CORRELATION-EFFECTS IN DISLOCATION PIPE DIFFUSION .1. THEORY

Impurity correlation factors for dislocation pipe diffusion are explored for two approximations of a simple dislocation model. The first uses the extended 'random-walk matrix' method to the approximation that the vacancy returns from a random direction if it leaves the third-nearest neighbour of the impurity. The second is an 'infinite-line' approximation in which all and only the sites in the dislocation pipe are included in calculation. Both approximations can be cast in the impurity form for correlation factors. Of the two approximations, the second appears more useful, since it may be used for all ratios of exchange frequencies for diffusion through a lattice and along dislocation pipes.