Temperature-stress phase diagram of strain glass Ti_48.5Ni_51.5

The temperature and stress dependence of the properties of a recently discovered strain glass Ti_48.5Ni_51.5, which is a glass of frozen local lattice strains, was investigated systematically. It was found that the ideal freezing temperature (T₀) of the strain glass decreases with increasing stress. When the stress exceeds a critical value σ_c(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 ∼20 K below T₀. Above T_CR, the sample showed superelastic behavior; however, below T_CR, the sample demonstrated plastic behavior. More interestingly, the σ_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.