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DATES COVERED (From - To) New Reprint - 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER High strength, nano-structured Mg–Al–Zn alloy W911NF-10-1-0512 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 611102 6. AUTHORS 5d. PROJECT NUMBER Baolong Zheng, Osman Ertorer, Ying Li, Yizhang Zhou, Suveen N. Mathaudhub, Chi Y.A. Tsao, Enrique J. Lavernia 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAMES AND ADDRESSES 8. PERFORMING ORGANIZATION REPORT NUMBER University of California - Davis Sponsored Programs 118 Everson Hall Davis, CA 95616 -8671 9. SPONSORING/MONITORING AGENCY NAME(S) AND 10. SPONSOR/MONITOR'S ACRONYM(S) ADDRESS(ES) ARO U.S. Army Research Office 11. SPONSOR/MONITOR'S REPORT P.O. Box 12211 NUMBER(S) Research Triangle Park, NC 27709-2211 58330-MS.1 12. DISTRIBUTION AVAILIBILITY STATEMENT Approved for public release; distribution is unlimited. 13. SUPPLEMENTARY NOTES The views, opinions and/or findings contained in this report are those of the author(s) and should not contrued as an official Department of the Army position, policy or decision, unless so designated by other documentation. 14. ABSTRACT The mechanical behavior and microstructure of nanocrystalline (nc) Mg AZ80 alloy, synthesized via a cryomilling and spark plasma sintering (SPS) approach are reported and discussed. The effects of cryomilling processing on chemistry, particle morphology, and microstructure of the Mg alloy powder are described and discussed. The experimental results show that cryomilling for 8 h yields nc Mg agglomerates, approximately 30m in size, with an internal average grain size of approximately 40 nm. The 15. SUBJECT TERMS Magnesium alloys, Nanocrystalline microstructure, Mechanical milling, Sintering 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 15. NUMBER 19a. NAME OF RESPONSIBLE PERSON a. REPORT b. ABSTRACT c. THIS PAGE ABSTRACT OF PAGES Enrique Lavernia UU UU UU UU 19b. TELEPHONE NUMBER 530-752-0554 Standard Form 298 (Rev 8/98) Prescribed by ANSI Std. Z39.18 Report Title High strength, nano-structured Mg–Al–Zn alloy ABSTRACT The mechanical behavior and microstructure of nanocrystalline (nc) Mg AZ80 alloy, synthesized via a cryomilling and spark plasma sintering (SPS) approach are reported and discussed. The effects of cryomilling processing on chemistry, particle morphology, and microstructure of the Mg alloy powder are described and discussed. The experimental results show that cryomilling for 8 h yields nc Mg agglomerates, approximately 30m in size, with an internal average grain size of approximately 40 nm. The mechanisms that are thought to be responsible for deformation twins that were observed in the cryomilled AZ80 powder are discussed. The cryomilled Mg powder was subsequently consolidated using SPS at 250, 300 and 350 ?C. The consolidated material consisted of a bimodal microstructure with nc fine and coarse grains formed in the SPS’ed Mg AZ80 microstructure. Inside of the coarse grains, nano-sized Mg17Al12 precipitates were observed. A maximum microhardness of 140 HV, compressive yield strength of 442.3 MPa, and ultimate strength of 546MPa are measured, which compare favorably to published values for conventional Mg alloys. REPORT DOCUMENTATION PAGE (SF298) (Continuation Sheet) Continuation for Block 13 ARO Report Number 58330.1-MS High strength, nano-structured Mg–Al–Zn alloy ... Block 13: Supplementary Note © 2011 . Published in Materials Science and Engineering, Vol. 528, (54), Ed. 0 (2011), (Ed. ). DoD Components reserve a royalty-free, nonexclusive and irrevocable right to reproduce, publish, or otherwise use the work for Federal purposes, and to authroize others to do so (DODGARS §32.36). The views, opinions and/or findings contained in this report are those of the author(s) and should not be construed as an official Department of the Army position, policy or decision, unless so designated by other