Prediction of ordered superstructure phase equilibria

The approach to phase diagram calculations has changed drastically within the last few years. Previously, mean-field models (regular solution, Bragg-Williams, concentration waves) were used almost exclusively. These models rely on two very poor approximations: supe5rposition of ’point’ probabilities for pair probabilities and an ideal solution model for the configurational entropy. Today, effective cluster interactions can be calculated from first-principles electronic structure methods: the superposition approximation is avoided, and cluster formulations for the entropy are available. As will be shown by recently computed examples, such cluster methods predict first-priciples phase diagrams that are often in excellent agreement with those determined empirically. The pater was presented at the International Phase Diagram Prediction Symposium sponsored by the ASM/MSD Thermodynamics and Phase Equilibria Committee at Materials Week, October 21-23,1991, in Cincinnati, OH. The symposium was organized by John Morral, University on Connecticut, and Philip Nash, Illinois Institute of Technology.