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Special nanostructures in Al-Mg alloys subjected to high pressure torsion

By Margaret Gordon,2014-02-18 22:23
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Special nanostructures in Al-Mg alloys subjected to high pressure torsionin,to,Al,AlMg,high,High,alloy

    Special nanostructures in Al-Mg alloys

    subjected to high pressure torsion

Availableonlineatwww.sciencedirect.com

    ScienceDirect

    PressTrans.NonferrousMet.Soc.China20(2010)20512056

    Transactionsof

    NonferrousMetals

    SocietyofChina

    WwW.tnlTISC.Cn

    SpecialnanostructuresinA1Mgalloyssubjectedtohighpressuretorsion

    LIUMan.ping(刘满平),HansJ.ROVEN,LIUXin.tao(刘新

    ),MaximMURASHK1N,

    RuslanZ.VALIEV,TamasUNGAR4

    ,

    LeventeBALOGH

    1.NationalEngineeringResearchCenterofLightAlloyNetForming, ShanghaiJiaoTongUniversity,Shanghai200240,China;

    2.DepartmentofMaterialsScienceandEngineering.

    NorwegianUniversityofScienceandTechnology,Trondheim7491.Norway; 3.InstituteofPhysicsofAdvancedMaterials,UfaStateAviationTechnicalUniversity,Ufa450000,Russia;

    4.DepartmentofMaterialsPhysics.E6tv6sUniversity,BudapestH-1518Hungary Received23October2009;accepted15August2010

    Abstract:DeformationtwinsandstackingfaultswereobservedinnanostructureA1

    Mgalloyssubjectedtohighpressuretorsion.

    Theseobservationsaresurprisingbecausedeformationtwinningshaveneverbeenobservedi

ntheircoarse.grainedcounterparts

    undernormalconditions.Experimentalevidencesareintroducedonnon.equilibriumgrainbound~es.deformationtwiuningsand

    partialdislocationemissionsfromgrainboundaries.Someofthesef.ean1rescanbeexplainedbytheresultsreportedfrom

    molecular-dynamicssimulationsofpureFCCmetals.Specia1emphasisislaidontherecentobservationsofhiglldensityhexagonal

    andrhombicshapednanostrucmreswithanaveragesizeof3nlnintheA1.Mgalloysprocessedbyhighpressuretorsion.Apossible

    formationprocessofthesenanostructuresisproposedbasedonmolecular-dynamicssimulations.

    Kevwords:aluminumalloys;severeplasticdeformation;highpressuretorsion;grainboundarysuctllre:deformationtwinning;

    nanostrucmres

    1IntrOductiOn

    Inthelastdecade.therehasbeenaconsiderable

    interestinthedevelopmentofbulknanostructure

    materialsprocessedbysevereplasticdeformation

    rSPD)[1-4].Thisinterestarisesbecausetheuseof

    differentSPDtechnologiesprovidenewopportunitiesfor

    developingnanostructuresinmetalsandalloyswim

    unusualpropertiesthatareveryattractiveforvarious

    structuralandfunctionalapplications[14].Although

    outstandingprogresshasbeenmadeinthisareainrecent

    years,thegenesisofthestructuralfeaturesin

    SPDprocessedmetalsisnotyetfullyunderstood1.

    3-5].Thesefeaturesarequitecomplexandthepresence

    ofnon-equilibriumgrainboundaries(GBs)[2-4],

    deformationtwins,stackingfaults(SFs)[610],severe

    latticedistortionsandothernanostructures[11-13]in

    thesematerialsmayhaveprofoundeffectsonthe deformationmechanismsandmechanica1behavior.High pressuretorsion(HPT)isoneofthemostpromisingSPD techniquesbecauseithasthepotentialtoproduce nanostructureswithgrainsize1essthan100nlrlr12].The purposeofthisworkistofurtherexploretheformation mechanismsofthetypicalnanostructuresinA1.Mg alloyssubjectedtoHPTbasedonourhigh.resolution ansmissionelectronmicroscopy(HRTEM)

    observatjons.

