Theory of Diffusion under Pressure

摘要

Several years ago, we suggested a new approach to solve the problem of the influence of elastic stress on the vacancy jump rate for atomic diffusion in crystals. It was based on the simple observation that a stress field alters the surrounding configuration and on the assumption that the height of the activation barrier should be altered accordingly. The change of activation barrier was shown to depend on the displacement field, the symmetry of the crystal, the atomic structure near the point defects and the interatomic potential. The knowledgeof this change makes possible to calculate the jump rate. The expression for the vacancy flux was obtained with the help of the "hole gas" method, by using jump rate. This method was later extended to binary systems for calculating the influence of stresses upon the diffusion fluxes, the expression of which were obtained in the approximation of regular solid solutions. This approach gives the possibility to investigate, among others, the influence of a uniform pressure on diffusion. The main characteristics of its effect on the diffusion process is the existence of an activation volume. In the case of pure metals and binary alloys the expressions for migration and formation volumes were obtained and the calculation for some BCC and FCC metals were made in the rigid lattice approximation. We were also able to calculate the temperature dependence of the migration volume, a result which had not been obtained previously (as for as we know). We are now building a general approach to calculate the activation volume for different mechanisms in solids.