材料科學(xué)基礎(chǔ)英文版課件_(13)-wenkub
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【正文】 osoDtxCCCC x ????Solution Since both cases reach the same carbon concentration at the same temperature, the left hand side of equation is the same. Therefore 221122 DtxDtx ?h4010522121222 ???? txxtExample Problem 2 Example Problem – 2 Solution )2(e r f2 )(2 )( 2121 DtxCCCCC x ????C1 = 5 wt%。Chapter 6 Diffusion in Solids Diffusion Introduction ? A phenomenon of material transport by atomic migration The mass transfer in macroscopic level is implemented by the motion of atoms in microscopic level ? Selfdiffusion and interdiffusion (or impurity diffusion) ? Topics: mechanisms of diffusion, mathematics of diffusion, effects of temperature and diffusing species on the rate of diffusion, and diffusion of vacancysolute plexes Demonstration of diffusion Before heat treatment After heat treatment Diffusion – Mechanisms (1) ? Vacancy diffusion ? Interstitial diffusion Two mechanisms: Diffusion – Mechanisms (2) ? Vacancy diffusion In substitutional solid solutions, the diffusion (both selfdiffusion and interdiffusion) must involve vacancies For selfdiffusion, the activation energy is vacancy formation energy + vacancy migration energy. ? Interstitial diffusion In interstitial solid solutions, the diffusion of interstitial solute atoms is the migration of the atoms from interstitial site to interstitial site Diffusion – Mechanisms (3) Position of interstitial atom after diffusion The activation energy is the migration energy of the interstitial atom. Mathematics of Diffusion (1) ? Steadystate diffusion – Timedependent process, the rate of mass transfer is expressed as a diffusion flux (J) AtMJ ?Mass transferred through a crosssectional area Diffusion time Area across which the diffusion occurs In differential form tMAJdd1?J = Mass transferred through a unit area per unit tim
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