    2Experimental

    TwoA1MgalloysincludinganA1.0.5Mgalloy

    (massfraction.%1andacommercialAA5182alloy A1.4.1Mg.0.35?【n.0.13Si0.32Fe(massfraction,%1

    receivedintheas.castandhomogenizedconditionwere subjectedtoHPTandtwistedfiveturnswitharotation speedoflr/minunderimposedpressureof6GPaat roomtemperature.ThedeformedHPTsampleshad dimensionsofd20mmx0.2mn1.Smalldiskswith diametersof3millwerepunchedfromtheouteredgeof Foundationitem:Project(50971087jsupportedbytheNationalNaturalScienceFoundation

    ofChina;ProjectsupportedbytheResearchCouncilofNorway undertheStrategicUniversityProgramonLightMetalsTechnology;Projects(67692,71594

    )supportedbytheHungarianNational

    ScienceFoundation

    Correspondingauthor:LIUMan-ping;Tel:+86-21-54742715;E

    mail:manping.1iu@sjtu.edu.cn;manping-liu@263.net DOI:10.1016/$1003-6326(09)6O416?7

    2052LIUMan-ping,etalfrrans.NonferrousMet.Soc.China20(2010)20512056

    theseH_PTsamples.Theequivalentstrainattheouter

edgeoftheHPTsampleswasabout906[2].The

    structura1characterizationwasperformedbyboth cOnventiOna1TEMandHrEMinaJEM-2010

    high.resolutionTEMoperatedat200kV.ThinTEMfoils werepreparedfromthesmalldisksbymeansofdisc grinding,dimplingandfinallyionpolishingwithAr+at anacceleratingvoltageof3kV.

    3Resultsanddiscussion

    3.1Grainboundarystructure

    0urpreviousworksconfirmthatthelowangle

    grainboundaries(LAGBs)intheHPTalloyscanbeiIl eithernonequilibriumorequilibriumstate[4].Fig.10) showsanHIEMimageofanequilibriumsubboundary

    withinalargegrainoftheHPTAA5182alloy.The misorientationbetweenthesetwosubgrainsisabout3.5.. Asshown,5perlectdislocationsof60.onthe(1l1) planearefoundtobealmostperiodicallydistributedon thesubboundary.Thesedislocationsaregeometrically Fig.1HRTEM[1T0]imagesofequilibriumsub-boundary (a)andnon-equilibriumsub-boundary(inverseFourierimage) (b)

    necessaryandthesubgrainboundarycanbedescribedby thesimplifiedFrankformula[4].Inotherwords,this subboundarydoesnotcontainextrinsicdislocationsand itisinanequilibriumstate.

    Fig.1fb)showsanexampleofanonequilibrium

    subboundarygeneratedwithinalargergrainintheHPT A10.5Mgalloy.Themisorientationanglehereisabout 1..Itisclearlyseenthattwotypesof60.dislocationsare

presentinthesubboundaryregiononthe(111)and

    (1l)planes,respectively.Basedonthesimplified Frankformula,severalextrinsicdislocationswhichare notgeometricallynecessaryexistinthesub.boundary region.Therefore,thissubgrainboundaryisinahigh energystate(ienon-equilibrium).

    Themechanismfordevelopmentofhighanglegrain boundaries(HAGBs)inSPDmaterialsisstillunclear[41. SomespecialHAGBswerefrequentlyobservedafter deformationinourpreviouswork[4,13].Fig.2(a)shows anexampleofanequilibrium?9GBintheHPTAA5182

    alloyl31.Asshown,the?9GBbetweengrainA2andC

    isclearlyconfirmedbytheHRTEMjmageandthe selectedareadiffraction(SAD1pattem(theinsetin Fig.2(a)).Theneighboringgrainsshareacommon 1101axisandhaveamisorientationof38.9.(theideal angleofZ9is38.94.,.Asdescribedbythecoincidence site1attice(CSL)modell3],1/9ofthelatticepointson

    thisgrainboundaryaresharedbytwoneighboring crystallaRicesandthedistortionoftheatomarrayonthe boundaryissmallerthanthatonanyotherrandomly orientedHAGBs.Therefore.theZ9boundaryhaslower boundaryenergyandcanbereferredtoasanequilibrium grainboundary[I3].

