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Eutectic

By Carmen Thomas,2014-07-18 02:26
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Eutectic

    Eutectic

    Dec.2007,Volume1,No.1(Monthly)JournalofMaterialsScienceandEngineering,ISSN1934-8959,USA

    ;EutecticstructureofA1.Sialloysolidifiedunderhighpressure ;XURuiiLIJiel.

    ;ZHA0Hailii.

    ;LIURi-pingl

    ;.

    ;WANGWenkuilJ2

    ;r.KeyLab.ofMetastableMaterialsScienceandTechnology,YanshanUniversity,Qinhuangdao066004:China

    ;2.CollegeofMaterialsScienceandEngineering,YanshanUniversity,Qinhuangdao066004,ChinaJ

    ;Abstract:TheexperimentofsolidificationofAIf1123)at.%

    ;SialloysunderhighpressurewascarriedoutwithaCS1Btype

    ;sixanvilhighpressureapparatus.Microstructureofthealloys ;solidifiedat5GPaisanalyzedbycomparingwiththatsolidified ;atambientpressure.Theresultsshowthatamuchamountof ;primary0【一A1phaseisobservedinthealloysolidifiedunderhigh ;pressureandtheformationmechanismofthe6eutectic

    ;structureisdifferentfromthatunderambientpressure.The ;eutectic3-Siphaseisfoundtobecoarsenedbypressure,andthe ;eutecticBSilamella,however,wererefinedwithincreasingthe ;siliconcontentat5GPa.

    ;Keywords:metalsandalloys;solidification;microstructure; ;highpressure;eutecticstructure

    ;1.Introduction

    ;Temperature(orcoolingrate),compositionand ;pressurearethemostimportantfactorsthathavegreat ;effectsonthesolidificationofmetalsandalloys.Inthe ;past,muchmoreeffortshadbeengiventotheeffectsof ;theformertwofactors,andSUCCESSfultheoriesabout ;equilibriumandnonequilibriumsolidificationhave

    ;beendeveloped.Theeffectofpressureonsolidification ;ofmetallicalloys,however,wasneglectedforalong ;timemainlybecauseofthelimitofequipments.

    ;Recently,som,

    ;eresearchesonsolidificationofmetals,

    ;elementalsemiconductorsandalloysunderhigher ;pressurehavebeenreported[1-61.Thediamond

    ;structureofgermaniumwassolidifiedintoamorphous ;,

;andmetastableGeIIIphasewhenquickly

    ;decompressed’,81.Inordertomakethemetalsand

    ;alloyssolidifyfarfromequilibrium,meltquenching, ;Acknowledgements:ThestudywassupportedtheNational ;NaturalScienceFoundationofChina(No.50325103). ;xuRui(1961),male,Ph.D.,professor;researchfields: ;solidificationandalloyphasediagramofmetallicmaterials. ;fluxingtreatment,andrapiddecompressionunderhigh ;Dressurewereinvestigated”.

    ;A1-Sialloyshavelongbeenstudiedfortheir ;industria1importance12.14.Theirmicrostructure

    ;consistsofprimaryoc-A1dendritesorprimaryf}.Siand ;theeutectic(a-Al+fl-Si)microstructuresdependingon ;composition.Itisnecessaryforincreasingthe ;industrialapplicabilitytomodifiedmicrostructureby ;addingelementssuchasTi,P,Na,Srl5_,directional

    ;solidificationl9.3].orrapidcoolingsuchasinsurface ;treatmentwithhigh.energybeams[24-261

    ;,whichcan

    ;resultindramaticincreaseinmechanicalpropertiesof ;thealloys.Thus,themorphologiesoftheprimaryand ;eutecticfl-Siphasesarepaidparticularattention’’

    ;23,27,28tOObtainarefinedmicrostructureandless ;faceted(;ISimorphologiesinordertoincreasethe ;strengthandductilityofthealloy.Recentresearches ;showthatthesoliduscurvesareraisedandshiftedright ;sideofA1?Sialloyphasediagramasafunctionof ;pressure[291

    ;,

    ;andaamountoftheprimaryo:-A1phase

    ;wasobservedinhypereutecticA1Sialloyassolidified

    ;underhighpressure3o].

    ;Inthepresentwork,highpressureisappliedto ;solidificationofA1Sialloytoobtainthe

    ;microstructurefarfromequilibriumandtoexplorethe ;solidificationmechanismofeutecticunderhigh ;pressurebycomparingwiththemicrostructure ;obtainedunderambientpressure.

