On equilibrium and stationary coefficients of grain boundary segregation

Grain boundary (GB) segregation is usually characterized by two different coefficients: either b, which is used in the isotherm of equilibrium adsorption (Langmuir, McLean etc.) or s, which can be determined as a ratio of grain boundary and bulk concentration in the immediate vicinity of grain boundary. It can be obtained in diffusion experiments, carried out in so-called B and C regimes. It is well known that GB segregation strongly influences the GB diffusion. It is generally believed that this influence is determined by two factors: bulk diffusion rate (fast bulk diffuser is also fast in the GB) and the segregation itself, which reduces the GB diffusion rate due to the fact that segregating impurity atoms occupy energetically favoured places in GB. Both factors were mentioned earlier many times. In spite of it their relative function is not clearly understood yet. In this paper the difference between segregation coefficients b and s and dependence of s-coefficient on GB and bulk diffusion parameters will be discussed on the base of experimental data and transition state theory.