材料科学基础英文版课件_(13)

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1、Chapter6DiffusioninSolidsDiffusion-IntroductionAphenomenonofmaterialtransportbyatomicmigrationThemasstransferinmacroscopiclevelisimplementedbythemotionofatomsinmicroscopiclevelSelf-diffusionandinterdiffusion(orimpuritydiffusion)Topics:mechanismsofdiffusion,mathematicsofdiffusion,effectsoftemperature

2、anddiffusingspeciesontherateofdiffusion,anddiffusionofvacancy-solutecomplexesDemonstrationofdiffusionBeforeheattreatmentAfterheattreatmentDiffusion–Mechanisms(1)VacancydiffusionInterstitialdiffusionTwomechanisms:Diffusion–Mechanisms(2)VacancydiffusionInsubstitutionalsolidsolutions,thediffusion(boths

3、elf-diffusionandinterdiffusion)mustinvolvevacanciesForself-diffusion,theactivationenergyisvacancyformationenergy+vacancymigrationenergy.InterstitialdiffusionIninterstitialsolidsolutions,thediffusionofinterstitialsoluteatomsisthemigrationoftheatomsfrominterstitialsitetointerstitialsiteDiffusion–Mecha

4、nisms(3)PositionofinterstitialatomafterdiffusionTheactivationenergyisthemigrationenergyoftheinterstitialatom.MathematicsofDiffusion(1)Steady-statediffusionTime-dependentprocess,therateofmasstransferisexpressedasadiffusionflux(J)Masstransferredthroughacross-sectionalareaDiffusiontimeAreaacrosswhichth

5、ediffusionoccursIndifferentialformJ=Masstransferredthroughaunitareaperunittime(g/m2s))MathematicsofDiffusion(2)Concentrationprofiledoesnotchangewithtime–steady-statediffusionconcentrationgradient=dC/dxdC/dxJe.g.thediffusionofatomsofagasthroughametalplateMathematicsofDiffusion(3)whereDisthediffusionc

6、oefficient(m2/s),showingtherateofdiffusionForthiscase,theunitofCisinmassperunitvolume,e.g.g/m3MinussignindicatesthediffusionisdowntheconcentrationgradientFick’sfirstlawThemathematicsofsteady-statediffusioninonedimensionisgivenbyForsteady-statediffusion,thediffusionfluxisproportionaltotheconcentratio

7、ngradientNegativeMathematicsofDiffusion(4)Nonsteady-statediffusionThediffusionfluxataparticularpointvarieswithtime.MathematicsofDiffusion(5)ThediffusionequationisrepresentedbyFick’ssecondlawCisafuncti