Evidence for crossover martensite in Ti₅₀Ni₄₅Fe ₅: An intermediate state between normal martensite and strain glass

We report that the crossover from normal martensite to strain glass in the TiNi-Fe alloy system does not proceed directly as indicated by previous studies but is actually mediated by an intermediate state named crossover martensite, appearing in the crossover composition of Ti₅₀Ni₄₅Fe ₅. The martensitic transition from the parent phase to the crossover martensite is a thermodynamic phase transition, being similar to a normal martensitic transition. However, its produced crossover martensite shows an obvious kinetic feature such as a time-dependent behavior in internal friction (aging effect), resembling that of the strain glass. The microstructure of the crossover martensite exhibits a special morphology of coexisting macro-sized martensite plates and nano-sized martensitic domains. The displaying of both thermodynamic and kinetic features in the crossover martensite demonstrates that it is an intermediate state between normal martensite and strain glass; this finding provides a complete physical picture for the crossover phenomenon from normal martensite to strain glass.