Structural and mechanical properties of NiTiAg shape memory alloys: ab-initio study

Abstract

The austenite phase of Ni50−x Ti50Ag x for different concentrations (x = 1, 1.5, 2, 3, 5, 7, 8, 10) has been analyzed in this work. The ground state, structural, and mechanical characteristics of Ni50−x Ti50Ag x for various concentrations are examined under ambient conditions. From total energy as well as formation energy calculations, it is found that the stability of Ni50−x Ti50Ag x austenite phase is increased up to x = 5 and after that it becomes unstable in nature. These results are in good agreement with our experimental results. The experimental and other theoretical results agree with the estimated lattice parameter values. The x-ray diffraction and differential scanning calorimetry reveal the phases formed and transformation characteristics of NiTiAg alloys. The elastic constants, bulk modulus, shear modulus, Poisson’s ratio, and elastic anisotropy factor of Ni50−x Ti50Ag x are examined at ambient conditions for different concentrations. From elastic constant calculations, it is also found that the austenite phase of Ni50−x Ti50Ag x is mechanically stable up to x = 5 and after that it does not obey Born–Huang criteria and becomes unstable. The Ni45Ti50Ag5 composition is found to be the stiffest and hardest material. According to Poisson’s ratio calculations, Ni50−x Ti50Ag x shows that the Ti–Ag, Ni–Ag bonding is more directed in nature. The austenite phase of Ni50−x Ti50Ag x is more incompressible because the Poisson’s ratio is nearly equal to the optimum value (0.5).