Precipitation microstructure and age-hardening response of an Mg-Gd-Nd-Zn-Zr alloy

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Materials Science and Engineering A

j ou rna l h o me p a g 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 age-hardening response of an Mg–Gd–Nd–Zn–Zr alloy

J.H. Li

^a,c,1

, G. Sha

^a,b,∗

, T.Y. Wang

^a

, W.Q. Jie

^c

, S.P. Ringer

^a,b

aAustralianCentreforMicroscopyandMicroanalysis,TheUniversityofSydney,MadsenBuildingF09,Sydney,NSW2006,Australia

bARCCentreofExcellenceforDesigninLightMetals,TheUniversityofSydney,Sydney,NSW2006,Australia

cStateKeyLaboratoryofSolidiﬁcationProcessing,NorthwesternPolytechnicalUniversity,Xi’an,710072,China

a r t i c l e i n f o

Articlehistory:

Received14December2010 Receivedinrevisedform11June2011 Accepted25October2011

Available online 3 November 2011

Keywords:

Magnesiumalloys Precipitation Agehardening Atomprobetomography Transmissionelectronmicroscopy

a b s t r a c t

PrecipitatesinanMg–3.6Gd–2.8Nd–0.6Zn–0.4Zr(wt.%)alloyagedfortimesupto70hat200^◦Chave beencharacterisedusingtransmissionelectronmicroscopyandatomprobetomography.Theprecipitate phasesknownas␤,␤and␤1occurduringthisageing.ThesoluteelementsNd,ZnandGdpartition signiﬁcantlyintotheseprecipitates.Theenhancedageinghardeningresponseafteraged70hismainly attributedtotheprecipitationof␤and␤1withanumberdensitybyafactorof10lessthanthatof␤ and␤precipitatesformedafter3hageing.

1. Introduction

Magnesiumalloysareattractiveduetotheirspeciﬁcstrength, providing potential for weight reduction in automotive and aerospaceapplications[1,2].TheMg–NdbasedalloyssuchasZM- 6 in Chinaand ML10 in Russia exhibit a strongage hardening responseandhavebeenusedinvariousstructuralairframecom- ponents.However,thehightemperaturemechanicalpropertiesof theseandotherMg–Ndbasedalloysareinadequatefortechnologi- calapplicationsabovetemperaturesof250^◦C.Thedevelopmentof Mgalloysforthesehighertemperatureapplicationsremainsasig- niﬁcantresearchtargetfortheinternationalmaterialscommunity [3–18]andthisisalsothegeneralsubjectofthiscontribution.

Heavyrareearth(HRE)elementssuchasNd,Gd,Y,Dy,Er,Sc,Tb andSm,havebeenusedwidelytoimprovethemechanicalproper- tiesofMgalloysatbothroomandelevatedtemperatures[3–12].

Indeed,mosthigh-strengthMgalloyssuchasWE54/43andQE22 containHREelements.ThelevelofalloyingadditionoftheHREele- mentsisacriticalconcerninalloydevelopmentanddesignbecause ofbothmaterialcostsandthedesiretohavethealloyaslightas

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

Tel.:+61290369050;fax:+61293517682.

E-mailaddresses:jie-hua.li@hotmail.com(J.H.Li),gang.sha@sydney.edu.au (G.Sha).

1 Currentaddress:ChairofCastingResearch,theUniversityofLeoben,Austria.

possible.Thishasdrivensigniﬁcantresearchinrecenttimes:(e.g.) Nieatal.[17]havedemonstratedthatsmalladditionsofZnstim- ulateanagehardeningresponseinMg–GdalloysthatpossessGd contentsthatarebelowthelevelordinarilyassociatedwithasig- niﬁcantageingresponse.Moreover,combinedadditionsofGdwith ZntoMg–Ndalloyshavebeenreportedtoimprovethemechani- calpropertiesofthealloys,particularlyatelevatedtemperatures [11,12].Itisexpectedthattheimprovementofmechanicalprop- ertiesoftheseMg–Gd–Nd–Znalloysshouldbecorrelatedtothe precipitatesmicrostructuresformedduringageingtreatment.To date,thereisalackofdetailedinvestigationstorevealtheevolu- tionofprecipitatesmicrostructureandthepartitioningofsolutes duringageingthesealloys.

