GRAI N-BOUNDARY DIFFUSION

Abstract
The more useful experimental techniques for determining grain-boundary diffusion are briefly described followed by a presentation of results that shed light on the models and mechanisms of grain-boundary and dislocation diffusion. Studies of the following grain-boundary diffusion phenomena will be considered ; anisotropy in grain-boundary diffusion, effect of orientation relationship on grain-boundary diffusion, effect of boundary type and dislocation dissociation, lattice structure, correlation and isotope effects, and effect of pressure on grain-boundary diffusion. A "pipe" mechanism of diffusion based on the well-established dislocation model seems most appropriate for small-angle boundaries. Open channels, which have atomic configurations somewhat like dislocation cores, probably play a major role in large-angle grain-boundary diffusion. Dissociated dislocations and stacking faults are not efficient paths for grain-boundary diffusion. Twist boundaries (screw dislocations) are less effective diffusion paths than tilt boundaries (edge dislocations). The isotope effect and pressure experiments and the computer modeling are consistent with a vacancy mechanism of diffusion by a rather well localized vacancy. The atomic migration is a three-dimensional process with jumps away from and back towards the dislocation as well as jumps along the dislocation core. Both direct observations and kinetic experiments suggest that the effective width of a boundary for grain-boundary diffusion is about two atomic planes. These results are discussed in detail in References 1 and 2.