Decomposition and ordering processes involving thermodynamically first-order order → disorder transformations

Clustering and ordering processes often occur concomitantly or synergistically during the decomposition of supersaturated solid solutions. However, the occurrence of spinodal decomposition in conjunction with an order → disorder transformation thermodynamically of first order under equilibrium conditions is generally not well understood. In this paper a simple graphical approach using free energy-composition diagrams and loci of thermodynamic instability delineating regions of continuous transformation is used to develop a framework for understanding the interplay between clustering and ordering occurring in metallic solid solutions where the ordered precipitate stems from a phase diagram configuration in which the ordered and disordered states are connected by a first-order phase transition. The approach is applied to the NiAl, NiTi and AlLi binary alloy systems in which the interaction of clustering and ordering appears to play an important role.