    Inparticular,non.equilibriumHAGBsarealso observedintheHPTalloys.AsshowninFig.2Co),avery highdensityofdislocationsisfoundnearanon. equilibriumHAGBintheHPTAA5182alloy.The neighboringgrainshaveamisorientationofabout18.5. whichtranscendsthelimitingangle15.ofaLAGB.The

    loca1dislocationdensitymeasuredfromFig.2Co1isas highas3.8xlOm_.whichisconsiderablyhigherthan theaveragedislocationdensityof1.3×10'rn-as

    measuredfromtheXraylineprofileanalysisinthesalTle sample[4].Inaddition,mostdislocationsinFig.2(b) appearasdipoles.Interstitialloops(markedbyblack circles)andvacancyloops(markedbywhitecircles)also existneartheGB(F2(b)).T1leintroductionofdipoles nearGBsislikelytoincreasethestoredelasticenergy[4] andmaketheGBenergyhigher.Therefore,itis reasonabletobelievethatgrainswithsuchseverelattice distortionandextremelydensedislocationsarestillina highenergystatethoughtheGBplaneisalmost straight.

    NonequilibriumGBsmightplayaroleinthe

    formationofdeformationtwinsandSFsshowninthe LIUMan.ping,etal/Trans.NonferrousMet.Soc.China20(2010)20512056

    Fig.2HRTEMimagesofequilibriumZ9GB(a)andhigh densityof60.perfectdislocationson(11T)planenear non-equilibriumHAGB(inverseFourierimage)Co) investigation[4,7].Thenonequilibriumdislocationsare

    probablycandidatesforemissionintopartials[7]andthe stressconcentrationscausedbytheseverelattice distortionsaresohighthattheycouldovercomeany energybarriersfornucleatingpartialdislocationsand twins[4,8,131.

    3.2Deformationtwinsandstackingfaults

    Ahighdensityofmicro.twinsandSFswasoften detectedwithinsmallergrainsandsubgrainswithsizes of20-50nn1.Thedensityvariesfrom100to100m-.

Fig-3(a)showsatypicalHIEMimageofsuch

    microtwinsandSFsobservedinthesubgrainA1inthe HPTAA5182alloy[131.ThewidthofthesubgrainiS about20nin.Theplanardefectsareindicatedbywhite arrows.Itisevidentthattheplanardefectshaveahabit planeof(1ll1,asthewhitesolidlineindicated.Thefast Fouriertransform(FFT)patterndemonstratesthein relationshipbetweenthetwinandmatrix.asshownin Fig.3(a).Thethicknessofthetwinsspansonlyl4

    2053

    Fig.3HRTEMimagesofhighdensityofmicro-twinsandSFs formedbypartialdislocationsemittedfromsubboundary13]

    (a),stackingfaultformedbytwo30.Shockleypartials dissociatedfromendon0.screwdislocation[5](b)and

    four-layertwinformedbydynamicoverlappingoffourSF ribbons[5(c)

    atomiclayersf0.21nm).ThesemicrotwinsandSFsare

    believedtoformbehindthemovingpartialdislocations whichareemittedfromthesubboundary[13].Therefore,

    itiSreasonabletoconcludethatsuchmicrotwinsandSFs 2054LIUManping,etal/Trans.NonferrousMet.

    Soc.China2o(2olo120512056

    observedinFig_3(a)formthroughtheheterogeneous mechanismaspredictedbythemolecular-dynamicsrMD1 simulationf141.

    InFCCmetals,stackingfaultsandtwinsformfrom thedissociationofeitherascrewdislocationora60. dislocation[5,8,l11.SFsformedbytwoShockley partialsOf30.dissociatedfromend.on0.screw dislocationswerefrequentlyobservedintheHPTalloys.

    FigI3fb)showsastackingfaultformedfromthe dissociationofascrewdislocationintheHPTAl0.5Mg

    alloy[5].IfBurgerscircuitsofthetwopartialswere drawnasindicatedinFig_3(b),itcanbeeasilyfoundthat thetwopartialsare30.Shockleypartials[5,8,11.Itis

    suggestedthatthestackingfaultbetweenthetwopartials isintrinsicandthewidthoftheSFis5.8nIn.

