ROLE OF ENTROPY IN ANISOTROPY OF GRAIN-BOUNDARY SEGREGATION

From the viewpoint of existence of linear dependence between entropy a nd 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 alp ha-iron reflect an 'ordering' tendency of the interfaces due to the pr esence of silicon. On the other hand, positive values of segregation e ntropy suggest a 'disordering' of the structure of these grain boundar ies by segregation of carbon and mainly of phosphorus, The effect of ' disordering' is especially high for special grain boundaries with comp act intrinsic structure, resulting in high values of segregation entro py. The slope of the dependence of enthalpy on entropy of solute segre gation at different grain boundaries can be considered as a compensati on temperature. Its existence implies that an anisotropy of grain boun dary chemical composition will possess a qualitatively different chara cter above and under this temperature, This anisotropy measured above the compensation temperature will be characterized by high values of g rain boundary concentration for special boundaries as compared to gene ral boundaries. (C) 1998 John Wiley & Sons, Ltd.