Enhancement in strength and superelastic cyclic durability by addition of Si in Ni–Ti–Cu–Zr alloy

In this study, the martensitic transformation and superelastic behaviors were investigated in (Ni_0.35Ti_0.30Cu_0.15Zr_0.20)_100-xSi_x (x = 0, 0.25, 0.5, and 1.0) alloys. After homogenization treatment, the microstructure consists of a B2 austenite matrix and a small fraction of secondary solidification particles at the grain boundaries. The concentration of Si in the B2 matrix increases with increasing Si content. The martensitic transformation temperature drops significantly as the Si content increases, i.e., an approximately 200 K decrease with the addition of 1.0 at% Si. The stress for inducing the martensitic transformation significantly increases with the increase in Si, whereas the strain to the final fracture decreases. After the cyclic loading test, the normalized stress for inducing the martensitic transformation increases with the addition of Si, whereas the residual plastic strain decreases. Thus, the Ni–Ti–Cu–Zr alloy with a minor addition of Si shows high potential for structural applications, owing to a good combination of strength and superelastic properties.