“Chemical” phase transition in alloys: Ordering–phase separation

It has been apparent in recent experimental studies that in the same alloy, there can be both the tendency toward ordering and the tendency toward phase separation; however, these tendencies each take place in the different temperature ranges. The transition from one tendency to another may be considered as a “chemical” phase transition “ordering–phase separation”, which changes the sign of chemical interactions between atoms of components and is the reason for two structural “order–disorder” phase transitions: “ordering–solid solution” and “solid solution–phase separation”. The solid solution regions in the accepted phase diagrams are considered as the regions, in which the tendency toward phase separation takes place, which at certain condition may result in the formation of the phase separation microstructure.