The effect of Zr on the transformation behaviors, microstructure and the mechanical properties of Ti-Ni-Cu shape memory alloys

The effect of Zr substituting for Ti in Ti_50-xNi₄₄Cu₆Zr_x alloys on the martensitic transformation behavior, microstructure, mechanical and shape memory properties was investigated systematically. The results showed that the Ti-Ni-Cu-Zr alloys were featured with the single step (B2⇋B19′) martensitic transformation. The transformation temperatures decreased linearly in approximate with the Zr content increasing up to 5 at.%. The M_p and A_p temperatures can be calculated by linear regression as: M_p (°C) = 51.565–11.733×; A_p (°C) = 69.695–10.408×, respectively. With further increasing of the Zr content, the transformation temperatures increased continuously in a linear relationship. The M_p and A_p temperatures can be expressed by linear regression as: M_p (°C) = 4.53x-33.38; A_p (°C) = 9.10x-24.12, separately. The Zr addition resulted in the reduction of the size of the Ti₂Ni type second phase. Instead, a Zr-rich zone at the neighborhood of the Ti₂Ni type second phase appeared owing to Zr addition. Based on the solution strengthening, the critical stress for the martensite reorientation and the tensile strength were significantly elevated, and further improving the shape memory effect. The complete recoverable strain of 4% and the largest tensile strain of 16.5% can be obtained in Ti-Ni-Cu-Zr alloy containing 3 at.% Zr.