Phonons in austenite and martensite NiTi crystals

Abstract
Here, the first results of ab-initio calculations of phonon modes in the austenite and martensite phases are presented. Using the local-density approximation, calculating the Hellmann-Feynman forces, and applying the direct method, phonon dispersion relations of the cubic austenite, orthorhombic and martensitic monoclinic phases of NiTi intermetalic compound have been calculated. In the austenite phase a soft mode at the M reciprocal lattice point has been found. The basin of the soft mode spreads from M point along the [1, 1, 0] crystallographic direction, and reaches the [1, 1, 1] direction. The M soft mode induces an unstable orthorhombic phase, which shows a low frequency acoustic mode, and facilitates a deformation to a final monoclinic stable martensite.