Effect of Cu content on martensitic transformation and shape memory behavior in Ti_31.5Hf₁₅Zr₅Ni_48.5−xCu_x alloys

The impact of Cu doping on the martensitic transformation and shape memory properties of a high-temperature shape memory alloy system, Ti_31.5Hf₁₅Zr₅Ni_48.5−xCu_x (x = 0, 5, 10, 15, 20), was investigated. Both the martensitic transformation temperature (M_s) and the transformation enthalpy decrease monotonically with increasing Cu content x, while the recoverable strain exhibits a saddle curve with a minimum value at x = 10. These Cu content-dependent behaviors were attributed to the constraint effect caused by the different size distribution of Ti₂Ni-like second phases. Additionally, a scaled linear relationship between M_s and several elemental features, including the modified valence electron concentration, electronegativity, and atomic volume, was established to describe the Cu content dependence of M_s across various Cu-containing TiNi-based systems. The experimental results presented herein, along with the proposed functional form, offer insights for the design of Cu-containing shape memory alloys.