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STUDIES_0

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    STUDIES

;ACTAMETALLURGICASINICA(ENGLISHLETTERS)

    ;Vo1.16No.5PP407-415October2003

    ;STUDIESoNPRIMARYCRYSTALLIZATIoNoFRAPIDI

    ;QUENCHEDAl-Ni-Cu-NdMEIIALLICGLASSBYUSING

    ;HIGHRESoLUTIoNTRASMISSIoN

    ;ELECTRoNMICRoSCoPY

    ;D.Xiao1,2,3),.

    ;?

    ;,

    ;D.Jacovkis2),

    ;Clavaguera3),

    ;ClavagueraMora2)

    ;andJ.Rodriguez.Viejo2)

    ;1)CollegeofMaterialsScienceandEngineering,Central-SouthUniversity,Changsha410083,China ;2)GrupdeFisicadeMaterialsI,DepartamentdeFisica,UniversitatAutonomadeBarcelona,08193 ;Bellaterra,Spain

    ;‘3)Fisicade1’EstatSolid,FacultatdeFisica,UniversitatdeBarcelona,Diagonal647,08028Barcelona, ;Spain

    ;Manuscriptreceived22October2002;inrevisedform9January2003

    ;?

    ;RapidlysolidifiedAls7NiTCu3Ndametallicglasses,preparedbyusingmeltspinning, ;weretreatedunder6Disothermalandnonisothermalregime.Theamorphousrib

    ;bonandtheannealedsampleswerecloselyexaminedbymeanso/differentialscan

    ;ningcalorimetric,conventionalX-raydiffractionandhighresolutiontransmission ;electronmicroscopywithselected-areaelectrondiffraction,withspecialinterestinpri

    ;marycrystallizationintootAlnanocrystallineparticles,inordertounderstandattic

    ;turalcharacteristicso/Al-basedamorphous/nanocrystallinealloys,andnucleation ;andgraingrowthmechanismonthenanometerscaleduringprimarycrystallization. ;Theresultsshowthat,theas-preparedribbonsareflyamorphousandhomogeneous ;inthemicronscale,butcontainhighdensityo/quenched-inclustersorcrystallite ;embryos.mncrystallizationmainlyleadstoformationoytwo-phasemixtureoy ;otAlcrystalandresidualamorphousphase.Theannealedribbonsexposedisother- ;mallyat11O0C/or5,130minutesandheatedcontinuouslyuptolessthan31O0C ;at4O~C/rainconsisto/largeamounto/QAl/eccrystalnanoparticlesdisperseduni

    ;,D”inanamorphousmatrix.However,averylittleamounto/finerorthorhombic ;Al3Niintermetalicsparticlesexistintheannealedribbonsheatedupto31O0During ;primarycrystallization.theleadingkineticmechanicstoimpedegrowtho/theQAl

    ;crystalissimpingement,insteadoygeometricimpingement.

    ;KEYWORDSrapidsolidification,Al-richamorphousalloy,crystallization,nu- ;cleationandgrowthmechanism

    ;1.Introduction

    ;t

    ;Forrecentlymanyyears,Al-richmetallicglasshasbeenpaidmoreandmoreattention ;inthenovelmaterialsresearchfield,notonlyduetoitshighstrength,hardnessandlow ;density,butalsobecauseanew-typeA1basednanophasecompositescanbeobtainedfrom

    ;itbypartialannealingdevitrification[1一引.

    ;TheAl-basednanophasecompositeshaveamoreattractivecombinationofmechan- ;icalandphysicalpropertiesthanthecorrespondingA1richamorphousphase,whichis

    ;attributedtotheirspecialstructuralcharacter.ThenovelA1basednanophasecomposites

    ;consistoflargeamount(20-30volumepercent)ofQA1crystalnanoparticles(5-20nm)dis

    ;perseduniformlyinanamorphousmatrix[6-8.Asanimportantwaytocontrolstructural

    ;

    ;

    ;i.408?evolutionofamorphousphase,lowtemperatureannealing,basedonprimarycrystalliza- ;tion,thefirststageofdevitrification,isstudiedwidely,andisusedtotreatthermally ;theA1richmetallicglassintonanophasecomposites[911.However.forrapidlyquenched

    ;l

    ;;A187Ni7Cu3Nd3metallicglass,manyfurtherstudiesshouldbedoneinordertounderstand ;jthemicroscopicmechanismofcrystallization,includingnucleationandgraingrowth,onthe ;nanometerscale,andtoseekforaneasycontrollingwaystoattainthedesirednanoscale

    ;structuresabove-mentioned.

