材料科學(xué)基礎(chǔ)英文版課件_(13)(已修改)
2025-01-30 20:28 本頁(yè)面
【正文】 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 time (g/m2 s)) Mathematics of Diffusion (2) Concentration profile does not change with time – steadystate diffusion concentration gradient = dC/dx dC/dx J . the diffusion of atoms of a gas through a metal plate Mathematics of Diffusion (3) xCDJdd??where D is the diffusion coefficient (m2/s), showing the rate of diffusion For this
教學(xué)課件相關(guān)推薦
鄂ICP備17016276號(hào)-1