nie,B.C.Muddle,ScriptaMater.10(1999)1089–1094.

[14]J.F.Nie,B.C.Muddle,ActaMater.48(2000)1691–1703.

[15] C.Antion,P.Donnadieu,F.Perrard,A.Deschampa,C.Tassin,A.Pisch,ActaMater.

51(2003)5335–5348.

[16]T.Honma,T.Ohkubo,K.Hona,S.Kamado,Mater.Sci.Eng.A395(2005)301–306.

[17]T.Honma,T.Ohkubo,S.Kamado,K.Hono,ActaMater.55(2007)4137–4150.

[18]J.H.Jun,B.K.Park,J.M.Kim,K.T.Kim,Adv.Mater.Res.26-28(2007)137–140.

[19]M.P.Moody,B.Gault,L.T.Stephenson,D.Haniel,S.P.Ringer,Ultramicroscopy 109(2009)815–824.

[20] J.M.Hyde,C.A.English,in:R.G.E.Lucas,L.Snead,M.A.J.Kirk,R.G.Elliman(Eds.), MicrostructuralProcessesinIrradiatedMaterials,MRS2000FallMeetingSym- posium,Boston,MA,2001,p.27.

[21]D.Vaumousse,A.Cerezo,P.J.Warren,Ultramicroscopy95(2003)215–221.

[22]G.Sha,R.K.W.Marceau,X.Gao,B.C.Muddle,S.P.Ringer,ActaMater.59(2010) 1659–1670.

[23]J.F.Nie,ScriptaMater.48(2003)1009–1015.

[24]C.L.Mendis,C.J.Bettles,M.A.Gibson,C.R.Hutchinson,Mater.Sci.Eng.A435-436 (2006)163–171.

[25]R.K.W.Marceau,G.Sha,R.Ferragut,A.Dupasquier,S.P.Ringer,ActaMater.58 (2010)4923–4939.

[26]R.K.W. Marceau,G. Sha, R.N.Lumley, S.P.Ringer, Acta Mater.58(2010) 1795–1805.

[27]T.Gorecki,in:F.J.Kedves,D.L.Beke(Eds.),Proc.Int.Conf.OnDiffusionin MetalsandAlloys(DIMETA-82),TransTechPublications,Switzerland,1983, p.365.

[28]S.Esmaeili,D.JLloyd,ActaMater.53(2005)5257.

[29]G.Barucca,R.Ferragut,F.Fiori,D.Lussana,P.Mengucci,F.Moia,G.Riontino, ActaMater.59(2011)4151–4158.

[30]S.Celotto,ActaMater.48(2000)1775–1787.

[31] Y.M.Zhu,A.J.Morton,M.Weyland,J.F.Nie,ActaMater.58(2010)464–475.

[32]J.F.Nie,K.Oh-ishi,X.Gao,K.Hono,ActaMater.56(2008)6061–6076.

[33] A.Galiyev,R.Kaibyshev,G.Gottstein,ActaMater.49(2001)1199–1207.