In this paper, transmission electron microscopy (TEM) and atom probe tomography (APT) have been employed to char- acterise precipitates microstructures formed during ageing a quinary Mg–3.6Gd–2.8Nd–0.6Zn–0.4Zr(wt.%)alloy.Quantitative APTanalysisaimstorevealthepartitioningbehaviourofsolutes.

Comprehensivestructuralinformationobtainedfromacombina- tionofTEMandAPTcharacterisationswillhelptoelucidatethe precipitationreactionsandtounderstandthenanostructurepro- vidinghardeningeffectinthealloy.

2. Experimentalprocedures

TheMg–3.6Gd–2.8Nd–0.6Zn–0.4Zr(wt.%)alloywasprepared from pure Mg (99.9%), Zn (99.9%), Nd (99.9%), Mg–28Gd and 0921-5093/$–seefrontmatter.Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

doi:10.1016/j.msea.2011.10.092

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2 J.H.Lietal./MaterialsScienceandEngineeringA534 (2012) 1–6

Fig.1.AgehardeningresponseofMg–3.6Gd–2.8Nd–0.6Zn–0.4Zr(wt.%)alloyaged at200^◦C.Forcomparison,theresultoftheGd-freealloyisalsoincluded.

Mg–33Zr (wt.%) master alloys in an electric resistance furnace undertheprotectionofananti-oxidizingﬂux,andthencastinto asandmould.Solutiontreatmentwasperformedinasaltbathat 520^◦Cfor18h,followedbyquenchingintocoldwaterandthen ageingin oilat200^◦Cfor upto100h. Vickershardnesstesting wasperformedusingaLECOHardnessTester(LV700AT)with10N loadand15sdwelltime.EachdatapointreportedinFig.1repre- sentsanaverageofatleast10measurements.Thefoilspecimens forTEMwere preparedby twinjetelectro-polishing ina solu- tionof25%HClO₄and75%methanolcooleddownto−40^◦Cwith avoltageof20V,andthenusinglow energybeamionthinning forsurfacecleanness.TEMexaminationswereperformedusinga CM12operatingat120kVandaJEOL-3000Foperatingat300kV.

Thesamplesforatomprobeanalysiswerecutandmechanically groundtosquarerodsofapproximately0.5mm×0.5mm×15mm, andthensharpenedbymicro-electro-polishing.APTanalyseswere performedusinganImagoLEAP^TM3000operatingataspecimen temperatureof20K,20%pulsefractionandunderultrahighvac- uumconditions.

Atom probe data sets were carefully reconstructed using an approach outlined recently by Gault and co-workers [19].

The maximum separation algorithm was employed to identify solute-richprecipitatesandtheconcentrationofthematrixwas measuredafter removingidentiﬁedprecipitates[20,21].Nd, Gd and Znwereselected as precipitationsolutesand a separation distance of 0.8nm and the minimum size of 15 solute atoms wereused intheprecipitateidentiﬁcation[22].A selectionbox analysismethod wasused tomeasure precipitate composition.

Suchmethodscanconvolutenon-systematicerrorswhereirreg- ularmorphologiesareinvolved.Selectionboxmethodsalsotend to minimise the inﬂuence of ion trajectory overlaps that can occurinmulti-componentsystemssuchashere,whereelements withwidely varying evaporative ﬁelds are involved. The aver- agecompositionofprecipitateswasmeasuredfromtheircentral 2nmregion.

3. Resultsanddiscussion

3.1. AgehardeningresponseoftheMg–Gd–Nd–Zn–Zralloy

Fig. 1 reveals the age hardening response of the quinary Mg–3.6Gd–2.8Nd–0.6Zn–0.4Zr(wt.%) alloy.For comparison, the resultof theGd-free alloyis alsoincluded. It is clearthat the age hardening response of the alloy containing 3.6wt% Gd is higher than that of Gd-freealloy. It should be noted that the

hardnessof thequinary alloyincreasedquicklyduring theﬁrst 1.5h at 200^◦C, witha hardnessincrease from 65HV of an as- quenched sample to 85HV of a sample aged for 1.5h. The 20HV hardness increment is equivalent to 30% of the initial hardness of the as-quenched sample. This fast age-hardening response canbedirectly correlatedtothestrongerpartitioning ofNdinMg–Ndbasedalloys[13].Thenitreachedaplateau-like range before reachingpeak hardnessafter 70h. Furtherageing led toover-ageing and a progressive decrease in hardness.On thebasis of theseresults, thesamples agedat 0h(as-quench), 3h, 14h and 70h were selected for TEM observationand APT analysis.

