Local decomposition induced by dislocation motions inside precipitates in an Al-alloy.

1.2 MB·English

by  Yang, B.

Local decomposition induced by
dislocation motions inside precipitates in
SUBJECTAREAS: an Al-alloy
METALSANDALLOYS
STRUCTUREOFSOLIDSAND
LIQUIDS B.Yang,Y.T.Zhou,D.Chen&X.L.Ma
MECHANICALENGINEERING
SURFACES,INTERFACESAND ShenyangNationalLaboratoryforMaterialsScience,InstituteofMetalResearch,ChineseAcademyofSciences,WenhuaRoad72,
THINFILMS 110016Shenyang,China.
Received Dislocationsincrystalsarelinearcrystallographicdefects,whichmoveinlatticewhencrystalsareplastically
28October2012 deformed.Motionofapartialdislocationmayremoveorcreatestackingfaultcharacterizedwithapartialof
alatticetranslationvector.Herewereportthatmotionofpartialdislocationsinsideanintermetallic
Accepted compoundresultinalocalcompositiondeviationfromitsstoichiometricratio,whichcannotbedepicted
7December2012 withanyvectorsoftheprimarycrystal.AlongdislocationslipbandsinsidethedeformedAl Cuparticles,
2
redistributionofCuandAlatomsleadstoalocaldecompositionandcollapseoftheoriginalcrystal
Published
structure.Thisfindingdemonstratesthatdislocationslipmayinducedestabilizationincomplex
8January2013
compounds,whichisfundamentallydifferentfromthatinmonometalliccrystals.Thisphenomenonof
chemicalunmixingofinitiallyhomogeneousmulticomponentsolidsinducedbydislocationmotionmight
alsohaveimportantimplicationsforunderstandingthegeologicevolvementofdeep-focusperidotitesinthe
Earth.
Correspondenceand
requestsformaterials
shouldbeaddressedto Precipitatehardeninganddispersionstrengtheninghavebeenwidelyappliedinmanyengineeringmaterials,
X.L.M.([email protected]. inwhichsmallparticlesofsecondphase(usuallywithcomplexcrystalstructure)playcriticalroleinresisting
cn) dislocationmotioninthematrix.Incaseofprecipitatehardening,thehardnessoryieldstrengthofanalloy,
which was experienced a higher temperature solution treatment, increases with the presence of small-sized
precipitationandtheseprecipitatesusuallyhavecoherentorsemi-coherentinterfacewiththematrix.Incontrast,
in dispersion strengthening alloys, dispersed second phases usually do not have coherent interface with the
matrix,suchasAl O -reinforcedAlcomposites.Quantity,morphology,anddistributionofthesesmallparticles
2 3
areofcriticalimportanceintuningmechanicalpropertiesofthematerials.However,inthepastdecades,more
andmoreexperimentalobservationsindicatethatthesmall-sizedsecondphasecanbedecomposedduringplastic
deformation,leadingtodegradationofmaterialproperties.Forinstance,whenAl-Cualloyswithh’precipitateis
plasticallydeformedatambienttemperature,dissolutionordecompositionoftheprecipitateswasdetected1,2.
Such a phase dissolution induced by plastic deformation has been observed in various Al-alloys in which
precipitatesplayacrucialroleinmechanicalperformanceofthealloys,suchasAl Fe particleinAl-Fe3,b’-
13 4
phaseinAl-Mg-Si4,c’-phaseinAl-Ag5,g’-phaseinAl-Zn-Mg6andd’-phaseinAl-Lialloy7,8.Inaddition,steels
provideanotherexamplewheredeformation-inducedphasedissolutionhasbeenrecognizedfordecades.Whena
pearliticsteel(consistingofcementite(Fe C)andferrite)isplasticallydeformedatambienttemperature,Fe C
3 3
particlesarefoundtorefineanddissolve9–12,typicallyinheavilycolddrawingofpearliticsteelsforproducing
extraordinarilystrongwires13.
Thedissolution ofthestrengthening particles inengineering alloysunder mechanical loadingunavoidably
leadstoadetrimentalconsequenceinthedesignedmicrostructureandperformanceofthematerials.Stability
controlofthestrengtheningparticlesreliesonathoroughunderstandingofthemechanismofdissolution,in
whichthebehaviorofdislocationsisbelievedtobethecentralissue.Interactionsbetweendislocationsandsmall
particles have been extensively studied and several mechanisms have been proposed, such as Orowan mech-
anism14–16andcuttingmechanism17,18.Inaddition,particularattentionhasbeenpaidtotheinterfacebetween
secondphaseandmatrix9–11,19–22.Althoughitisknowthatmotionofpartialdislocationinorderedstructuresmay
removeorcreateanti-phaseboundaries,dislocationbehaviorsinsidetheprecipitateswithcomplexstructureslike
thoseinAl-alloysarenotwellunderstood.Dislocationstructureatatomisticscale(particularlyatthecoreofa
dislocation)iscrystalstructure-dependent.Hence,dislocationdynamicsincomplexstructuresmaydifferfun-
damentallyfromthatinmonometalliclattices.
