Effect of Zr on the properties of (TiZr)Ni alloys from first-principles calculations

The effect of Zr on the martensitic transformation (MT) behavior and mechanical properties of (Ti_0.5-xZr_x)Ni_0.5 alloys is investigated by calculating the elastic constants and elastic moduli in the B2 phase as a function of x for 0⩽x⩽0.2 . The calculations are performed using the coherent potential approximation implemented within the framework of the exact muffin-tin orbitals method. We find that the theoretical elastic properties correlate well with the behavior of the MT. With increasing Zr concentration, the anisotropy of the alloy decreases, indicating that the nonbasal plane shear on which the modulus C₄₄ plays an important role, dominates and, therefore, a monoclinic martensitic phase should result. The experimental Zr content dependence of the MT temperature is paralleled with the calculated C₄₄ versus Zr content. The theoretical elastic moduli demonstrate that the (TiZr)Ni alloys, with Zr distributed randomly on the Ti sublattice, are intrinsically ductile, which suggests that the poor ductility of these alloys may be ascribed to some other factors, for example, impurities, precipitation, and grain boundaries.