Martensitic transformation in amorphous-crystalline Ti-Ni-Cu and Ti-Hf-Ni-Cu thin ribbons

Martensitic transformations and strain variation in amorphous-crystalline thin ribbons of Ti₅₀Ni₂₅Cu₂₅, Ti_40.7Hf_9.5Ni_44.8Cu₅ and Ti_40.7Hf_9.5Ni_39.8Cu₁₀ alloys were investigated. The amorphous-crystalline state in the samples was produced by an interruption of the isothermal crystallization in DSC apparatus. The volume fraction of the crystalline phase V_cryst in the samples was varied from 10 to 100%. Transformation temperatures in amorphous-crystalline thin ribbons of Ti₅₀Ni₂₅Cu₂₅ alloy depended only slightly on the crystalline volume fraction. On the contrary, the strong dependence of the martensitic transformation temperatures on the volume of crystalline phase was obtained in Ti_40.7Hf_9.5Ni_44.8Cu₅ and Ti_40.7Hf_9.5Ni_39.8Cu₁₀ alloys. Moreover, it was found that the intervals of the direct transformation are extremely narrow, whereas the intervals of the reverse transformation are unusually wide. As the crystalline volume fraction rises, both the direct transformation interval and the reverse one decrease, and the transformation temperatures increase. It was shown that increase of the Cu-content in Ti_40.7Hf_9.5Ni_{49.8 -x}Cu_x (x = 5, 10%) alloys results in decrease of the transformation temperatures, of the temperature interval of the reverse transformation on heating and of the hysteresis, while the temperature interval of the direct transformation on cooling widens