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1、Indealingwithmicrostructures,itissometimesconvenienttousethetermmicroconstituent（微观组织）,thatis,anelementofthemicrostructurehavinganidentifiableandcharacteristicstructure.Forexample,inthepointminset,Figure3.14,therearetwomicroconstituents,namely,primaryαandeutecticstr
2、ucture.Thustheeutecticstructureisamicroconstituenteventhoughitisamixtureoftwophases,becauseithasadistinctlamellarstructure,withafixedratioofthetwophases.Itispossibletocomputetherelativeamountsofbotheutecticandprimaryαmicroconstituents.Sincetheeutecticmicroconstituen
3、talwaysformsfromtheliquidhavingtheeutecticcomposition,thismicroconstituentmaybeassumedtohavecompositionof61.9wt%Sn.Hence,theleverruleisappliedusingatielinebetweentheα-(α+β)phaseboundary(18.3wt%Sn)andtheeutecticcomposition.Forexample,considerthealloyofcompositionC4’i
4、nFigure3.16.ThefractionoftheeutecticmicroconstituentWeisjustthesameasthefractionofliquidWLfromwhichittransforms,or(3.10) Furthermore,thefractionofprimaryα,Wα’,isjustthefractionoftheαphasethatexistedpriortotheeutectictransformation;or,forFigure3.16,Figure3.16Thelead-
5、tinphasediagramusedincomputationsforarelativeamountsofprimaryαandeutecticmicroconstituentsforanalloyofcompositionC4’(3.11)Thefractionsoftotalα,Wα(botheutecticandprimary),andalsooftotalβ,Wβ,aredeterminedbyuseoftheleverruleandatielinethatextendsentirelyacrosstheα+βpha
6、sefield.Again,foranalloyhavingcompositionC4’,(3.12)and(3.13)Analogoustransformationsandmicrostructuresresultforalloyshavingcompositionstotherightoftheeutectic(i.e.,between61.9and97.8wt%Sn).However,belowtheeutectictemperature,themicrostructurewillconsistoftheeutectic
7、andprimaryβmicroconstituentsbecause,uponcoolingfromtheliquid,wepassthroughtheβ+liquidphasefield.When,forcase4(representedinFigure3.14),conditionsofequilibriumarenotmaintainedwhilepassingthroughtheα(orβ)+liquidphaseregion,thefollowingconsequenceswillberealizedforthem
8、icrostructureuponcrossingtheeutecticisotherm:(1)grainsoftheprimarymicroconstituentwillbecored,thatis,haveanonuniformdistributionofsoluteac