documentation. Approved for public release; distribution is unlimited. MaterialsScienceandEngineeringA528 (2011) 2180–2191 ContentslistsavailableatScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea High strength, nano-structured Mg–Al–Zn alloy BaolongZhenga,OsmanErtorera,YingLia,YizhangZhoua,SuveenN.Mathaudhub, ChiY.A.Tsaoc,d,EnriqueJ.Laverniaa,∗ aDept.Chem.Eng.Mater.Sci.,UniversityofCalifornia,Davis,OneShieldsAvenue,Davis,CA95616,USA bWeaponsandMaterialsResearchDirectorate,U.S.ArmyResearchLaboratory,AberdeenProvingGround,MD21005,USA cDept.Mater.Sci.Eng.,NationalChengKungUniversity,Tainan,Taiwan,ROC dFrontierMaterialsandMicro/NanoScienceandTechnologyCenter,NationalChengKungUniversity,Tainan,Taiwan,ROC a r t i c l e i n f o a b s t r a c t Articlehistory: Themechanicalbehaviorandmicrostructureofnanocrystalline(nc)MgAZ80alloy,synthesizedviaa Received8July2010 cryomillingandsparkplasmasintering(SPS)approacharereportedanddiscussed.Theeffectsofcry- Receivedinrevisedform omillingprocessingonchemistry,particlemorphology,andmicrostructureoftheMgalloypowderare 23November2010 describedanddiscussed.Theexperimentalresultsshowthatcryomillingfor8hyieldsncMgagglom- Accepted24November2010 erates,approximately30(cid:2)minsize,withaninternalaveragegrainsizeofapproximately40nm.The mechanismsthatarethoughttoberesponsiblefordeformationtwinsthatwereobservedinthecry- omilledAZ80powderarediscussed.ThecryomilledMgpowderwassubsequentlyconsolidatedusing Keywords: SPSat250,300and350◦C.Theconsolidatedmaterialconsistedofabimodalmicrostructurewithncfine Magnesiumalloys Nanocrystallinemicrostructure andcoarsegrainsformedintheSPS’edMgAZ80microstructure.Insideofthecoarsegrains,nano-sized Mechanicalmilling Mg17Al12precipitateswereobserved.Amaximummicrohardnessof140HV,compressiveyieldstrength Sintering of442.3MPa,andultimatestrengthof546MPaaremeasured,whichcomparefavorablytopublished valuesforconventionalMgalloys. © 2010 Elsevier B.V. All rights reserved. 1. Introduction thehighesttensilestrengthofcastMgalloysreportedthusfaris 414MPa,achievedinaMg–18.2Gd–1.9Ag–0.34Zr(wt.%)alloy[8]. Mg alloys provide an opportunity to engineer light-weight However,thisMgalloysystemcontainslargeamountsofrare-earth structures, and interest in these materials has increased as one andnoblemetalelementsforsolidsolutionstrengtheningresulting consequence of the global energy crisis. Mg alloys have advan- inhighcostsformanypracticalapplications. tages when used as structural materials, such as an attractive Grainrefinementviasevereplasticdeformationmethodsrepre- combination of low density of 1.74g/cm3 (35% lighter than Al, sentsanotherrecentlyproposedapproachtoachievehighstrength about78%lighterthanFe)andhighspecificstrength,alongwith in Mg alloys. It has been well established that nanocrystalline gooddampingcapacity,castability,weldability,machinabilityand (nc) metals and alloys with grain sizes less than 100nm gener- recyclability [1–3]. Thus, Mg alloys have been widely used as ally exhibit significantly improved strength, an observation that replacementforAlalloys,aswellassteelinweight-critical,auto- hasbeenpartiallyattributedtothewellknownHall–Petchmech- motiveandaerospacestructuralapplications.Mgalloyshavealso anism[9,10].Consequently,therearesomestudiesdescribingthe beenusedasanalternativetopolymericmaterialsintheelectronic synthesisofncMganditsalloysusingmechanicalalloyingatroom andcomputerindustries[1,4].Despitesomeencouragingresults,it temperature [11,12]. Other severe plastic deformation methods, isacknowledgedthatwidespreadapplicationofMgalloyshasbeen suchasequalchannelangularpressing(ECAP)[13–15]andhigh hindered,inpart,bytheirrelativelylowstrength. pressuretorsion(HPT)[16],havebeenreportedtoprovidesamples StrengtheningofMgalloysviatheintroductionofsolidsolu- with no porosity and straightforward processing routes. How- tion atoms and grain refinement additives represent effective ever, these studies reveal that structural evolution