Lattice stability, elastic constants and macroscopic moduli of NiTi martensites from first principles

The elastic constants that describe the fundamental elastic properties of NiTi martensites are unknown today. We present results of ab initio calculations of the ground-state energies and the relative mechanical stability of B19, B19′ and B33 (a theoretically predicted ground state from recent ab initio studies). It is demonstrated that shear stresses of the order of 1 GPa are sufficient to mechanically stabilize B19′ against B33. The full sets of elastic constants and the associated macroscopic elastic parameters (Young’s, shear and bulk moduli, Poisson ratios) are determined for the first time for B19′ and B33 NiTi. The predicted macroscopic Young’s modulus of B19′ based on the first-principles results is an order of magnitude larger than the values currently assumed in micro or continuum mechanical modeling studies. Yet the results are in good agreement with novel experimental data and, furthermore, resolve a long-standing issue in the well-known Müller–Achenbach–Seelecke model by predicting Young’s modulus of martensite to be larger than that of austenite.