The diffusion of 63Ni in high-purity polycrystalline material was studied, at temperatures ranging from 973 to 1258K (table 89), by using radio-tracer and serial sectioning techniques. Because the grain size could not be completely stabilized by pre-diffusion heat treatments, parts of the grain boundaries could migrate during diffusion annealing. The tracer penetration profiles were treated by using a model for diffusion in polycrystals which contained both stationary and mobile grain boundaries. As well as the grain boundary diffusivity, the average velocity of moving grain boundaries and the activation energy for boundary migration were also deduced from the effect of boundary motion upon the profile shape. It was found that the grain boundary diffusion of Ni was anomalously fast; with an activation energy of 155.2kJ/mol. This behavior was consistent with that of Co in Nb, and implied that the diffusion of both elements in Nb was governed by an atomic mechanism which involved both vacancies and interstitials.

I.M.Razumovski, J.Mishin, C.Herzig: Materials Science and Engineering A, 1996, 212[1], 45-50