How to compare superficial and intergranular segregation? A new analysis within the mixed SMA-TBIM approach

The driving forces for intergranular segregation are much less known than t he ones for super ficial segregation. In order to investigate the relation between grain-boundary and surface segregation, we have calculated the segr egation energy for one impurity in both dilute limits for the Cu-Ag system. This is performed for the (001) surface on one hand and for the symmetrica l tilt grain boundary Sigma = 5 (210) [001] on the other. These numerical r esults, obtained using N-body interatomic potentials derived from the secon d moment approximation (SMA) of the tight-binding scheme, are then discusse d within an analytic model, the tight-binding Ising model (TBIM). This allo ws us to determine the relative role of the different driving forces in sup erficial and intergranular segregation. In particular competition or synerg y between site and size effects strongly differ between surface and grain-b oundary segregation. This study illustrates the efficiency of the mixed SMA -TBIM approach to predict and to analyse interfacial segregation. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. Ali rights reserv ed.