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Figure1|(a)HAADFimageshowingthemicrostructuralcharacteristicsoftheas-receiveddual-phasematerial.TheparticlesembeddedinAlmatrixare
tetragonalh-Al Cuprecipitates,verifiedbyX-raydiffractionandaseriesofelectrondiffractionpatterns.(b)HAADFimageofanareawithinasingle
2
crystallineAl Cuprecipitatealong[001]crystallographicdirection(seeinset).(c)and(d)areelementalmapsofAlandCu,respectively,correspondingto
2
theareasquaredin(b).BothAlandCuarehomogeneouslydistributed.(e)Micrographshowinganarrayofdislocationsintheprecipitateinducedby
deformation.(f)Bright-fieldTEMimageshowingslipbandson(110)crystallographicplanesinanAl Cuparticle.(g)HAADFimageofthesameareaas
2
thatin(f)showsclearcontrastvariationalongtheslipband.Thebottom-leftinsetisasectionalongthisslipbandmarkedwithblackrectangle.Theup-left
insetisanEDPof[110]zoneaxisfromtheareamarkedwithablackcircle.
Results confirmed by the homogeneous contrast of the high-angle-angu-
WechoseanAl-4wt.%Cualloyinthisstudy.Thealloyexperienceda lar-dark-field(HAADF) imageinTEM(Fig. 1b),aswellasbythe
solution treatment at 520uC followed by ageing at 300uC for electrondiffractionpattern(InsetinFig.1b).ItisnotedthatHAADF
48 hours,asshowninFig.S1.Theas-receivedsampleiscomposed images provide strong contrast associated with the local variety of
of tetragonal h-Al Cu precipitates embedded in Al matrix, Fig. 1a chemical compositions and/or thickness contribution23. Elemental
2
and Fig. S2. Each of the Al Cu precipitates is a single crystal, as mapsofAlandCuinthesinglecrystallineAl Cu(Fig.1c,1d)show
2 2
Figure2|(a)HAADFimageofasectionalongatypicalslipband.ItisseenthatthemorphologyofCu-richareasrangesfromcirculartoelliptical,and
sometimesirregular.(b)and(c)areAlandCumaps,respectively,obtainedfromtheareamarkedwithawhiterectanglein(a).Adarkerringin(b)
correspondingtothebrighterringin(c)ispoorofAlandrichofCu.(d)isEDSprofilesshowingthelocalcompositionfluctuationalongtheslipband.
ThescanningrouteforthisEDSprofileisperpendiculartotheslipbandmarkedwithawhiteverticallinein(a).(e)Cu-mapalongtheslipband,wherethe
intensityofCuisvaried.(f)[110]EDPtakenintheareaclosetotheslipband.(g–i)EDPstakenatdifferentCu-poorareasalongtheslipband.TheseEDPs
canbenolongerindexedtotetragonalAl Cuduetodecompositionandstructuralcollapse.
2
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Figure3|(a)HREMimageobtainedfrom[110]directionshowingadislocationarraywithaslipsystem[001](110).Burgerscircuitisoutlinedforeach
dislocation,whichexhibitsthelatticedisplacementof1/2[001]inthisprojection.(b)Highmagnificationimageofthepartialdislocation(leftpartialin
(A)).Stackingfaultintherightofdislocationcore(‘‘H’’)isnotresolvedsincetheatomicprojectionsof(001)and1/2(001)planesarethesame.
(c)Atomicstructuralmodelofthepartialsviewedalongthedislocationline.Here,atomsinthreesub-layers(x50,0.096,0.25)areselectedforprojection
inordertodistinguishthetwocores.Thepartialchangesthestackingofthecrystal(verticaldirectioninthisschematicrepresentation),i.e.iteither
removesorcreatesastackingfault.Thewholeconfigurationoftwopartialsandstackingfault(anellipsisbetweentwocoresaremarked)iscalledan
extendeddislocation.