3.2. TEMcharacterisationofprecipitatesinthe Mg–Gd–Nd–Zn–Zralloy

Fig.2showsatypicalbrightﬁeld(BF)TEMimageandthecor- respondingselectedareaelectrondiffraction(SAED)patternand energydispersiveX-ray(EDX)spectratakenfromtheas-quench Mg–3.6Gd–2.8Nd–0.6Zn–0.4Zr (wt.%) alloy.Only some particles containingZr(Fig.2c)arerarelydistributedinthe␣-Mgmatrix.

No otherprecipitatesarepresent inthemicrostructure(Fig.2a and b). These Zr particles were believed to play an important roleinthegrainreﬁnementofMgalloy,buthavenogreateffect ontheprecipitationstrengthening due totheirsparsedistribu- tion.

Fig.3showsaseriesofrepresentative[0110]_MgBFTEMimages andthecorrespondingSAEDpatternsofsamplesagedat200^◦Cfor 3h,14hand70h.Notwithstandingthewell-knowndifficultiesin achievinglarge,uniformlythinregionsoffoilinMgalloyscontain- ingmultipleHREs,thismicroscopyconfirmsthepresenceofafine anduniformdispersionofprecipitatesinalloftheseageingcon- ditions.Afterageingfor3h,theSAEDpatternin[0110]_Mgzone axisoftheMgmatrix,asshowninFig.3b,wasobservedcontain- ingtheweakdiffractionstreaksat1/2(2110)_Mgand1/2(2114)_Mg (markedwithawhitearrow),indicatingthat␤precipitatesformed inthealloy.The␤phasepossessesaDO₁₉crystalstructurewith ahexagonalunitcellofa=b=0.64nmandc=0.52nm[8]andit wasreportedtohaveastoichiometryMg3X[10].The␤precipi- tateshaveageneralhabitplaneparallelto{21 10}Mg.Inaddition tothoseweakdiffractionstreaksof␤precipitates,theextremely weakdiffractionof␤precipitatesat1/2(2112)_Mg,markedwitha blackarrowinFig.3b,wasobservedinthe[0110]_MgSAEDpattern.

Thissuggeststhatalownumberdensityof␤precipitatesco-exist withthemorenumerous␤precipitatesatthisstageofageing.The

␤phaseisknowntohaveabase-centredorthorhombicunitcell a=0.640nm,b=2.223nm,c=0.521nm[8]andstoichiometryMg5X [10].

Afterageingfor14h,diffractionintensityat1/2(2112)_Mgwas stronger in the[0110]_Mg SAEDpatterns (markedwitha black arrow in Fig.3d),suggesting that more ␤ precipitatesformed.

Afterageingfor70h, aweakdiffractionspotclearlyadjacentto 1/2(2110)_Mg,as marked witha black box in Fig.3f, indicates thepresenceof␤1 inthemicrostructure.Thisphasepossessesa face-centredcubicunitcellwitha=0.72nmandstoichiometryof Mg3X[6,8,10].Theorientationrelationshipsbetweentheprecipi- tatessuchas␤,␤,␤1andthematrixareinagreementwiththose reportedpreviously[6,8,10].Thus,theprecipitationsequencein thisquinaryMgalloyduringageingissupersaturatedsolidsolution (SSSS)→␤(D019)→␤(bco)→␤1(fcc).␤phase(aface-centred cubicunitcellwitha=2.223nmandstoichiometryofMg₃X[6])has beenconsideredtotheequilibriumphaseinthealloy,itmayeven- tuallyprecipitate,althoughitwasnotobservedover70hageingat 200^◦C.

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Fig.2.TEMbrightﬁeldimage(a),correspondingSAEDpattern(b)andEDXanalysis(c)takenfromtheas-quenchMg–3.6Gd–2.8Nd–0.6Zn–0.4Zr(wt.%)alloy.

