Energy barriers and hysteresis in martensitic phase transformations

We report results from a systematic program of alloy development in the system TiNiX, X = Cu, Pt, Pd, Au, to pursue certain special lattice parameters that have been identified previously with low hysteresis. We achieve λ2=1λ2=1, where λ2λ2 is the middle eigenvalue of the transformation stretch matrix, for alloys with X = Pt, Pd, Au. In all cases there is a sharp drop in the graph of hysteresis vs. composition at the composition where λ2=1λ2=1. When the size of the hysteresis is replotted vs. λ2λ2 we obtain a universal graph for these alloys. Motivated by these experimental results, we present a new theory for the size of the hysteresis based on the growth from a small scale of fully developed austenite martensite needles. The energy of the transition layer plays a critical role in this theory. Overall, the results point to a simple systematic method of achieving low hysteresis and a high degree of reversibility in transforming materials.