    ;l

    ;iHighresolutiontransmissionelectronmicroscopy,asanefficientmethodtoobservedirectlymicro

    -andnanostructureofthematerials,

    ;hasbeenwidelyappliedtocharacterize

    ;lnanoscalestructureofmetallicglassanditscorrespondingnanophasecomposites,andto ;lstudytheirstructuralevolutionduringthermalexposure.Thepurposeofthisexperiment ;istoinvestigatedevitrificationprocessofrapidlyquenchedAl87Ni7Cu3Nd3metallicglass, ;withspecialinterestincharacterizingnanoscalestructuresbeforeandafterthermalex-? ;posureandunderstandingitsprimarycrystallizationbehavior,includingnucleationand ;growthmechanicsofaA1crystalliteparticles,intheamorphousmatrixduringthermal ;exposurebyusinghighresolutiontransmissionelectronmicroscopy.’

    ;2.Experimental

    ;Rapidlysolidifiedlongribbonswerepreparedinasinglecopperrollmeltspinner,whose

    ;coolingratewashighenoughtoformmetallicglass.Theglassribbonsweretypicallyless ;Ithan3mmwideand30#mthickwithchemicalcompositionofAl87Ni7Cu3Nd3. ;Calorimetricexothermalbehavioroftheaspreparedsampleswascharacterizedbyusing

    ;Perkin-ElmerDSC..7differentialscanningcalorimeterunderpureAratmosphere.The ;sampleswereheatedcontinuouslyupto500.Cataheatingrateof40and160.C/min. ;Structuralcharacterizationandphaseidentificationoftheaspreparedandpartially

    ;annealedsamplesweredonebymeansofconventionalX-raydiffraction(XRD)andhigh ;resolutiontransmissionelectronmicroscopy(HRTEM)withselectedareaelectrondifffrac-

;tion(SAED).

    ;TheXRDandHRTEMspecimenswerecutfromtheaspreparedribbons.Theirhea

    ;treatmentwasalsocarriedoutinaPerkin-ElmerDSC7indifferentheatingregime:con-

    ;itinuouslyupto310.Cat40.C/min,andisothermallyat110.Cfor5and130min,heating/coolingratebe

    ing160.C/min.ConventionalXRDexperimentswereperformedinaSIEMENSD500X-raydiffraction?meterwithmonochromaticCuradiation(A=0.15418nm)overa20anglerangeof30._

    ;l50.atastepof0.03..TheHRTEMfoils,preparedbyionmillinginaliquidnitrogen ;(N2)bathtoavoidcrystallizationduringthinning,wereexaminedusingaCM30high

    ;iresolutiontransmissionelectronmicroscopeat300keV.HRTEMimageswereanalyzedbyusingDigitalMicrographsoftwareandSAEDpatternsbyprocessdiffractionsoftware

    .3.ResultsandAnalysis3.1StructuralcharacterizationofrapidlysolidifiedAls7Ni7Cu3N

    metallicglassesFig.1showstypicalHRTEMbrightfieldimagesanditscorrespondingSAEDpatterns ;oftheasspunA187Ni7Cu3Nd3ribbons.Nosignsofcrystallinephasepresentoverthe ;wholefigure,seeninFig.1a,exceptonlymazecontrasttypicalforamorphousstructure, ;I

    ;I

    ;:

    ;

    ;,

    ;

    ;.

    ;409.

