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 Ti50-xNi44Cu6Zrx 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 Mp and Ap temperatures can be calculated by linear regression as: Mp (°C) = 51.565–11.733×; Ap (°C) = 69.695–10.408×, respectively. With further increasing of the Zr content, the transformation temperatures increased continuously in a linear relationship. The Mp and Ap temperatures can be expressed by linear regression as: Mp (°C) = 4.53x-33.38; Ap (°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. [Display omitted] •Both Mp and Ap firstly decrease and then increase with Zr content increasing.•Moderate Zr addition can improve the thermal cycling stability.•The size of Ti2Ni type second phase is reduced due to Zr addition.•Zr addition increases the critical stress for the reorientation of martensite variants.•The TiNiCuZr3 alloy shows the largest SME strain and tensile fracture strain.