of Mg during approaches actively being researched and implemented. To that warmECAPproducesinhomogeneousgrain/subgrainmicrostruc- effect,muchprogresshasbeenachievedinthedevelopmentofhigh tures with sizes ranging around 1–20(cid:2)m and ultimate tensile strengthMgalloysthroughsolidsolutionstrengtheningusingvari- strength(UTS)valuesof329MPa[15]. oustypesofsoluteatoms[5–7].Reviewoftheliteratureshowsthat Morerecently,thecryogenicmillingprocesshasattractedcon- siderableinterest,primarilyasaresultofitsabilitytogeneratenc andnon-equilibriumstructuresinrelativelylargequantities(i.e., ∗ upto35kg).Inspectionofthescientificliteratureshowsthatthis Correspondingauthor.Tel.:+15307524964. E-mailaddress:[email protected](E.J.Lavernia). techniquehasbeenwidelyusedtosynthesizencmetalsandalloys 0921-5093/$–seefrontmatter© 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.11.073 B.Zhengetal./MaterialsScienceandEngineeringA528 (2011) 2180–2191 2181 suchasAl[10,17,18],Ni[19],Fe[20],andTi[21].Thecryomilling processisthatitallowsfabricationofbulkmaterialsfrompowders techniquepossessesseveralcharacteristicsthatdistinguishitfrom usingafastheatingrateandshortthermalexposurecycleatlow theconventionalmechanicalalloyingprocess,includingrelatively temperaturesandtherebylimitsgraingrowthascomparedtothe high strain rates, large cumulative strains, and a cryogenic envi- commonlyusedhotpressing(HP)andhotisostaticpressing(HIP) ronment and temperature [10]. In related studies [10,18], it has approaches. been reported that the milling time required to reach the final The present study seeks to provide insight into two impor- grainsizewassignificantlyshorterinthecaseofcryomillingrela- tant questions. First, what is it the influence of cryomilling on tivetothatrequiredofconventionalmillingperformedatambient an hcp system, as represented by Mg? Second, is it possible to temperature,whichwasattributedtothesuppressionofrecovery take advantage of a rapid sintering approach, as represented by mechanismsatcryogenictemperatures[22].Moreover,reviewof SPS,toretainthencstructureinMgandtherebyattainincreased thescientificliteraturerevealsthatpublishedstudiesonthecry- strength?Accordingly,wesynthesizedncMgAZ80alloypowders omilling of nc Mg alloys remain essentially non-existent, which usingthecryomillingtechniquefollowedbyconsolidationofthese providesamotivationforthepresentwork. powdersintobulksamplesusingSPS.Microstructuralevaluation Thepoorcoldworkabilityofthehexagonalclosepacked(HCP) was performed using X-ray diffraction (XRD), scanning electron crystalstructureofMgislargelyresponsibleforitslowformability microscopy (SEM), and transmission electron microscopy (TEM). atroomtemperature,andhencehaslimitedtheutilizationofMg The microstructure and mechanical behavior of the SPS consoli- asawroughtmaterial.AmongvariouscommercialMgalloys,the datedsamplesweretheninvestigatedusingstandardASTMtesting familycomprisingtheMg–Al–Znternarysystem(i.e.,labeledasAZ methods. The underlying deformation mechanisms are also dis- alloys),iswidelyusedforindustrialapplications.Asanexample,Mg cussedinanefforttoelucidatefundamentalphenomenainncMg AZ80nominallycontains8.0wt.%Aland0.5wt.%Znwiththeaddi- alloys. tionofasmallamountofMnasanalloyingelement.Itisamedium strength Mg alloy with good corrosion resistance and very good 2. Experimentalprocedures forging capability [23,24]. Therefore, the Mg AZ80 alloy system wasselectedandprocessedusingacryomillingandsparkplasma 2.1. CryomillingofMgAZ80powder sintering (SPS) synthesis route in this work. SPS is an attractive consolidationtechniquethatwasoriginallydevelopedforsintering ThestartingmaterialusedinthisstudywasgasatomizedMg structuralceramics,metalsandcompositematerials[25].TheSPS AZ80powderwithachemicalcompositionof7.83wt.%Al,0.47wt.% processisapressureandpulsedcurrentassistedsinteringprocess Zn,and0.16wt.