Temperature-stress phase diagram of strain glass Ti48.5Ni51.5

The temperature and stress dependence of the properties of a recently discovered strain glass Ti48.5Ni51.5, which is a glass of frozen local lattice strains, was investigated systematically. It was found that the ideal freezing temperature (TO) of the strain glass decreases with increasing stress. When the stress exceeds a critical value a,(T), the pseudo-B2 strain glass transforms into B19' martensite. However, the stress-strain behavior associated with such a stress-induced transition showed a crossover at a crossover temperature T-CR, which is similar to 20 K below To. Above TCR, the sample showed superelastic behavior; however, below TCR, the sample demonstrated plastic behavior. More interestingly, the sigma(c) vs. temperature relation for unfrozen strain glass obeys the Clausius-Clapyeron relationship, whereas that for frozen strain glass disobeys this universal thermodynamic law. A phenomenological explanation is provided for all the phenomena observed, and it is shown that all the anomalous effects come from the broken ergodicity of the glass system and a temperature-dependent relative stability of the martensitic phase. Based on experimental observations, a temperature-stress phase diagram is constructed for this strain glass, which may serve as a guide map for understanding and predicting the properties of strain glass. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.