First-principles phonon-based model and theory of martensitic phase transformation in NiTi shape memory alloy

Remarkable shape memory property of Nitinol (NiTi) originates from a diffusion-free reversible phase transition it exhibits upon cooling from the cubic austenite to a low-symmetry martensite structure. Atomistic mechanisms of this martensitic transformation (MT) and consequent emergence of its various low-symmetry structures are not understood yet. Starting with first-principles density functional theoretical calculations, we present here a phonon-based model and its statistical mechanical analysis to obtain atomistic insights into martensitic phases and transformation in NiTi. We uncover seven order parameters that are relevant to the MT in NiTi. From Monte Carlo simulations of an effective model Hamiltonian derived to capture its low energy landscape, we determine its soft phonons and establish the cell-doubling M5, mode as the primary order parameter. Using Landau theoretical analysis, we show that relative strengths of its third-order coupling with secondary order parameters (e.g. strain) determine the symmetry of low-T structures emerging at its MT. These couplings can be used as the descriptors of stability of martensitic phases that will guide the strategies to improve the shape memory of NiTi through substitutional alloying.