%Mn.Thestartingpowder,asshowninFig.1(a)and utilizingon–offDCpulseenergy.OneoftheadvantagesoftheSPS (b),exhibitedanaverageparticlesizeof55(cid:2)m,distributedinthe Fig.1. SEMmicrographsshowingthemorphologyof(aandb)as-receivedgasatomizedand(candd)cryomilledMgAZ80powder. 2182 B.Zhengetal./MaterialsScienceandEngineeringA528 (2011) 2180–2191 Table1 imens (4mm×4mm×4mm) were electrodischarge-machined CryomillingMgpowderusingthelargeattritorwithliquidAr. (EDM). Prior to testing, the surfaces of all test specimens were Experimentalconditions polishedtoremoveanyoxidelayerspresent.Compressionstudies wereperformedatacrossheadvelocityof0.001mm/suntilfailure. Mill SzegvariAttritor,1-S,UnionProcessCo. Container(tank) Stainlesssteel Atmosphere Liquidargon 3. Resultsandanalysis Millingtemperature −185.9◦C,1atm rpm 180 3.1. CryomilledAZ80powder BPR 60:1 Balls 6.4mmdiameter,stainlesssteel(20kg) Powder MgAZ80(350g) 3.1.1. Particlesizeandmorphology Millingtime 8h Fig.1(c)and(d)showsthetypicalparticlemorphologiesofMg Az80powderastheyevolvedafter8hcryomilling.Thefinalparticle sizedistributionwashomogenousandrangedbetween10(cid:2)mand range10–106(cid:2)mwithasphericalmorphology.Oxidephases,with 40(cid:2)mwithanirregular,butclosetosphericalmorphology.During discontinuousandnon-uniformdistribution,wereobservedonthe theearlystagesofcryomilling,themorphologyofalargepropor- Mg AZ80 powder surfaces, as commonly reported for atomized tionofpowderparticlesismodifiedfromsphericaltodisc-likedue metalpowders. tothesevereplasticdeformationinducedbyball–powder–ballcol- The cryogenic milling experiment was performed with liquid Arattemperatures−186◦C.Amodified1-SSzegvari-typeAttritor lisionsandshearingasreportedinrelatedstudies[10,26,27];thisis followedbyaperiodofcoldweldingandfracturingofMgparticles. (UnionProcess,Inc.,Akron,OH)wasusedinastainlesssteeltank Astheparticlesworkharden,theyfracturemorereadily;hencethe andanimpellerwithstainlesssteelballs(withdiameterof6.4mm). averageparticlesizetendstodecreaseandasthesphericalmor- Liquid Ar was continuously introduced into the tank during the phologyisrecoveredwithincreasingmillingtime.Thisobservation millingbyatemperaturecontrollerwithanattachedthermocou- is consistent with the mechanism Benjamin and Volin proposed pletomaintainaconstantliquidlevelinthetank.TheMgpowders toexplainparticlesizeevolutionduringmechanicalmilling[28]. werecryomilledfor8hwithaball-to-powderratio(BPR)of60:1 Fig.2showsthecrosssectionofthecryomilledMgpowders,indi- (w/w)andanimpellerrotationspeedof180rpm.Theexperimen- catingacryomillingmechanismofcoldwelding(w)andfracturing talparametersareshowninTable1.Inorderpreventatmospheric (f).Cryomillingupto8hcorrespondstothestagewheretheweld- contamination and to mitigate the hazards associated with the ingandfracturingprocessesarecompetitiveandcontinuous.The reactivity of Mg, the powders were always handled in an inert decreaseintheaverageparticlesizesindicatesastageoffracture atmosphereusingaclosedtransfercontainerandanArglovebox. dominance.Thissuggestionisconsistentwiththepresenceofaliq- uidArenvironment,whichatatemperatureof−186◦Cislikelyto 2.2. SPSofncMgAZ80 hinderrecoveryprocesses,andtherebyfacilitaterapidhardening andfracture.Thecoldweldingandfracturingprocessesarestrongly Bulk nc Mg AZ80 alloy samples were fabricated via SPS from dependent on the powder characteristics and milling conditions thecryomilledpowders.Powdersobtainedaftercryomillingwere [10,28]. consolidatedbyanSPS-825SDR.SINTERapparatus(SPSSyntexInc., Theeffectofmillingtimeontheaverageparticlesizeprovides Japan)withmaximumpulseDCoutput12Vand8000Aundera an indication of the rates at which cold welding and fracturing vacuumcondition(lowerthan6Pa).PriortoSPSprocessingthecry- occurduringcryomilling.Thetimevariationofthetotalnumber omilledpowderswereloadedinagraphitedie(20mmdiameter) NofparticlesintheMgsystemcanbewrittenas[29]: inanArgloveboxtominimizeoxidationofMgpowder.SPSwas pwehrifloeramheedataitngdirfafetereonft1t2e0m◦pCe/mraitnu,raessinwtietrhin3gmprinesosufrheooldfi1n0g0tMimPea NNo =exp(˛f −(cid:2)p˛w)t (1) wereused,aslistedinTable2.Thepurposeofusingdifferentsin- teringtemperatureswastoinvestigateitsinfluenceondensityand wheretheterms˛fand˛wrepresenttheprobabilityofafractureor weldingeventoccurringinaspecificimpact,changingasafunction mechanicalbehavior. Thecross-sectionalmicrostructureofcryomilledpowdersand ofmillingtimeandparticlemorphology,and(cid:2)pisthetimebetween fracturesurfacesoftheSPSfabricatedbulkncMgAZ80samples impactsforanaverageparticle,andNoistheinitialtotalnumberof particles.InthecaseofHCPMgonlytwoindependentslipsystems were observed using a SEM operated at 5kV. Thin foils for TEM areactiveatambienttemperatureandhenceevenfewerslipsys- observationswerepreparedviamechanicalgrindingandpolish- temswillbeactiveattheliquidArtemperatureof−186◦C[3,30], ing to a thickness of about 30(cid:2)m, followed by ion milling to a hencetheirinherentlylowductility.Therefore,theprobabilityofa thicknessofelectrontransparencywascarriedoutusingaGatan fracture,˛,ofHCPMgalloysishigherthanthatcorrespondingto precision ion polishing system (PIPS) 691 with an Ar accelerat- f BCCandFCCalloys,leadingtoarapidincreaseintheparticlenum- ing voltage of 4kV from both sides until perforation occurred. berN.Thisleadstoarateofdecreaseinparticlesizewhichishigher XRD with CuK-(cid:3) radiation was utilized for phase identification thanthatcorrespondingtowidelystudiedAlsystems.Theaverage andgrainsizecalculations.Thecompressionbehaviorofthecon- particlesizeofMgAZ80powderaftercryomillinginliquidArfor solidatedbulkmaterialsatambienttemperatureswasmeasured 8his17.8(cid:2)m,buttheaverageparticlesizeofcryomilledpureCu usinganInstron8801universaltestingmachine(Norwood,MA) powder,forexample,is34.2(cid:2)m,andtheaverageparticlesizeof equipped with a video extensometer. Cubic compression spec- cryomilledAl5083powderis91.2(cid:2)m[27]. Table2 3.1.2. Chemicalanalysis ProcessconditionsforSPSsinteringcryomilledMgAZ80powder. Chemical analysis of the as-received and the milled powders Sample Powder Temperature(◦C) Pressure(MPa) Time(min) wascarriedoutbyacommerciallaboratory(LuvakInc.,Boylston, 0 Gasatomized 350 MA).Thechemicalanalysisresultsforoxygen,nitrogen,iron,mag- 1 350 nesium,aluminum,zinc,andmanganesecontentsaresummarized 100 3 2 Cryomilled 300 inTable3.TheamountofArwasnotmeasuredgiventhedifficul- 3 250 tiesassociatedwithAranalysis.Theresultsrevealanincreasein B.Zhengetal./MaterialsScienceandEngineeringA528 (2011) 2180–2191 2183 Fig.2. ThecrosssectionofthecryomilledMgpowders,indicatingacryomillingmechanismofcoldweldingandfracturing. theamountofnitrogenandironduringcryomilling,butaslight ingequation[33]: (cid:2) (cid:3) decrease in the amount of oxygen. Powder contamination is an B2 K(cid:4) B inherentcharacteristicofmechanicalmilling,andthismayarise = +16e2 (2) fromeitheroftheprocessingmediaand/ortheatmosphere.The tan2(cid:3)o D tan(cid:3)osin(cid:3)o Fecontaminationstemsfromthewearofthestainlesssteelmilling where(cid:3)oisthepositionofpeakmaximum,andKisafactor,being balls,tank,andshaftduringcryomilling.Thenitrogenisintroduced taken as 0.94, (cid:4) is the wavelength of the X-ray radiation, (cid:3)o is into the powder during powder exposure to air, because of the theBraggangleofapeakinradians,Distheaveragegrainsize, highsurfaceareaofcryomilledMgpowderanditshightendency eisthemicrostrain,andBistheintegralwidth.Byperforminga toreactwithnitrogen[10].Thesurfaceareapriortoandfollow- leastsquaresfittoB2/tan2(cid:3)oagainstB/(tan(cid:3)osin(cid:3)o)forallofthe ingcryomillinghasbeenmeasuredandtheresultsshowa10-fold measured peaks of a sample. From the slope, K(cid:4)/D, the average increaseinsurfacearea,A,andsurfacetovolumeratio,A/V,after grainsizeiscalculatedtobe39.5nmforMgAZ80powdersafter8h cryomilling[31].Theincreasedsurfacearea,incombinationwith cryomilling. thechemicalactivityofMgfacilitatessurfaceadsorptionofgaseous AsshownintheXRDpatternsinFig.3,Mgpeaksareevident atoms.Theslightdecreaseinoxygenanalysisisnotconsideredtobe