材料科学基础英文版课件_(12)

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1、Chapter5ImperfectionsinSolidsLecturer:ShenhuaSONGOutlinePointdefectsPointdefectsinmetalsPointdefectsinceramicsLineardefectsDislocationsPlanedefectsExternalsurfacesGrainboundariesTwinboundariesStackingfaultsPhaseboundariesVolumedefectsPores,cracks,inclusions,andsecondphasesPointDefects–PointDe

2、fectsinMetals(1)1.VacanciesandInterstitials(self-interstitials)ThermalequilibriumvacanciesandinterstitialsQuenching-inducednon-equilibriumvacanciesandinterstitialsIrradiation-inducednon-equilibriumvacanciesandinterrstitialsDeformation-inducednon-equilibriumvacanciesFrenkelpair:vacancy+interst

3、itialSchottkydefect:movinganatomtothesurfaceproducesavacancyPointDefects–PointDefectsinMetals(2)Formation–leadingtoanincreaseofinternalenergy–freeenergyincrease(G=H-TS,H=U+PV,U-H-G)Formation–leadingtoanincreaseofentropy(randomness)–freeenergydecreaseAtagiventemperature,theremustbeaconce

4、ntrationofpointdefectsatwhichthefreeenergyofthesystemhasaminimumvalue.Whythereisathermalequilibriumconcentrationforvacanciesorinterstitials(self-interstitials)PointDefects–PointDefectsinMetals(3)DerivationofequilibriumvacancyconcentrationwhereHvistheincreaseinenthalpypermoleofvacanciesintrod

5、uced(thevacancyformationenergy)TheincreaseinenthalpyforintroducingXvmoleofvacanciesinonemoleofmaterial(Xv:themolefractionofvacancies):Theincreaseinentropy:whereSvistheincreaseinthermalentropypermoleofvacanciesintroducedTheincreaseinthefreeenergyofthesystemduetothevacancyformation:G=H-TSPo

6、intDefects–PointDefectsinMetals(4)ThemolarfreeenergyofthecrystalcontainingXvmoleofvacancies:whereGAisthemolarfreeenergyofthevacancy-freecrystalTheequilibriumconcentrationofvacanciescanbecalculatedbythecondition:PointDefects–PointDefectsinMetals(5)Or(G=H-TS)isinunitsofthemolefractionandcana

7、lsoberepresentedby=Nv/NwhereNvisthenumberofvacanciesatequilibriumforacertainamountofmaterialandNisthetotalnumberofatomicsitesPointDefects–PointDefectsinMetals(6)~3formostmetalse.g.iron(Fe):Hvis~1.4eVperatom,themeltingpointsis1808K(1535oC),th