    ;andtheSAEDpattern,asshownintheinsetofFig.1a,onlyconsistofabroadhalo ;typicalforamorphousphase.TheresultsuggeststhattheasquenchedAls7Ni7Cu3Nd3

    ;ribboniscomposedofhomogeneousandfullyamorphousstructurehardlywithdetectable ;crystallite.However,itisworthtonotethat,althoughtheasquenchedAls7Ni7Cu3Nd3

    ;ribbonexhibitsauniformamorphousmicrostructureintheWhole,asseeninFig.1b,ahigh ;numberdensityofquench-inclustersperhapsoriginatedfromrapidsolidificationprocess, ;arefoundinsomeregions,andtheclustersdistributeuniformlyintheregions,andeven ;lessthan2%nanometerquenchincrystalliteembryosembeddedintheamorphousmatrix. ;whichareeasiertoappearintheairsideoftheasspunribbons.Thisresultindicatesthat.

    ;thereexistsmoreorlesscompositionalorstructuralheterogeneityonthenanometerscale ;intheribbon.

    ;Fig.1BrightfieldHRTEMmicrographsandtheirSAEDpatterns(showninthe ;insets)fromtheas-quenchedAlsrNirCu3Nd3metallicglassribbons,show- ;ingfullyamorphousstructure(a),butwith’highlydenseclusters(b).

    ;3.2TwomainexothermictransformationsduringdevitrificationoftheAls7NiTCu3Nd3 ;metallicglassesundercontinuousheatingregime

    ;Fig.2showssometypicalDSCcurves(differentialscanningcalorimetercurves)of ;theAls7Ni7Cu3Nd3metallicglassesforcontinuousheatingatheatingratesof40and ;160.C/min.Twomainexothermiccrystallizationpeaksareobservedoneachofthecurves, ;whichcorrespondtotwomaintransformationsduringdevitrification.ThefirstiSbroad ;andasymmetric.rangingfrom110to310.Cwithpeaktemperatureof155.Conthecurve ;of40.C/min.ItcorrespondstoprimarycrystallizationleadingtoprecipitationofAlcrys

;talfromtheamorphousmatrixll3lb1.Thesecondpeaksaresharpandsymmetric.which

    ;correspondtofurthertransformationoftheresidualamorphousphaseintomultiphase

    ;mixtureofAlplussomeintermetalliccompounds,rangingfrom310to360.Cwithpeak ;temperatureof345.Conthecurveof40.C/min.Thesecondpeaksplitintotwosub-peaks ;onthecurveoflowheatingrate.Withheatingrateimproving,theexothermicpeaksshift ;towardshighertemperatureandtwosplitsubpeaksbecomeoverlapped,andevencom-

    ;pletelycoincidentfor160.C/min.Itindicatesthattworeactionsthatarenotcompletely ;separatedoccurintheresidualamorphousphase.

    ;Fig.3revealssomeconventionalX-raydiffractionpatternsfromtheaspreparedribbon

    ;andthesamplesheatedcontinuouslyuptovarioustemperatures.TheXRDpatternof ;

    ;.

    ;410.

    ;Fig.2DSCcurvesoftheas-spunAlsrNirCu3Nd3

    ;ribbonsundercontinuousheatingregime

    ;atheatingrateof40.C/min.

    ;2dea.

    ;Fig.3TypicalXRDpatternsoftheas-prepared

    ;AlsrNirCuzNdaribbonanditsannealed

    ;samplesheatedcontinuouslyuptovarious

    ;temperaturesat40.C/min.I

    ;aspreparedsampleshowstypicalamorphoushalocharacteristics,butwithadetectable ;shouldernearby28?44.,whichgenerallymayresultfromatomicclustersorAlcrystalline

    ;embryos.TheresultiSinagreementwithHRTEMmicrographsandSAEDpatternsin ;Fig.1.Whenheatinginthetemperaturerangelowerthan310.C,onlyfccO/Alcrystalpar-

    ;ticlesprecipitatefromthefullyamorphousmatrix.Uponincreasingheatingtemperature. ;thefullyamorphousphasedevelopsgraduallyintotwo-phasemixture,typicallyconsisting ;ofnanometersizedaluminumcrystalembeddedhomogeneouslyintheresidualamorphous ;matrix.Whenheatingatthehighertemperatures,someintermetallicphasesprecipitate ;rapidlyintheformofdispersionoreutecticmixtureintheamorphousribbons.Hence, ;whenheateduptohigherthan310.Ctheannealedribbonsdevelopgraduallyintomulti- ;phasemixturemicrostructuresofQAlandsomeintermetalliccompounds,mainlyAlNi

    ;(orthorhombic,a=0.6681,b=0.7368,c=0.4814nm)t16J,AlllNd3(orthorhombic,a=0.4359, ;b=1.2924,c=1.0017nm)t?J,andAI8CuaNd(tetragonal,a=0.8789,c=0.5143nm)[1.

