02-Introduction-to-Materials-Science-and-Engineeri

材料科学与工程专业英语IntroductiontoMaterialsScienceandEngineering2020/5/20合肥工业大学材料学院2MicrostructureWhendescribingthestructureofamaterial,wemakeacleardistinctionbetweenitscrystalstructureanditsmicrostructure.Theterm“crystalstructure”isusedtodescribetheaveragepositionsofatomswithintheunitcell(晶胞),andiscompletelyspecifiedbythelatticetype（点阵类型）andthefractionalcoordinates（相对坐标）oftheatoms.Inotherwords,thecrystalstructuredescribestheappearanceofthematerialonanatomiclengthscale.Theterm“microstructure”isusedtodescribetheappearanceofthematerialonthenm-cmlengthscale.Areasonableworkingdefinitionofmicrostructureis“thearrangementofphases（相）anddefects（缺陷）withinamaterial.2020/5/20合肥工业大学材料学院3MicrostructureManytimes,thephysicalpropertiesand,inparticular,themechanicalbehaviorofamaterialdependonthemicrostructure.Microstructureissubjecttodirectmicroscopicobservation,usingopticalorelectronmicroscopes.Inmanyalloys,microstructureischaracterizedbythenumberofphasespresent,theirproportions,andthemannerinwhichtheyaredistributedorarranged.Themicrostructureofanalloydependsonsuchvariablesasthealloyingelementspresent,theirconcentration,andtheheattreatmentofthealloy.2020/5/20合肥工业大学材料学院4Microstructure1.3.6.1PhaseDiagrams（相图）Muchoftheinformationaboutthecontrolofmicrostructureorphasestructureofaparticularalloysystemisconvenientlyandconciselydisplayedinwhatiscalledaphasediagram,alsooftentermedanequilibriumorconstitutionaldiagram.Manymicrostructuresdevelopfromphasetransformation,thechangesthatoccurbetweenphaseswhenthetemperatureisaltered(ordinarilyuponcooling).Thismayinvolvethetransitionfromonephasetoanother,ortheappearanceordisappearanceofaphase.Phasediagramsarehelpfulinpredictingphasetransformationsandtheresultingmicrostructures,whichmayhaveequilibriumornonequilibriumcharacter.2020/5/20合肥工业大学材料学院5MicrostructureTheunderstandingofphasediagramsforalloysystemsisextremelyimportantbecausethereisastrongcorrelationbetweenmicrostructureandmechanicalproperties,andthedevelopmentofmicrostructureofanalloyisrelatedtothecharacteristicsofitsphasediagram.Inaddition,phasediagramsprovidevaluableinformationaboutmelting（熔炼）,casting（铸造）,crystallization（结晶）,andotherphenomena.2020/5/20合肥工业大学材料学院6MicrostructureBinaryIsomorphousSystems（二元匀晶）Theeasiesttypeofbinaryphasediagramtounderstandandinterpretisthatwhichischaracterizedbythecopper-nickelsystem(Figure1.19).Threedifferentphaseregions,orfields,appearonthediagram,analpha(a)field,aliquid(L)field,andatwo-phasea+Lfield.TheliquidLisahomogeneousliquidsolutioncomposedofbothcopperandnickel.Theaphaseisasubstitutionalsolidsolution(置换型固溶体)consistingofbothCuandNiatoms,andhavinganFCCcrystalstructure.Attemperaturesbelowabout1080℃,copperandnickelaremutuallysolubleineachotherinthesolidstateforallcompositions.Thecopper-nickelsystemistermedisomorphous（同行的）becauseofthiscompleteliquidandsolidsolubilityofthetwocomponents.2020/5/20合肥工业大学材料学院7MicrostructureFigure1.19Thecopper-nickelphasediagram2020/5/20合肥工业大学材料学院8MicrostructureForabinarysystemofknowncompositionandtemperaturethatisatequilibrium,atleastthreekindsofinformationareavailable:(1)thephasesthatarepresent.(2)thecompositionsofthesephases.(3)thepercentagesorfractionsofthephases.Theproceduresformakingthesedeterminationswillbedemonstratedusingthecopper-nickelsystem.2020/5/20合肥工业大学材料学院9MicrostructurePhasesPresentTheestablishmentofwhatphasesarepresentisrelativelysimple.Onejustlocatesthetemperature-compositionpointonthediagramandnotesthephase(s)withwhichthecorrespondingphasefieldislabeled.Forexample,analloyofcomposition(WNi=60%;WCu=40%)at1100℃wouldbelocatedatpointA(Figure1.19).Sincethisiswithintheαregion,onlythesingleaphasewillbepresent.2020/5/20合肥工业大学材料学院10MicrostructureCompositionofEachPhaseThefirststepinthedeterminationofphasecompositionsistolocatethetemperature-compositionpointonthephasediagram.Differentmethodsareusedforsingle-andtwo-phaseregions.Ifonlyonephaseispresent,theprocedureistrivial:thecompositionofthisphaseissimplythesameastheoverallcompositionofthealloy.Foranalloyhavingcompositionandtemperaturelocatedinatwo-phaseregion,thesituationismorecomplicated.Inalltwo-phaseregions(andintwo-phaseregionsonly),onemayimagineaseriesofhorizontallines,oneateverytemperature;eachoftheseisknownasatieline（连接线）,orsometimesasanisotherm.2020/5/20合肥工业大学材料学院11MicrostructureThesetielinesextendacrossthetwo-phaseregionandterminateatthephaseboundarylinesoneitherside.Tocomputetheequilibriumconcentrationsofthetwophases,thefollowingprocedureisused:(1)Atieline（连接线）isconstructedacrossthetwo-phaseregionatthetemperatureofthealloy.(2)Theintersections(交叉点)ofthetielineandthephaseboundariesoneithersidearenoted.(3)Perpendiculars（垂直的）aredroppedfromtheseintersectionstothehorizontalcompositionaxis,fromwhichthecompositionofeachoftherespectivephasesisread.2020/5/20合肥工业大学材料学院12MicrostructureAmountofEachPhase(TheLeverLaw)杠杆定律Insingle-phaseregions,theamountofthesinglephaseis100%.However,intwo-phaseregionswemustcalculatetheamountofeachphase.Ifthecompositionandtemperaturepositionislocatedwithinatwo-phaseregion,thetielinemustbeutilize