First-principles determination of the structure, elastic constant, phase diagram and thermodynamics of NiTi alloy

We have investigated the structural and elastic properties of Ni, Ti and NiTi alloy from first-principles calculations. Elastic constants are calculated from the evaluation of the relationships of the energy and small strains using density-functional technique. From the elastic constants, other elastic properties are also obtained. Comparing with the experimental data, our results are satisfactory. Within quasi-harmonic Debye approximation, the phase diagram and thermodynamic properties of NiTi alloy are predicted successfully. The predicted phase boundary between the austenite B2 phase and martensite B19′ phase can be described as a third order polynomial T=287−1.4155P+0.1206P2−6.4190×10−4P3. We have also predicted some thermodynamic properties of NiTi, such as the thermal equation of state, thermal expansivity and thermal pressure.