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1、材料科学基础FundamentalofMaterials Lessonthirty 6 8DiffusionMechanism Flick slawinmicroscopicformOnedimensionaldiffusion probabilityofatomjumpingtonearestposition jumpfrequencyperiod simplecubic Twodimensionaldiffusion Threedimensionaldiffusion FCC FordiffusionofinterstitialatomsinBCC Fortetrahedralinters
2、titialForoctahedralinterstitial DiffusionMechanismsDirectexchangeofneighboringatoms Atommustacquireadditionalenergy calledtheactivationenergy beforetheyjumpfromnormalstatetoactivatedstate thesaddlepoint Thismechanismseemstobeunrealisticforlarge Interstitialmechanismtobeeffectivefordiffusionininterst
3、itialsol sol Vacancymechanism Atomsjumpdirectlyintonearbyvacantsite beingsmall Differentkindofatomshasdifferent chemical affinityforvacancy CalculationofD P1 probabilityofatomacquiringadditional 6 9Empiricalrulefordiffusion FactorsinfluencingdiffusionTemperatureandthediffusioncoefficient D Theactiva
4、tionenergyfordiffusionofspeciesunderconsideration R ThegasconstantT TheabsolutetemperatureD0 Thepre exponentialterm CompositionSize Grainsize DefectsColdwordingorirradiationAcceleratediffusionUp hilldiffusionmaytakeplace Structure Dinclosedpackedstructuresissmallerthanin open structures Diffusionisa
5、nisotropyincrystalsotherthancubic Thirdelements Noeffect Acceleratebyformingfinegrainsorcausinglatticedistortion Retardbyformingcompound Curiouseffect Forself diffusion Example Designanironmembranethatwillallownomorethan1 ofthenitrogentobelostthroughthemembraneeachhour whileallowing90 ofthehydrogentopassthroughthemembraneperhour Thetotalnumberofnitrogenatomsinthecontaineris Then minimumthicknessofthemembrane maximumthickness Soironmembranebetween0 0128and0 0729cm ExamplesandDiscussions Exercise Thankyou 30
