ATOMISTIC VS PHENOMENOLOGICAL APPROACHES TO GRAIN-BOUNDARY SEGREGATION - COMPUTER MODELING OF CU-AG ALLOYS

Experimental investigations of grain boundary segregation, for example the Auger electron spectroscopy (AES), as well as the associated thermodynamic analyses, are principally macroscopic. On the other hand, computer modeling of grain boundary segregation provides a very detailed picture of the atomic structures of segregated boundaries. In order to investigate the relationship between macroscopic and microscopic approaches we have performed molecular statics and Monte Carlo simulations of grain boundaries in Cu-Ag alloys in which Ag segregates strongly to grain boundaries. On the atomic level segregation is inhomogeneous, the segregated region extends several lattice spacings away from the boundary and the segregation process cannot be described in the framework of simple thermodynamic analyses. Energetic as well as entropic interactions between solutes play important role and structural transformations may be induced by segregation. However, on the macroscopic level the picture is much simpler. The average concentration of the segregant in the boundary region follows the McLean's isotherm but no obvious relationship between the ''effective'' segregation enthalpy and atomistically determined segregation enthalpies can be established. Furthermore, the Auger analysis assuming a homogeneous distribution of the segregant gives correct average concentrations but for the case when the segregant is confined to a single atomic layer. Hence, macroscopic analyses of the segregation are insensitive to complex atomic level processes and although they give correct average concentrations of the segregant, no conjectures regarding the atomic level processes associated with segregation can be made on the basis of such studies.