    ;3.3StructuralevolutionduringprimarycrystallizationoltheAls7NiTCu3Nd3metallic ;glassesonthenanometerscale

    ;Fig.4showstypicalHRTEMbrightfieldimagesandtheircorrespondingSAEDpat

    ;ternsfromtheannealedspecimenaftervariousisothermalexposures.Whenisothermally ;heatedat110.C,Alcrystalnucleiform,multiplyandgrowrapidlyintheas-quenched ;AI87NiTCu3Nd3metallicglass.After5minutes,asshowninFig.4a,thenucleihaved ;velopedintohighlydensenearlysphericalQAlcrystalnanoparticles.1essthan5nmin

    ;diameter,whichdisperseuniformlyinthehomogeneousamorphousmatrix,withrandom ;orientation.Atthistime,theQAlcrystala~lountisverysmall,asshownintheinsetof

    ;Fig.4a.Tworingscanbeseeninthefigure,whichcorrespondtothe(111)and(200)planes

;oftheQAlcrystal,respectively.Afterfurtherheatingisothermallyfor130min,theQAl

    ;crystallineparticleshavegrownobviously,seeninFig.4b.andtheamountofa.Alcrystal ;phasealsoincreasesobviously,whilethatofamorphousphasedecreasesintheannealed ;ribbon,seenintheinsetofFig.4b,inwhichthefiveringscorrespondto(III),(200), ;..誊差一

    ;o,II|.olIz

    ;

    ;?

    ;?

    ;1

    ;.

    ;411.

    ;(220),(311),and(222)planesoftheQAlcrystal,respectively.Atthistime,theannealed ;ribbonhasdevelopedintotypicalprimarycrystallizedmorphologies.Themicrostructure ;consistsofahighnumberdensityofnanoscale(about68am1crystalparticlesembedded

    ;dispersedlyintheamorphousmatrix.

    ;Forheatingcontinuouslyupto310.C,closetbtheonsettemperatureofthesecondpeak ;onthecontinuousscanningDSCcurveat40.C/min,thetypicalmorphologicalcharacter ;oftheannealedspecimen.asshowninFig.5.iSverysimilartothatofthesampleannealed ;isothermallyat110.Cfor130minutes.However.theAlcrystalparticlesfurthergrow ;Fig.4BrightfieldHRTEMmicrographsandtheirSAEDpatterns(shownin ;theinsets)fromtheisothermallyannealedspecimensofrapidlysolidi

    ;fledAl87Ni7Cu3Nd3metallicglass:(a)at110.Cfor5min;(b)at110.C ;for130min.

    ;slightly,gettingto5-15nmindiame-

    ;ter,andmoreamorphousphasecrys

    ;tallizesinto0Alcrystalphase.which

    ;iSalsoindicatedbysharperCOntrast

    ;ofpolycrystallinediffractionringpat

    ;tern.seenintheinsetofF4g.5,with

    ;fiveclearrings,correspondingtoA1

    ;(111),(200),(220),(311),and(222)

    ;planesoftheQAlcrysta1.respec

    ;tively.However.intheribbonstill

    ;remainslargeamountofamorphous

    ;phase.bywhichtheQAlnanocrys

    ;talswereseparatedwitheachother.

    ;Duringprimarycrystallization

    ;theprecipitateparticlesfromthe

    ;amorphousmatrixaremainlyQAl

    ;nanocrystals.Hence,thepartially

    ;annealedribbonsconsistoftwo-phase

    ;mixedstructureofQAlcrystaland

    ;theresidualamorphousphase.

;Fig.5BrightfieldHRTEMmicrographand

    ;itsSAEDPattern(inset)fromthe

    ;annealedspecimensofrapidlysolidi

    ;fledAls7Ni7Cu3Nd3metallicribbonaf-

    ;terheatedcontinuouslyupto310.Cat

    ;40.C/min.

