Effect of Composition and Applied Stress on the Transformation Behavior in Ni_XTi_80−XZr₂₀ Shape Memory Alloys

This study demonstrates the effect of Ni content and applied stress on the microstructure and shape memory properties of a range of Ni_XTi_80−XZr₂₀ (X = 50–51 at.%) shape memory alloys, which were produced via arc melting. Uniaxial constant force thermal cycling, differential scanning calorimetry, microhardness, and microscopy were used to evaluate the alloys in the homogenized, solutionized, and aged (500–550 °C) conditions. Transformation temperatures decrease linearly with increasing Ni content for the solutionized and aged samples, while the data demonstrate a rebound in transformation temperatures in the high-Ni alloys in the homogenized condition. In general, hardness increases with Ni content as a result of solution strengthening in the case of the solutionized condition, and precipitation strengthening in the aged conditions. During uniaxial constant force thermal cycling, the austenite finish temperature (A_F) increased with increasing stress at a higher rate in the near-stoichiometric compositions than in the high-Ni compositions. Transformation strains also increased with stress, with the stress rate and the magnitude of the peak transformation strain heavily dependent on Ni content. Residual strain decreased with Ni content due to the strengthening effects of the solid solution and H-phase precipitates.