Intercrystalline diffusion of cobalt in polycrystalline tungsten : I. Theory of diffusion of substitutional atomic probes in the core of crystallite-conjugation regions and adjacent zones

A theoretical model is proposed for time and temperature evolution of the occupancies q of states in the zone of intercrystalline (grain-boundary) diffusion of a monatomic polycrystal. The complex temperature dependence of occupancies q can be described only if we go beyond the traditional model of diffusion fluxes in the zone of intercrystalline diffusion. The Gol'dshtein-Fisher model of intercrystalline diffusion is modified by introducing a pumping zone where the dilfusion coefficient is much larger than that of bulk diffusion. It is shown that, in a certain temperature range, the only reason for variation of the occupancies q is the temperature dependence of the coefficients of segregation of atomic probes for the grain boundaries. In this temperature range of diffusion annealings, we can determine the enthalpy of activation for segregation and the relative width of the pumping zone. The pumping coefficients can be determined from the high-temperature part of the Arrhenius plot of 1nq vs. 1/T. The experimental conditions nccessary for observation of the time and temperature evolution of the occupancies of states q, considered in this paper in the zone of intercrystalline diffusion in monatomic polycrystal are discussed.