    AspredictedbytheMDsimulation[7],atwincan bealsoformedbythehomogeneousmechanism

    involvingthedynamicoverlappingofthestackingfaults ofdissociateddislocationsinthegraininteriors. Deformationtwinsformedbysuchhomogeneous mechanismwereobservedbyHRTEMintheHPT

    alloys[45,13].Fig.3(c)showsadefo?rrnationtwinwith athicknessof4atomicplanes(about1nm)51.Thetwin

    wasformedbythedynamicoverlappingof4SFsof dissociateddislocationsonadjacentslipplanes.Thetwin growsthickerbyaddingmoreSFsoneithersideofit. Sucha4.1ayertwinformedbyfourSFribbonshasnever beenexperimentallyobservedbvHIEM.The2-layer versionofsuchtwinswasfirstobservedbyZHUetalin 2003[7].Interestingly,asindicatedbythewhite''in Fig-3(c),ahighdensityof60.perfectdislocationsexists aroundthetwinboundary.Thesedislocationsare believedtoberelatedtothetwinformationprocess[7]. Recently,MDsimulationsbyS,YGENH0VENet

    al[15]reportedthatgeneralizedplanarfaultenergy rGPFE)curvessignificantlya-ectthepartial- dislocationmediateddeformationprocesses.Our

    experimentalfindingssuggestthattheMDsimulations

    basedonGPFEcurvescouldexplaintheformationof perfectdislocations,SFsandtwinsintheHPTAIMg

    alloys.However,furtherinvestigationsarenecessaryto revealthedominantdislocationactivi~,inthe nanostructureA1Mgalloy.

    3.3Specialnanostructures

    Surprisingly,highdensityhexagonalandrhombic shapednanostructures(referredasspecialnanostructures)

    Mgalloys. werefrequentlyobservedintheHPTAI

    Thesespecialnanostructuresareanalogoustoour previousobservationsinacommercialA1??Mg-Sialloy processedbyequalchannelangularpressing[11]. Figs.4(a)(b)showRTEMimagesofthespecial

    nanostructuresobservedinsideagrainwithasizeof about50nmintheHPTAA5182alloy.Asshown,an

    Fig.4HRTEM[1Tolimagesshowingplanarhexagonaland rhombicshapednanostructures(a)andspecialnanostructuresat highermagnificationwithinsetshowingFFT(b) extremelyhighdensityofplanardefectsexistswithinthe grain.Theaveragesizeisabout3nlTlandthelocal densityisabout10m-.AsrevealedbytheFFTofthe imagesinFig.4(b),theseplanardefectsarepresentalong twofl11,planesandone(001)planeandaretherefore referredtoas{l11}and(001)interfaces,respectively. Aninterestingfeatureintheseimagesisthattheseplanar deflectsaDpearwithtwokindsofregularshape,oneis rhombicandtheotherishexagonal[11].

    Thesesurprisingnanostructuresmaybecausedby reactionsbetweendislocationsondifferentslip

systems[6,ll,16].Highdensityperfectdislocations

    (Fig.2(b)),partialsandSFs(Fig.3)observedintheHPT alloysareprobablycandidatesforthereactions.As predictedbvMDsimulations[6],whentwopartialsof twoSFsondifferentslipplanesmeet,theycanreactto fornlatriangularstructurewithanangleof70.53.or 109.47..Therefore.thepresentrhombicstructurescould beformedbythereactionofsuchfourSFs[11]. Furthermore,twotrailingpartialscanreactwitha LIUManping,etal/Trans.NonferrousMet.Soc.China20(2010)20512056

    stair-roddislocationtoformaperfectdislocationcapable ofglidingon{001}planes[16].Thus,thehexagonal structurescouldbeformedbytheslipofperfect dislocationson{001}planesafterthetriangular structuresform11].

    Thedislocationreactionscanbeunderstoodbythe unfoldedThompson'StetrahedronshowninFig.5.Based onMDsimulationsbvYAMAK0Vetal[6],thepossible formationprocessofthesenanostructuresiSillustratedin Figs.6(a)(e).Theformationprocessprobablyincludes thefollowingsequentstages.

    11Onepairoftwointrinsicstackingfaults(ISFs) aregeneratedfromthedissociationoftwoperfect dislocations1/21101])and1/2011]C)ontwo

    2055

    f11T)and(111)slipplanes,asshowninFig.5and Fig.6(a).

    D—?J[)y+).4

    C

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