2. Intergranular diffusion mechanisms

Jumping atoms in a joint generally involve point defects, vacancies and interstitials, present at equilibrium at any temperature other than 0 K. To describe the elementary mechanisms of diffusion at joints, it is necessary to examine the possible modes of exchange between atoms and these defects, and then to determine which is the predominant mode, and its kinetics, for a given material in a defined temperature range. This is the subject of this chapter, in which we attempt to identify operational mechanisms, mainly in the case of self-diffusion, better known than hetero-diffusion, depending on the type of material: metallic, ionic or covalent. We will then see that these mechanisms can also differ from one grain boundary to another and, for the same boundary, according to the direction of propagation of the diffusing element.

Three mechanisms allow a net flow of...