Investigation of Ni-63 diffusion along stationary and moving grain boundaries in Nb

Grain boundary diffusion of Ni-63 in high purity polycrystalline niobium has been studied in the temperature range 973-1258 K by the radiotracer serial sectioning technique. Since the grain size in the material could not be completely stabilized by pre-diffusion heat treatment, some part of the grain boundaries could migrate during the diffusion anneals. The tracer penetration profiles were treated using the model of diffusion in polycrystals containing both stationary and moving grain boundaries developed in our previous work. Along with the grain boundary diffusivity, also the average velocity of moving grain boundaries and the activation energy of boundary migration in Nb were determined from the effect of boundary motion on the profile shape. Grain boundary diffusion of Ni in Nb is anomalously fast and exhibits an activation energy of 155.2 kJ mol(-1). This behaviour is well consistent with that of Co diffusion in Nb studied previously and implies that diffusion of both elements in Nb is governed by a combined atomic mechanism involving both vacancies and interstitials.