2材料科学基础英文版课件_（十三）.pptVIP

Chapter 6Diffusion 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 Self-diffusion 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 vacancy-solute complexes Demonstration of diffusion Diffusion – Mechanisms (1) Vacancy diffusion Interstitial diffusion Diffusion – Mechanisms (2) Vacancy diffusion In substitutional solid solutions, the diffusion (both self-diffusion and interdiffusion) must involve vacancies Diffusion – Mechanisms (3) Interstitial diffusion In interstitial solid solutions, the diffusion of interstitial solute atoms is the migration of the atoms from interstitial site to interstitial site Mathematics of Diffusion (1) Steady-state diffusion Time-dependent process, the rate of mass transfer is expressed as a diffusion flux (J) Mathematics of Diffusion (2) Concentration profile does not change with time – steady-state diffusion Mathematics of Diffusion (3) Mathematics of Diffusion (4) Nonsteady-state diffusion The diffusion flux at a particular point varies with time. Mathematics of Diffusion (5) The diffusion equation is represented by Mathematics of Diffusion (6) Solutions of diffusion equation Mathematics of Diffusion (7) For t0, Cx=Cs at x=0 Cx=Co at x=?? Therefore Mathematics of Diffusion (8) For t=0, Cx=C1 at x0 Cx=C2 at x0 Therefore Mathematics of Diffusion (9) Mathematics of Diffusion (10) Mathematics of Diffusion (11) Factors Affecting Diffusion (1) Factors Affecting Diffusion (2) Factors Affecting Diffusion (3) Example Problem - 1 Example Problem - 2 Example Problem – 2 Other Diffusion Paths “Short-circuit” diffusion paths: dislocations, grain boundaries, external surfaces, Along these paths, the diffusion is much faster