Lattice dynamics of NiTi with defect-induced short-range disorder

Nickel–titanium (NiTi) is a shape-memory alloy that undergoes a reversible martensitic transformation between the high-temperature B2 austenite phase and the low-temperature B19' martensite phase. Lattice dynamics plays a crucial role in this transformation, making the study of phase stability at finite temperature essential. We examine how point defects, such as vacancies and antisites, affect the lattice dynamics and free energy of the B2 structure at finite temperature. Short-range disorder induced by the defects was quantified using the Warren–Cowley parameters up to fourth nearest neighbors. Molecular dynamics simulations were combined with Harmonic Ensemble Lattice Dynamics (HELD) to determine effective harmonic force constants up to fifth nearest neighbors in the Born–von Kármán model. HELD was integrated within the Utilities To Execute Pipelines (UTEP) framework to streamline the generation and analysis of large-scale datasets. Our results corroborate that the B2 phase is dynamically stable at finite temperature, as shown by multiple studies. Increasing temperature leads to phonon stiffening at the M high symmetry point in all cases. Vacancies have a minor effect on the phonon dispersions, but antisites shift phonon modes to higher energies and induce hybridization between acoustic and optical modes at the X high symmetry point. All defects increase the internal energy and the entropic term, but might be stable at high temperature.