Role of entropy in anistropy of grain boundary segregation

Abstract

From the viewpoint of existence of linear dependence between entropy and enthalpy of grain boundary segregation, the meaning of the entropy term is discussed. It is suggested that negative values of segregation entropy for silicon segregation at individual grain boundaries in α-iron reflect an ‘ordering’ tendency of the interfaces due to the presence of silicon. On the other hand, positive values of segregation entropy suggest a ‘disordering’ of the structure of these grain boundaries by segregation of carbon and mainly of phosphorus. The effect of ‘disordering’ is especially high for special grain boundaries with compact intrinsic structure, resulting in high values of segregation entropy. The slope of the dependence of enthalpy on entropy of solute segregation at different grain boundaries can be considered as a compensation temperature. Its existence implies that an anisotropy of grain boundary chemical composition will possess a qualitatively different character above and under this temperature. This anisotropy measured above the compensation temperature will be characterized by high values of grain boundary concentration for special boundaries as compared to general boundaries. © 1998 John Wiley & Sons, Ltd.