aratherhomogeneousdistributionofbothelements.Compositional Semi-quantitativecharacterizationofthecompositionvariations
fluctuations in the measured areas are within 65,68 at% Al and inadeformedAl Cuparticleisperformedbyelementalmappingand
2
correspondingly31,35at%Cu. X-ray energy dispersive spectrometer (EDS) analysis. Local areas
Plastic deformation of the sample was performed by using the withacontrastinhomogeneityalongtheslipbandintheHAADF
surface mechanical attrition treatment (SMAT) under vacuum of imageareselected(Fig.2a),withaparticularattentiontotheAland
631022 Pa at liquid nitrogen temperature for 60 minutes with a Cudistribution.Theelementalmapsoftheareamarkedwithawhile
vibratingfrequencyof50 Hz.SMATproducesagradientvariation rectangle in Fig. 2a are shown in Fig. 2b and 2c. In the Al map
ofplasticstrain(andstrainrate)inthetreatedsurfacelayer24–26.The (Fig.2b),adarkerringisobserved.Atthecorrespondingposition,
overalldeformationlayerintheSMATsampleisabout40mmthick aring with brightercontrast is seen in theCu map(Fig. 2c). This
(Fig. S3), in which Al Cu particles at different depths from the meanstheringareaispoorofAlandrichofCu.SuchaCusegrega-
2
treatedsurfacearedeformedwithdifferentplasticstrains.TEMana- tionleavesaCu-poorareainitsvicinity.Furthermore,EDSprofiles
lysisindicatesthat,inadditiontotheAlmatrix,Al Cuprecipitates (Fig.2d)scannedalongtheverticallineinFig.2aalsoindicatethata
2
are also plastically deformed and strain-induced dislocations are composition fluctuation happened. The wavy counts indicate a
observed inside the precipitates (Fig. 1e). Semi-quantitative X-ray remarkablejumpofchemicalcompositions.Detailedstatisticmea-
diffractionanalysiswithalowscanningangleof3ushowsthatthe surementsfromadozenofEDSspectraindicatethatthecomposition
volumefractionofAl Cuprecipitatesinthetopmostdeformedlayer intheCu-richareasrangeswithin38,46at%Cuand53,62at%Al.
2
(of about 2 mm thick) is reduced by 35.6% after SMAT (Fig. S4), ThiscorrespondstoAl/Curatioof1.16,1.63,deviatedpronounc-
indicatingthatalargevolumeoftheprecipitateshasbeendissolved edly from the Al Cu stoichiometric ratio (251). By comparison of
2
asaresultofheavyplasticdeformationinthetopmostlayer. HAADF images and thecorresponding EDSprofiles, onemay see
Inthedeepdeformedlayer(30,40mmfromthetreatedsurface), thattheareawithabrighterHAADFcontrastisalwaysrichofCu,
presenceofslipbandsalong(110)crystallographicplaneinsidethe whilethedarkerareaispoorofCu.Suchanelementalredistribution
deformed Al Cu particles is one of the salient features, as seen in canbeoverviewedclearlyinaCumapping(Fig.2e),whereCu-rich
2
bright-field TEM images (Fig. 1f). HAADF image (Fig. 1g) of the andCu-poorareasarealternatelypresentalongtheslipband.
sameareaasthatinFig.1fshowsthatthecontrastalongtheslipband A series of electron diffraction experiments were carried out to
is rather inhomogeneous (the most remarkable areas are up-ward identify the structural evolution in the area corresponding to the
arrowed).Thissignifiesthatthechemicalcompositionalongtheslip decomposition.Forcomparisons,the[110]electrondiffractionpat-
bandisdeviatedfromthatofunformedstate.Acloseobservation,as tern(EDP)takenintheareaclosetotheslipbandisshown(Fig.2f).
in theinset ofFig. 1g, reveals a more distinct contrast where each It is interesting to note that when electron beam is shifted in a
darkerareaisattachedwithabrighterellipticaldomain. parallel approach to the Cu-poor areas along the slip band, the
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Figure4|(a)SchematicofthecomputationalcellusedforMDsimulations.Thedislocationisindicatedwith‘‘H’’;andthestackingfaultarea(intheleft
ofthecore)isalsomarked.(b)CellforMDsimulation,wherepartialdislocationisinvolved.(c–e)Snapshotsoftheatomicconfigurationat14,000,
200,000,and400,000timesteps,respectively.In(b–e),thegreenspheresdenoteAlandthebrownspheresCuatoms,respectively.(f)Atomistic
configurationsprojectedalong[110]directionwhereperfectarea,partialcore,andstackingfaultarerepresented.(g)and(h)showacontinuousmotion
ofdislocationleavesbehindCudisplacements(red-arrowed)aroundtheslipplane.(i–m)MDsimulationindicatesthatthetwopartialstendtomove
towardeachother(seethelengthchangingofthedouble-arrowedbarsinthefigure).Coloredaccordingtothedisplacementvalues(pffidffiffixffiffi2ffiffizffiffiffiffiffidffiffiyffiffiffi2ffizffiffiffiffiffidffiffiffizffiffi2ffiffi),
theplotsdisplayanevolutionofCudisplacementat6,000,10,000,16,000,30,000,and240,000timesteps,respectively.In(i–l),thedisplacementvaluesare
from0to1A˚;whilein(M)therangeis0,7A˚.Theadditionalpeaksbetweenthetwopartialsin(j,k,l)arebelievedtoresultfromthelocaldistortionunder
stressinthestackingfaultarea.
