SOLIDIFYINGSTRUCTURESOFAl_2O_3SHORTFIBER/Al- SiALLOYMATRIXCOMPOSITEPREPAREDBYSQUEEZECAST ING¥Zhang,Siqi;Huang,Jinsong;Shen,Jian;Lu,B in;Xu,Guofu(DepartmentofMaterialsScienceand Engineering,CentralSouthUniversityofTechnol ogy,Changsha410083)Abstract:Al2O3shortfiber /Al-Sialloymatrixcompositewaspreparedbysque ezecasting.Solidifyingstructuresofthecompos itewereinvestigated.Resultsshowedthatprimar ysiliconandeutecticsilicongrainsinthecompos iteareobviouslyrefined.Keywords:composite,s hortfiber,Al-Sialloy,solidifying,structures ,squeeze,casting1INTRODUCTIONThesqueezecast ingisanimportantmethodtoproducemetalmatrixc omposite.Toinvestigatethesolidifyingcharact eristicofthiscompositeisofgreatimportanceto thecontrolofitssolidifyingprocessandstructu res,andtotheimprovementofitsmechanicalprope rties.Mortensenetal[1]haveinvestigatedtheso lidifyingprocessofSiCandAl2O3fiber/Al-Cuall oysmatrixcomposites.Clyneetal[2,3]haveinves tigatedtheeffectofδAl2O3fiberonthesolidifyi ngstructuresofaluminiumalloymatrixcomposite andfoundsiliconprecipitateonthesurfaceofδ-A l2O3fibersinthecomposite.Themainpurposeofth isworkistoexaminethereasonsofsilicongrainre finementinα-Al2O3shortfiber/Al-Sialloymatri xcomposite.2EXPERIMENTALAl2O3shortfiberused inthisworkmainlyconsistsofα-Al2O3containing 15%SiO2.TheratioL/D(Length/Diameter)ofthefi beris7to15.ThematrixisanAl-11Si-0.8Cu-0.7Mg -0.9Nialloy.Theshortfibersweremadeintoprefo rms,thenAl2O3/Al-Sialloymatrixcompositewasp reparedbysqueezecasting.Thesqueezcastingpro cessisschematicallyshowninFig.1.Theshortfib ervolumefractionVfis24%inthecomposite.Fiber distribution,siliconphasesizeanddistributio ninthecompositewereobservedbyopticalmicrosc opeandSEM.Thehardnessofreinforcedfieldandun reinforcedfieldinthecompositewasmeasured.Fi g.1schematicdiagramofsqueezecastingprocess( a)─pouring;(b)─mouldassemblingandpressing;( c)─mouldunloading①ReceivedMarch24,1995;acce ptedMay17,19953RESULTSANDDISCUSSIONThemacro structuresofthecompositeareshowninFig.2,whi chindicatesthatthecombinationofthepreformwi ththematrixalloyisgood,buttheinterfacebetwe enreinforcedfieldandunreinforcedfieldisdist inct.ThehardnessofthecompositeisshowninTabl e1.Comparingthehardnessdata,itcanbefoundtha tthematrixalloyisobviouslyreinforcedbythefi ber.Table1HardnessofthecompositeSolidifying microstructuresareshowninFig.3andFig.4.Fig. 3showsthatsolidifyingstructuresofthematrixa lloyinthereinforcedfieldandtheunreinforcedf ieldmainlyconsistedofα-phase,primarysilicon andeutectic(α+Si),andprimarysiliconandeutec ticsilicongrainsareobviouslyrefinedintherei nforcedfield.Fig.4indicatesthatsiliconispre cipitatedonthesurfaceofthefibers,whichagree swiththereportedresultsinrefs.[2,3].Fig.4al soindicatesthatthesurfaceofsomefibersbecome srough,whichwouldbeproofofthereactionofAlan dMginthematrixwithSiO2inthefibers.Fromthevi ewpointofthermodynamics,thefollowingreactio nscanoccur:3SiO2+4Al=2Al2O3+3Si(1)Fig.2Macr ostructuresofthecompositeabove─unreinforced field;below─reinforcedfieldFig.3Opticalmicr ographsofthecomposite,×200(a)─reinforcedfie ld;(b)─interfacefield;(c)─unreinforcedfield Fig.4SEMmicrographofthecompositeSiO2+2Mg=2M gO+Si(2)Reducedsiliconisconservedonthesurfa ceofthefibersormovesfromthesurfaceofthefibe rstothematrixalloymeltwiththehelpofthemeltc onvection.Inthecasethatpouringtemperatureis suitable,thesiliconisnotresolvedornotallres olved.Reducedsiliconmayactasnucleiofsilicon crystals.Thisisoneoftheimportantreasonsforg rainrefinementoftheprimarysiliconandtheeute cticsiliconinthereinforcedfield.Someprimary silicongrainsandeutecticsilicongrainsareobs ervedaroundfibers(Fig.3(a)and(b)).Occurrenc eofthisphenomenonhasthreepossiblereasons.As above,thereducedsilicononthefibersurfaceact sasnucleioftheprimarysiliconandtheeutectics ilicon,andgrowsaroundthefibers.Siliconcryst alsmayimmediatelybeprecipitatedfromthematri xalloymeltandgrowuntiltheycontactwiththefib ers,i.e.thefibermaypreventsilicongrainsfrom growing.Inaddition,α-Al2O3structureisrhombohedral,itslatticeparamtersareα=5.13,α=55°17'[4].Siliconstructureisface-centeredcubic,itslatticeparameterisα=5.43.Thelatticemisfit
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