    ;2.Experimental

    ;PureA1(99.999%)andpureSi(99.999%)were ;usedtopreparethemasterA1-(1323at.%,Sialloysin

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    ;difiedunderhighpressure

    ;avacuumarcmeltingfumace.Theexperimentsof ;highpressuresolidificationofthealloyswerecarried ;outonaCS-1Btypesixanvilhighpressureapparatus

    ;withpyrophylliteassealingmaterialandpressure ;mediumandBNasflux.Thesample,with3mmin ;diameterand4mminheight,washeatedbyelectrical ;currenttothetemperatureof1400.Candkeptfor5 ;minutestomeltcompletelythesampleat5GPa.F0r ;comparison,thecoolingrateofabout200K/swasused ;f0rthebothsamplessolidifiedunderhighandambient ;pressure.Themicrostructuresofthesampleswere ;observedbyopticalmicroscope,scanningelectron ;microscope(SEM).

    ;3.Experimentalresultsanddiscussion

    ;Fig.1MicrostructuresofAI21at.

    ;%Sialloysolidified

    ;undernormal(a)andhighpressure(5GPa)(b)

;ItcanbefoundfromFig.

    ;1(b)thatthe

    1323at.%Sialloy.different ;rmcrostructureoftheA1

    ;fromthatsolidifiedunderambientpressurewhere ;primarynubblySiisobservedasshowninFig. ;1(a),

    ;iscomposedof(eutecticandflnumberofAJ

    ;dendritesforwitchthecrystalstructuresare ;determinedbyXraydiffractionwhenthealloyis ;solidifiedat5GPa.Itsuggeststhatoe-A1dendritic ;phaseisprimaryinhypereutecticA1Sialloysolidified

    ;atpressure.Theeutecticpointsignificantlyshifts ;towardthefl-SisideinA1Sialloyphasediagram.

    ;Fig.2SEMmorphologiesofeutecticphasein ;Al-13at.%Sialloysolidified(a)undernormal ;and(b)highpressure(5GPa)

    ;TheirregulareutecticmicrostructureinA1Si

    ;alloyssolidifiedunderambientpressurecanbefound ;duetohighcoolingrate.Thelamellarstructuresof ;(eutecticswasgenerallyformedintheeutectic ;AlSialloyssolidifiedunderambientpressure. ;The

    ;typicalsiliconstructureisobservedintheeutectics ;asshowninFig.2(a).Duringtheeutecticgrowth.the ;fl-siliconlamellaetendtogrowinaspecificdirection ;onlybecauseofthefacetedgrowingfeature,sothatthe ;unalignedneighbouringlamellaewilleitherdivergeor ;converge.Duetothedifficultyinchangingthe ;growthdirectionofSi,branchingandterminationof ;45

    ;EutecticstructureofAl-Sialloysolidifiedunderhighpressure ;growthoccursinthesolidificationsothattheredoes ;notexistauniquelamellarspacing.Consequentlythe ;growthfrontofA1-Sialloysofeutecticcomposition ;hasbeenshowntobeveryjagged[331andtheeutectic ;cellsaredifficulttobeobservedbecauseoftheir ;complicatedmorphologies.Recentlytherelationship ;betweenthelamellarspacingandsolidification ;parametersofirregulareutectichasbeeninvestigated ;theoreticallvandexperimentally33.

    ;Fig.3SEMmorphologiesofeutecticphaseinAI21

    ;at.%Sialloysolidifiedunderhighpressure(5GPa) ;Themicrostructuresofeutecticaregreatly ;changedbythepressureeffectwhenthealloysare

;effectsonthegrowthmechanismofeutecticinAISi

    ;alloys.

    ;Solidificationunderahigherpressurecanleadthe ;nucleationrateofeachphaseofA1Sialloytoahigher

    ;level[30.

    ;Simultaneouslythediffusivityofatoms

    ;decreaseswithanincreaseofpressure,itresultsinthe ;increaseoftheactivationenergyfordiffusionofatoms ;andalsotheincreaseoftheactivationenergyfor ;growthofcrystals,sohighpressuremakesthegrowth ;rateofcrystalsreduce.Sothestructuresofalloy ;solidifiedunderhighpressurecouldberefined.Butin ;thispaper,themorphologiesoftheeutecticobserved ;bySEMshowsthatitisnotrefinedasshowninFig.2. ;TheeutecticSiphaseisfoundtobecoarsenedby ;pressure,whichalsosuggeststhatthegrowth ;mechanismofeutecticunderhighpressurehasbecome ;differentfromthatundernormalpressure. ;Itcanbeseenthatunderhighpressuretheeutectic ;Silamella,however,wererefinedwithincreasingthe ;siliconcontent,asshowninFig.2(b)and3.The ;eutecticSiphaseofA113at.%Sialloysolidified

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