aftercryomilling,indicatingthatthemilledMgpowdersprimar- statisticallyrelevant,giventhedifficultiesassociatedwithchemi- ilyconsistofasingleMgphase.Mostofthealloyingelementsof calanalysisforthiselement.Theoxygeninthecryomilledpowder AlandZnatomsinthecryomilledMgarethoughttobeinsolid ispresentasbrittleoxideparticles,whichareuniformlydispersed solution,whilesomesmallpeakscorrespondingtoMgoxideMgO throughoutthemicrostructure[32].Forotheraddedelements(i.e., were found possible due to the reaction with oxygen when cry- AlandZn)alloyingoccursandchemicalhomogeneitywasachieved omilledMgpowderwasexposedtoairduringXRDtesting.Cast after8hcryomilling,asdiscussedinthephaseanalysisthatfollows. AZ80 alloy usually contains a large fraction of Mg Al precipi- 17 12 tatesalongthegrainboundaries.Thesmallpeakscorrespondingto theintermetalliccompoundofMg Al werealsoshowninthe 17 12 3.1.3. Grainsizeandphaseidentification XRDcurveofstartinggasatomizedMgAZ80powder.Thesepeaks XRD profiles for as-received gas atomized Mg AZ80 powder areleastevidentandalmostdisappearedinthe8-hcryomilledMg andcryomilledMgAZ80powdersareshowninFig.3.Significant powderXRDcurve.Theformationoffineoxideparticlesmaycon- peakbroadeningfromstrongpeakswasobservedforcryomilled sideredasresultsfromthegasatomizedpowderandtheinteraction MgAZ80powder,ascomparedtothoseofthegasatomizedpow- betweenMgandOfromthesurroundingenvironment. der.Thispeakbroadeningisattributedtothereducedcrystallite sizeandthehighinternalresidualmicrostrainintroducedduring cryoThmeilliinnigti.ally sharp diffraction peaks of the Mg phase signifi- m m m mC -- MMgg17Al12 m m CARM p8ohwrder cantlybroadenedaftercryomilling.Thestrainbroadeningmaybe O- MgO m m closelyapproximatedbyaGaussianfunction,whereastheeffectsof smallcrystallinesizedistributionsmorecloselyresembleaCauchy m broadeningprofile.Whenboththeeffectsareresponsibleforpeak u.) broadeningthecombinedrelationmaybeexpressedbythefollow- sity (a. O CM 8hr O m m n e Table3 Int C ThechemicalanalysisresultsofthegasatomizedandcryomilledMgAZ80powders (wt.%). Elements Gasatomized Cryomilled C C C GA Powder CC CCC Magnesium 90.8 89.5 Aluminum 7.83 7.6 Zinc 0.47 0.46 30 40 50 60 70 80 Oxygen 0.074 0.065 2-theta (º) Nitrogen 0.029 1.18 Manganese 0.16 0.15 Fig.3. TheXRDpatternofas-receivedgasatomizedandcryomilledMgAZ80pow- Iron 0.023 0.058 ders. 2184 B.Zhengetal./MaterialsScienceandEngineeringA528 (2011) 2180–2191 Fig.4. TEMmicrographofcryomilledMgAZ80powders:(aandb)bright-filedimageofMgpowderafter8hmilling,(c)itscorrespondentSAED,and(d)histogramsofgrain sizedistributionofcryomilledncMgAZ80powders. The microstructure of the as-cryomilled powders was also Fig.4(d)showstheoverallgrainsizedistributionforcryomilledMg investigated in detail using transmission electron microscopy AZ80powders.Amajorityofthegrainsweredistributedinthesize (TEM).Fig.4showstheTEMmicrographsandselectedareaelectron rangeof20–60nm,whilesomecoarsegrainsweredocumentedas diffraction(SAED)patternofthenano-structuredMgpowderafter largeas120nm. cryomilling.Equiaxedgrainsdistributedinthesizerangefrom10 to120nmandseparatedwithwelldefinedgrainboundarieswere 3.2. SparkplasmasinteredMgAZ80 observed.Theaveragegrainsizecalculatedistobe41nmbased onmorethan300measuredgrains.Thisresultisconsistentwith 3.2.1. Microstructurecharacterization thegrainsizemeasuredfromthebroadeningofXRDpatterns.As TheSPSprocessrepresentsamultiple-fieldprobleminwhich showninFig.4(c),theSAEDpatternoftheMggrainsshowsaring the electric, thermal and displacement (i.e., shrinkage) fields are pattern,indicatingthattheindividualgrainsarencinsizeandsepa- intimatelycoupledviaamaterialresponse.Thisproblemisstrongly ratedbyhigh-anglegrainboundarieswitharandomorientation.It nonlinear since each field interacts with each other and affects isinterestingtonotethatsomedeformationtwinswereobserved thepropertiesofSPS’edmaterials.InSPStheas-sintereddensity asindicatedbythearrowinFig.4(b),similartothedeformation is dominated