Entropic stabilization of austenite in shape memory alloys

This paper discusses the phenomenon of entropic stabilization of the austenite phase in SMAs and explicitly demonstrates it using Molecular Dynamics (MD) and a three-dimensional all-atom potential energy model whose equilibria crystal structures correspond to commonly observed SMA phases. A new technique is used to carefully select a model so that it is likely to lead to entropic stabilization of a B2 cubic austenite from a B19 orthorhombic martensite. This is accomplished by using a detailed branch-following and bifurcation (BFB) parametric study of the Morse pair potential binary alloy model. The results of the MD simulation clearly demonstrate the entropic stabilization of the B2 austenite phase at high temperature. Analysis of the dynamics of the B2 austenite phase indicates that its stabilization may be viewed as a result of individual atoms randomly visiting the B19 and αIrV phases (with only occasional visits to the B2 and L10 phases). This occurs without long-range correlations in such a way that each atom's time-average configuration corresponds to the B2 structure.