Martensitic transformations and superelastic behavior at low temperatures in Ti_50-xNi_40+xCu₁₀

In the present study, Ni content dependence of martensitic transformation behavior in Ti_50-xNi_40+xCu₁₀ alloys (x = 0.03.0) was investigated, and then the superelastic behavior of a x = 2.4 alloy was examined at low temperatures. The Ti_50-xNi_40+xCu₁₀ alloys are in B2 single-phase up to x = 2.4 at 1373 K. The B2/B19 and B19/B19A transformation temperatures decrease with increasing x and the lowest martensitic transformation starting temperature of 125K was detected. In the alloys with Ni compositions over x = 1.4, a pre-martensitic "intermediate" phase was detected by thermoanalysis measurements. While the middle eigenvalue λ₂ is close to 1.0, which was found to be independent of the Ni content, the thermal hysteresis, T_hys increases with increasing Ni content. This is explained by the drastic decrease in transformation entropy change. Although starting to drastically increase at temperatures below 100 K, the temperature dependence of stress hysteresis in compression testing is basically smaller than that in Ti_48.2Ni_51.8 binary alloy.