A theoretical model was developed for the temporal and temperature dependences of the occupancies of states in the grain-boundary diffusion zone of a monatomic polycrystal. It was noted that the complex temperature dependence of the occupancies could be described only by extending the traditional model for diffusion fluxes. The Goldshtein-Fisher model for intercrystalline diffusion was modified by introducing a so-called pumping zone in which the diffusion coefficient was much larger than that for bulk diffusion. It was shown that, within a certain temperature range, the sole reason for variations in the occupancies was the temperature dependence of the segregation coefficients of atomic probes in the grain boundaries. The activation enthalpy of segregation, and the relative width of the pumping zone, could be determined within the temperature range of diffusion annealing. The pumping coefficients could be deduced from the high-temperature part of the Arrhenius plot of occupancy versus temperature.

Intercrystalline Diffusion of Co in Polycrystalline Tungsten I: Theory of Diffusion of Substitutional Atomic Probes in the Core of Crystallite-Conjugation Regions and Adjacent Zones. S.M.Klotsman, M.I.Kurkin, V.N.Kaigorodov, V.V.Dyakin: Physics of Metals and Metallography, 1998, 85[2], 135-9