by both the packing density as well as the rate of twinsobservedinthencMg–Tialloy[34].Withintwindomains shrinkage.Fig.5showsthevariationofdisplacementofagraphite thelatticeisshearedandreorientedanditisseveralnanometersto die punch units as a function of heating temperature for differ- sub-micrometersinthickness.Deformationtwinsareeasilyformed entsinteringtemperatureandpowders.The1stshrinkingregime indeformedcoarsegrainedMgalloysduetothelowstackingfault occurredwithincreasingtemperaturewhiletheappliedpressure energy(SFE)ofpureMg(measured78mJ/m2 [35]andcalculated of80MParemainedconstant.The2ndshrinkingregimeoccurred around30mJ/m2 [36])incombinationwithalimitednumberof with increasing pressure up to 100MPa while the heating tem- independentslipsystems.Moreover,alloyingMgwithAlandZn, perature was kept constant. The cryomilled Mg AZ80 powder as in Mg AZ80, also effectively decreases the SFE, thereby facili- beganshrinkingatabout65◦C,whereasthegasatomizedpowder tatingtwinningasitcouldbearesultofdecreasingSFE[37].The begantoshrinkat115◦Cduetotheirdifferentphysicalresponse SFE in Mg–(3–9wt.%) Al alloys is 5.8–27.8mJ/m2, and decreases (i.e., thermal stability), geometry (i.e., different surface area and withincreasingAlcontent[38].Thepresenceofhighlocalstresses morphology)anddistributionofphasespresent(i.e.,nitridesand duringcryomilling,incombinationwithlimitedavailabilityofslip oxides).Theobservedshrinkageisrelatedtothecomplexdefor- systemsandadecreasedSFEareallfactorsthatwillcontributeto mationbehaviorofeachparticle,whichalsodependsoninternal theactivationofthemainpyramidalslipsysteminMg,andtwin- defects(i.e.,grainboundaries,dislocations,twins,vacancies,etc.), ningwilltakeplacebyplanedisplacementandatomicshifting[39]. as well as the local pressure and temperature at the interface B.Zhengetal./MaterialsScienceandEngineeringA528 (2011) 2180–2191 2185 2.0 ples.Grainsizesdistributedinthesizerangeof200nmto1(cid:2)m were observed at the interface area between the nano-grained CM 300 ºC powder particles, effectively leading to a bimodal nanostructure CM 350 ºC intheMgAZ80materials.Tothateffect,inthecaseofthesample 1.5 CM 250 ºC SPS’edat350◦C,themeangrainsizeofthefinegrain(FG)matrix m) 2nd shrinking: Constant T, increase P ismeasuredtobeapproximately68.9nm,whichisslightlylarger m ment ( 1.0 GA 350 ºC tlihmanitetdhagtroafinthgeraosw-cthry.oInmiclolemdpMargispoonw,dtheer,mpreeasnumgraabilnyssiuzgegeosfttinhge e coarsegrained(CG)region(total5%invol.)attheinterfacesofpar- c a ticlesismeasuredtobe495.3nm.Thecorrespondinghistograms spl 1st shrinking: Constant P, increease T for the grain size distributions of SPS’ed Mg AZ80 materials are Di CM powder 250 ºC showninFig.8.Thegrainsattheparticleinterfacesappeartohave 0.5 CM powder 300 ºC CM powder 350 ºC experiencedmorecoarseningwhencomparedtothoseinsideofthe GA powder 350 ºC particles,leadingtotheobservedbimodalnanostructure.Thepres- CM - Cryomilled ence of nano-pores, approximately 100–200nm in size was also GA - gas atomization 0.0 documentedasshownwitharrows(P)intheTEMmicrographof 0 100 200 300 400 Figs7(a)and(c),and9(a),althoughthemeasureddensitycanreach Temperature (ºC) 1.83g/cm3,whichishigherthanthatoftheoreticaldensityofAZ80 (1.81g/cm3)presumablyattributabletothepresenceofoxidepar- Fig.5. Displacementvariationofdiepunchasafunctionofheatingtemperature. ticlesorotherelementcontaminationintheSPS’edsamples.The densityincreaseswithSPSsinteringtemperatureincreasingfrom betweenparticles.Thelowershrinkagethresholdobservedforthe 250◦C to 350◦C. The density was measured with the method of cryomilledpowderisthoughttoberelatedtotheincreasedden- hydrostatic weighing in an ethyl alcohol solution. The origin of sity of interfaces that are introduced during deformation which thenano-poresfoundintheSPS’edMgAZ80materialsisnotcom- mayreadilyaccommodateanddissipatestrainsduringloading.In pletelyunderstoodatpresent.Onepossiblesourcefortheobserved