材料科学导论(英文课件)--青岛科技大学diffusion【荐】.pdf

Chapter 6 Diffusion Diffusion - how do atoms move through solids? Chapter Outline Diffusion mechanisms Vacancy diffusion Interstitial diffusion Impurities The mathematics of diffusion Steady-state diffusion (Fick’s first law) Nonsteady-State Diffusion (Fick’s second law) Chapter Outline Factors that influence diffusion Diffusing species Host solid Temperature Microstructure What is diffusion? Diffusion is material transport by atomic motion. Inhomogeneous materials can become homogeneous by diffusion. For an active diffusion to occur, the temperature should be high enough to overcome energy barriers to atomic motion. Diffusion for Cu-Ni alloying Interdiffusion (or impurity diffusion) occurs in response to a concentration gradient. Self-diffusion is diffusion in one-component material, when all atoms that exchange positions are of the same type. To jump from lattice site to lattice site, atoms need energy to break bonds with neighbors, and to cause the necessary lattice distortions during jump. This energy comes from the thermal energy of atomic vibrations (Eav ~ kT). Materials flow (the atom) is opposite the vacancy flow direction. Interstitial diffusion is generally faster than vacancy diffusion because bonding of interstitials to the surrounding atoms is normally weaker and there are many more interstitial sites than vacancy sites to jump to. Requires small impurity atoms (e.g. C, H, O) to fit into interstices in host. Diffusion Flux The flux of diffusing atoms, J, is used to quantify how fast diffusion occurs. The flux is defined as either in number of atoms di