EDPsdisplaycompletelydifferentcharacteristics(Figs.2g–2i).The inbetween.Theseparationdistancebetweentwopartialsisinarange
EDPscanbenolongerindexedtotetragonalAl Cu.Inotherwords, of10,20nm.TheleftpartialinFig.3aismagnifiedasseeninFig.3b,
2
1
dislocationmotionalongtheslipbandinsidetheAl2Cuprecipitate wherealatticedisplacementof [001]canbeidentifiedinthispro-
resultsinnotonlychemicaldecompositionbutalsoalatticestruc- 2
jection. The tetragonal Al Cu crystal has eight sub-layers stacked
ture collapse, which differs fundamentally from the scenario in 2
along ,110. direction, namely, A B CB A B CB (schematically
monometalliccrystals. 1 1 2 2 2 1
showninFig.S5).Aluminumatomsinlayer‘‘A’’formanetworkof
hexagonsandthealuminumatomsinadjacentlayer‘‘B’’islocated
Discussion
abovethehexagonalcenters.Layer‘‘C’’constitutedofcopperatoms
Dislocation characteristics in the body-centered tetragonal Al Cu
2 positionedbetweentwo‘‘B-layer’’.Thepuckered‘‘BAB’’layerscan
crystal have been studied earlier by diffraction contrast analysis
beconsideredasasandwichstructure,andaglidingby1/2[001]will
andhigh-resolutionimaging27,28.DislocationswithBurgersvectors
1 resultintheatomsinlayer‘‘B’’movetotheedgesofhexagonsoflayer
of ,111.,[001],and[100]wereidentified,andfaultson(110)and ‘‘A’’,whichshouldbeextremelyunstable.Thusaslipshouldoccur
2
(001)planeswerealsoobserved.InourSMATsample,dislocations between‘‘B’’and‘‘C’’layers.Basedonthisreasoning,thecorestruc-
associatedwiththeslipsystemof[001](110)arefrequentlyobserved. tureateachpartialisproposedasshowninFig.3c,whereatomsin
Highresolutionimaging(Fig.3a)indicatesthatsuchadislocationis threesub-layersareillustrated(anellipsisisintroducedinthestack-
usuallydissociatedintotwopartialdislocationswithastackingfault ingfaultsareabetweentwocores).
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Detailedmoleculardynamical(MD)simulationswereperformed thetop.ATecnaiG2F30transmissionelectronmicroscope,equippedwithHAADF
totherightpartialinFig.3ainordertomonitoratomisticprocesses detectorandX-raydispersivespectrometersystems,wasusedat300kVfordiffrac-
tion,HAADFimaging,andcompositionanalysis.MDsimulationswithLarge-scale
during dislocation motion. In the computational cell used in MD
Atomic/MolecularMassivelyParallelSimulator(LAMMPS),areperformedusingthe
simulations(Fig.4a),thepartialdislocationiscreatedbyremoving embeddedatommethod(EAM)interatomicpotential.
twoadjacentatomichalfplanesfromthecrystalstructure(Fig.4b).
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Methods
AnAl-4wt.%Cualloywasselectedandexperiencedasolutiontreatmentat520uC
followedbyageingat300uCfor48hours,theas-receivedsampleiscomposedof Acknowledgements
tetragonalh-Al2CuprecipitatesembeddedinAlmatrix.X-raydiffractionsatlower WethankProfessorK.LuforthecriticalcommentsonthisworkandMr.X.Shiforhis
scanningangleof3uwereperformedtodeterminephasesinthe2mmthicklayerfrom assistanceinsampletreatmentandMr.S.C.WangforhisassistanceinX-raydiffraction.
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ThisworkissupportedbytheNationalNaturalFoundationofChinaandMinistryof Competingfinancialinterests:Theauthorsdeclarenocompetingfinancialinterests.
Science&TechnologyofChina(2009CB623705).
License:ThisworkislicensedunderaCreativeCommons
Attribution-NonCommercial-NoDerivs3.0UnportedLicense.Toviewacopyofthis
Authorcontributions: license,visithttp://creativecommons.org/licenses/by-nc-nd/3.0/
X.L.M.designedtheoverallstudyandwrotethepaper.TEMexperimentswereperformed Howtocitethisarticle:Yang,B.,Zhou,Y.T.,Chen,D.&Ma,X.L.Localdecomposition
byB.Y.andY.T.Z.(equallycontributedintheexperiments).D.C.carriedoutthetheoretical inducedbydislocationmotionsinsideprecipitatesinanAl-alloy.Sci.Rep.3,1039;
calculations.Allauthorscontributedtothediscussionofthedata. DOI:10.1038/srep01039(2013).
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