additiontoahigherpackingdensityandsurfaceenergy,cryomilled pores is thought to be the physical entrapment of Ar in the cry- powdersinherentlyhaveahighfractionofcontactinterfacesthat omilledMgAZ80powder.Eventhoughthereisnoneorverylimited are likely to promote higher local current density and electrical chemicaladsorptionorsolubilityofArinMgalloys[3],itispossible resistanceduringSPSprocessing.Therefore,thesmallercryomilled forArtohavebeenphysicallyadsorbedonthesurfaceofMgalloys particleswillundergofasterdeformationanddensificationrelative duringcryomillinginliquidArenvironment.Thecontinuousfold- tothegasatomizedparticles. ing,coldworkingandfractureofmetallicsurfacesduringmilling XRDstudieswerecarriedouttoidentifyexistingphasesaswell inaliquidArenvironmentallfacilitateinternalgasentrapment. as variations in grain size in the SPS’ed samples. Fig. 6 shows a Assuch,increasedmillingtimeswillleadtoadditionalexposure comparisonbetweentheXRDprofilesoftheSPS’edbulksamples ofcleanmetalsurfaces/interfacestotheArenvironmentresulting and the cryomilled Mg powders. The experimental results show toincreasesinArconcentrationinthemilledpowder.Thecombi- that the grain size of Mg AZ80 increases during SPS process as nationofanessentiallyzerosolubilityofArintheMglattice,with theXRDpeaksofSPS’edsamplesarenarrowerwhencomparedto thermalactivationoftheAratomsduringSPSprocessingarelikely thoseoftheas-cryomilledpowder.Thespecificvariationofgrain topromotecoalescenceoftheAratomsandtheeventualnucle- sizewasalsoobservedwithTEM.Theresultsshowthepresence ationofpores.Therefore,degassingofcryomilledMgAZ80powder oftheintermetalliccompound,body-centeredcubicMg17Al12 in isnecessarytoremoveanyentrappedArpriortoconsolidationof alloftheSPS’edbulksamples,indicatingthatMg17Al12precipita- the cryomilled powder. The vacuum level that is present during tionandgrowthoccurredduringSPSprocessingpresumablydue SPSislimitedtothe2–6Parange,andhenceitisunlikelytopro- tothermalandstrainactivation[40]. motecompletedegassingoftheArduringsintering.Hence,anygas Fig. 7 shows the typical TEM micrographs and corresponding presentattheinterfacesofthecryomilledpowderislikelytoform selectedareaelectrondiffraction(SAED)patternforSPS’edMgsam- isolatedvoidsduringdensification. Inside of the coarse grains, nano-sized Mg Al precipitates 17 12 (30–90nm) were observed. These are illustrated with arrows in Fig.9(a).EnergyDispersiveX-raySpectroscopy(EDX)quantitative analysis,incombinationwithTEM,wasusedtomeasuretherel- ative amounts elements in precipitated particles. The results, as showninFig.9(b),indicatethattheprecipitatedparticlescontain moreAlthanthatofmatrix,whichimpliesthattheprecipitated particlesarepossiblyMg Al intermetallicphase,basedonphase 17 12 diagramconsiderationsandpublishedresults[24,41,42].Therel- ative concentrations of Mn and Zn are also higher around the precipitatedparticlesthanthatinthematrix. Thephenomenaresponsiblefortheformationoftheobserved bimodalnanostructureintheSPS’edMgAZ80canberationalized on the basis of the conditions associated with the SPS process, whichinvolvesanelectricalsparkdischargephenomenon.Inpar- ticular,spatialvariationsinthethermalandstressfieldsarelikely toconditionsthatcanlocallypromotegraingrowth.DuringSPS, apulsedcurrentpassesthoughbothdieandconductivepowder, whichwillpromotethermalenergyandmasstransfer.Thethermal energygeneratedinthegraphitediefromJouleheating,istrans- Fig.6. XRDpatternsofSPS’edcryomilledMgAZ80powder. ferredtothepowderviaradiationandconduction;however,local 2186 B.Zhengetal./MaterialsScienceandEngineeringA528 (2011) 2180–2191 Fig.7. TEMmicrographofSPS’edMgAZ80bulks:bright-filedimage,anditscorrespondentSADof(aandb)fineand(candd)coarsegrain. a 500 b 30 28 26 400 24 22 20 300 18 s s 16 unt unt 14 o o C 200 C 12 10 8 100 6 4 2 0 0 0 20 40 60 80 100 120 140 160 180 200 200 300 400 500 600 700 800 900 1000 Grain Size (nm) Grain Size (nm) Fig.8. HistogramofgrainsizedistributionofSPS’edMgAZ80in(a)fineand(b)coarsegrainarea.