GRAIN BOUNDARY NON-EQUILIBRIUM SEGREGATION IN STEELS

Abstract
A model quantifying the kinetics and magnitude of non-equilibrium and equilibrium grain boundary segregation in metallic solids is described. The major input parameters for the model are : vacancy-impurity binding energies ; diffusion data for impurities and complexed impurity-vacancy pairs ; vacancy formation energies ; and grain size. The principal thermal treatment parameters controlling non-equilibrium segregation are shown to be solution treatment temperature and the cooling rate employed to reduce the material to room temperature. The values of these parameters leading to maximum searegation differ greatly depending upon which impurity element in the matrix is being considerid. Examples of the extent of silicon and phosphorus grain boundary segregation expected in steels as a function of austenitising temperature, cooling rate and subsequent tempering will be shown. Experimental measurements of grain boundary segregation of phosphorus in a microalloyed steel will be described. These measurements are made with the high spatial resolution microanalytical method known as field emission gun scanning transmisdion electron microscopy (FEGSTEM). Supplementary data concerning the mechanical properties of ferritic/martensitic steels will be presented. These data provide indirect evidence of grain boundary segregation influencing mechanical properties such as impact fracture energy. Recommendations are made about how to alleviate non-equilibrium and equilibrium grain boundary segregation in steels.