Phase diagram calculation: past, present and future

Abstract

The past, present and future of phase diagram calculations for multicomponent alloys are reviewed and assessed. The pioneering studies of Van Laar and Meijering in the first half of the 20th century led to the use of phase equilibrium information as a supplement to single phase thermodynamic property data in these calculations. The phenomenological modeling or the Calphad approach is the primary focus of this review due primarily to its great success in calculating multicomponent phase diagrams for technological applications. In this approach, thermodynamic descriptions of multicomponent alloys are obtained by appropriate extrapolations of descriptions obtained for the lower order systems, viz., the constituent binaries and ternaries. Some shortcomings of the Calphad route to obtaining phase diagrams are pointed out. These include (a) the inability of first generation software to always automatically calculate the stable phase diagram of a system given a thermodynamic description and (b) the use of some inappropriate thermodynamic models, particularly those used for ordered phases. The availability of second generation software eliminates the first shortcoming and a physically more realistic model, the cluster/site approximation, has been formulated which is more suitable for describing the thermodynamics of ordered alloys. The results obtained to-date using the new software and the new model open up new avenues for calculating more reliable multicomponent phase diagrams for technological applications.