3.3. APTcharacterisationofprecipitatesintheMg–Gd–Nd–Zn–Zr alloy

Fig. 4 provides a series of three-dimensional atom maps recorded from atom probe experiments on specimens of the Mg–3.6Gd–2.8Nd–0.6Zn–0.4Zr(wt.%)alloyafterageingat200^◦C for(a)3h(b)14hand(c)70h.Itisclearthatallthreeofthemain soluteelements,Nd,GdandZnweredirectlyimagedwithinthe precipitates.Usingthezonelinesandpolesobservedfromthefield desorptionimageavailableinatomprobemicroscopy,theview directionsofthetomogramsprovidedcanbespecificallydefined, e.g.theviewdirectionofimagesinFig.4a–cis[0001]Mg.Byexam- iningasmallregimemarkedwithwhitesolidlinesinFig.4aata highmagnification,atomicplanes(0110)_Mgand(0111)_Mgwere clearlyresolvedinFigs.5aandb.Threeelongatedprecipitatesare present in thesmall volume.A platelet precipitate habitingon {0110}Mg isprobably␤.Thissuggestionisconsistentwiththe habitplane{0110}Mg of␤ inMg alloysreportedin literatures [16,18].Arodprecipitatehabitingon{2110}Mgonlyseparatedby oneortwoatomicplanesfromthe␤plateletmaybeidentifiedas

␤,asseeninFig.5b.Byexaminingtheprecipitatesfromdifferent viewdirections,wefoundthatmostprecipitateswereelongated withtheirlongitudinalaxisparallelto[0001]_Mg.Thisobservation isinagreementwithpreviousTEMobservationsfor␤,␤and␤1

[6,10].

Fromthe[0001]_Mgviewdirection,thesizeoflargeprecipitates wasfoundtoincreasewithageingtime(Fig.4),whichisnoteasyto seefromthewholeanalysedvolumeduetotheoverlapofthehigh numberdensityofprecipitates(Fig.4b).Mostoftheprecipitates

formedbetween3hand70hageinghaveelongatedmorphology along[0001]_Mg,whichareeffectiveobstaclestobasalslip[23].It isalsonoteworthythatthenumberdensityofprecipitatesappears tobehighestinthesampleaged3handthatthisprogressively decreaseswhencomparedtothenumberdensitiesofprecipitates afterageingfor14hand70h.Ouranalysisoftotalnumberdensity revealsthattherearelessprecipitatesbyafactorof10inthesam- pleaged70hat200^◦Ccomparedtothesampleaged3handyetthe hardnessis10%higherinthe70hsample.Thedecreaseinprecipi- tatenumberdensityisincontrasttoanincreaseinhardnesswith increasingageingtimefrom3hto70h,probablyimplyingthatthe differentprecipitatesformedunderdifferentageingtimehadthe differentpotencyinproducingstrengtheningeffect.Giventhatthe formationofmore␤and␤1precipitateswiththeincreaseofageing timeasrevealedbyTEMcharacterisationdescribedpreviously,we canconcludethatthe␤and␤1aremorepotenthardeningobsta- clesthanthe␤phaseinthealloy.Interestingly,similarconclusion hasbeendrawninafewotherMg–REalloysincludingWE54alloy aged at 250^◦C [6] and Mg–2.0Gd–1.2Y–1.0Zn–0.2Zr (at.%)alloy agedat225^◦C[10],wherethepeak-agedmicrostructurecontains predominantlythe␤and␤1phases.

Theaveragecompositionsoflargeprecipitatesformedatdif- ferent ageing time are shown in Fig. 6a. Since APT chemical compositionanalysisindicatedthatsoluteconcentrationincrease withprecipitatesize,andTEMSAEDanalysisconﬁrmedtheco- existenceof␤and␤,␤and␤1indifferentageingconditions,it isreasonabletoassumeherethatlargeprecipitatemeasuredat3h shouldbecorrespondingto␤,andlargehigh-solute-concentration precipitatesobservedat70hcorrespondto␤1.TheGdconcentra- tionwithinthelargeprecipitateswaseffectivelyunchangedduring

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Fig.3. [0110]_MgbrightﬁeldTEMimagesandcorrespondingSAEDpatternsfromthemicrostructureofMg–3.6Gd–2.8Nd–0.6Zn–0.4Zr(wt.%)alloysamplesagedat200^◦C forvarioustimes:(aandb)3h,(candd)14hand(eandf)70h.

Fig.4.CombinedatommapsofNd(red),Gd(blue)andZn(green)obtainedfromMg–3.6Gd–2.8Nd–0.6Zn–0.4Zr(wt.%)alloysamplesagedat200^◦Cfor(a)3h,(b)14hand (c)70h,inaviewdirectionclosetothe[0001]Mgzoneaxis.(Forinterpretationofthereferencestocolorinthisﬁgurelegend,thereaderisreferredtothewebversionof thearticle.)