    ;

    ;.

    ;412.

    ;3.Morphologicalcharacterizationandphaseidentificationofthenanocrystalparticlesin ;theannealedribbonservariousthermalexposures

    ;Fig.6showstypicalmorphologiesoftheOlA1crystalsintheribbonsaftervarious

    ;isothermalannealing,andtheirfastFouriertransform(FFT).Afterthedigitalizedmor

    ;phologicalimagesareprocessedandenhancedbyusingfastFouriertransform,filtering, ;andinversefastFouriertransform,theyidentifymoreclearlytheparticlesasOlA1fcc

    ;crystalformedduringprimarycrystallization.

    ;Fig.6HRTEMbrightfieldmicrographsoftheQA1particlesintheisother

    ;mallyannealedspecimens,andthefastFouriertransform(FFT)shown

    ;intheinsets,correspondingtoA1(200)planes,respectively(a)heated ;isothermallyatii0.Cfor5min;(b)heatedisothermallyatii0.Cfor

    ;130min.

    ;t

    ;Whenisothermallyheatedatl10.Cforshortertime,thefineOlAlcrystalparticlesex-

    ;hibitnearlysphericalshapeandareisolated,surroundedbytheresidualamorphousphase. ;Afterfurtherheatingisothermallyfor130min.theAlcrystalparticlesgrowobviously,

    ;andsomeofthemexhibitadevelopingtendencytowardsdentriteshape.However,the ;nanocrystalparticlesstillareinanisolatedstate,exceptaveryfewparticlesappearing ;tocontactgeometricallywitheachother,asbeingshowninFig.7.Infact,evenifheated ;athightemperatureupto310.C.seldomoccursthiskindofphenomenonthattheOlAl

    ;crystalparticleshitdirectly.Theseresultsindicatethatduringprimarycrystallization,. ;softimpingement,indirectinteractioncausedbysolutediflusionconcentrationfieldsur- ;roundingtheOlAlcrystalparticles.isaleadingkineticmechanicstoimpedeadvanceof ;theOl—Alcrystallineparticlesinsteadofgeometricimpingement.’

    ;AlthoughmostofthenanocrystalparticlesintheannealedribbonsareOlAlcrystal

    ;solidsolutionphase,averylittleamountofA13Niintermetallicphasebegiastoappear ;intheresidualamorphousmatrixoftheannealedribbonafterexposedthermallyupto ;310.C,whichcanbeshownintheinsetofFig.5throughanalyzingcarefullybyusingDigital ;Micrograph.ExistenceoftheAl3NinanometerparticlesismoreclearlyrevealedinFig.8. ;Thenewlyappearedintermetalicsisveryfine,lessthan5nmindiameter,withmuchlarger ;spacingoftheplanesthantha_toftheOlAlsolidsolution,about0.389nm,corresponding

    ;tothef101)planesoftheA13Nicrystalphase.

    ;

    ;?

    ;413?

;Fig.7HRTEMbrightfieldmicrographofthe

    ;A1fccparticlesinthespecimensheated

    ;isothermallyat110.Cfor130rain,andits

    ;FFTshownintheinset.correspondingto

    ;A1(200)planes.

    ;4.Discussions

    ;Fig.8HRTEMbrightfieldmicrographofthe

    ;finerparticlesintheamorphousmatrixof

    ;thespecimenheatedcontinuouslyupto

    ;310.Cat40.C/rain.anditsFFTshown

    ;intheinset,respectively,showingthatthe

    ;A13Niintermetallicswithlargerspacingof

    ;theplanesbeginstoappear.

    ;PrimarycrystallizationiSanintegratedprocessofnucleationandgrowthinthedisor- ;deredsystemaswellassolidification.However,duringprimarycrystallization,structural

    ;evolutionbecomesmoredependentondiffusionofslowsoluteatoms,duetothephase ;transformationoccurringinthesolidstatestiffamorphousmatrix.Hence,diffusionof ;soluteatomswithlowestdiffusionCOecientisconsideredasthemostimportantfactorto ;affectstructuralmorphologi?

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