Coexisting Ordering and Phase Separation in Binary Fcc Alloys

Abstract

ABSTRACTConsideration of only nearest neighbour pairwise interactions Vij in a binary alloy leads to the classification of the system as ordering (unlike near neighbours) or clustering (like near neighbours), depending upon the sign of Vij However, this simple classification loses meaning when multi-atom correlations, many-body interactions or a longer range interaction are considered. For example, the first nearest neighbour interaction may favour ordering while the second, which may be of comparable magnitude, may favour clustering. By extending the Bragg-Williams model to include second near-neighbour interactions in fcc alloys, it is shown that a miscibility gap may form in the region of the orderdisorder solvus, leading to a complicated sequence of atomicrearrangement upon slow cooling. Despite the well-known failings of the point approximation when applied to fcc alloys, the results are shown to be consistent with the unusual behaviour exhibited by some systems.