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Fig.5.Highresolutionimagesofasmallregion(asmarkedwithwhitesolidlinesinFig.3a)obtainedfromMg–3.6Gd–2.8Nd–0.6Zn–0.4Zr(wt.%)alloysamplesagedat200^◦C for3h,inviewdirectionscloseto[2 110]_Mg(a)and[0001]Mg(b)zoneaxesrespectively.

Fig.6.Soluteconcentrationsoflargeprecipitates(a)andmatrix(b)inMg–3.6Gd–2.8Nd–0.6Zn–0.4Zr(wt.%)alloysamplesagedat200^◦Cfrom3hto70h.

70hofageingat200^◦C.However,theZnandNdconcentrations withintheprecipitatesincreasedwithincreasingageingtime.The Nd:Znratiooftheprecipitateswasapproximately1.6:1through- outtheageingsequenceexaminedhere,whichissimilarto,though slightlylessthanthe2:1ratioreportedfromTEM-basedenergy dispersiveX-rayspectroscopy(EDXS)microanalysisof␤precipi- tatesinasimilaralloy[16].TheprecipitateswererichestinNdof allsolutesandtheconcentrationofthiselementincreasedfrom

∼11 to∼15at.%duringageing.Thisisalsoclosetotheaverage compositionof15at.%measuredbyEDXS[16].Acombinedcon- centrationofMgandZnis84at.%,andacombinedconcentration ofNdandGdis16at.%inlargeprecipitates(likely␤)formedafter ageingfor3h.Thisgivesthelargeprecipitateswithastoichimetry of(Mg,Zn)₅(Nd,Gd).Thestoichimetryisconsistentwith␤having Mg₅NdreportedinbinaryMg–Ndalloy[13].Incontrast,thecom- positionofthelargeprecipitatesformedafterageingfor70hthat werethoughttobe␤1phaseonthebasisoftheSAEDanalysis,Fig.3, gives(Mg,Zn)₄(Nd,Gd).ThisisincontrastwithMg₃Xproposedby previousresearch[6,8,10].FurtherAPTinvestigationisnecessary tomeasurethechemistryof␤1 inover-agedsample.Ithasbeen reportedthattheprecipitatesformedinbinaryMg–Ndalloyspos- sessarangeofcompositionsfromMg₅NdtoMg₉Nd[16,18].We concludethatthetransformationsfrom␤to␤and␤1arerelated tofurtherenrichmentofNd,ZnandGdsolutesintheprecipitates.

Thesoluteconcentrationsinthematrixdecreasedduringageingat 200^◦Cfrom3hto70h,asindicatedinFig.6b.Theseresultsindicate thatsigniﬁcantsoluteshavepartitionedintotheprecipitates.The lowconcentrationsofNd,GdandZnmeasuredinthematrixafter ageingfor70hindicatethattheequilibriumsolubilityofNdshould belessthan0.13at.%inthisMgalloyandthatthesolubilityofGd shouldbelowerthan0.23at.%.ThismayexplainwhyGddoesnot seemtopartitionintoanyofthe␤,␤,or␤1precipitatesphases observedintheselowGdcontentalloys[16].

4. Conclusion

TheprecipitatesequenceintheMg–3.6Gd–2.8Nd–0.6Zn–0.4Zr (wt.%)alloyduringageingat200^◦Cover70his␤→␤→␤1.The precipitateshavea lamella-likemorphologywithalongitudinal axisparallelto[0001]Mg.ThesoluteelementsNd,ZnandGdpar- titionsigniﬁcantlyintotheprecipitates(␤,␤ and␤1).Thepeak hardnessat70hisincoincidencewithalownumberdensityofpre- cipitates(␤and␤1)inthemicrostructureofthealloybutwitha highfractionofthetotalsolutesassociatedwithprecipitation.The enhancedageinghardeningresponseafteraged70hwasmainly attributedtotheprecipitationof␤and␤1precipitateswithanum- berdensitybyafactorof10lessthanthatof␤and␤precipitates formedafter3hageing.

Acknowledgments

The authors are grateful for scientiﬁc and technical input and support from the Australian Microscopy & Microanalysis ResearchFacility(AMMRF)nodeattheUniversityofSydney.J.H.

LialsowishestothanktheChinaScholarshipCouncilforﬁnancial support. Thiswork is partly supportedby theDoctorate Foun- dationofNorthwesternPolytechnicalUniversityundergrantno.

(CX200705).

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