Temperature dependence of NiTi martensite structures: Density functional theory calculations

Temperature dependences of the stability and structural parameters of the martensite phase in nickel‑titanium shape-memory alloys are studied by applying a harmonic approximation within density functional theory. By analyzing detailed free energy landscapes of martensite structures with various monoclinic angles, the transformation barrier between the reported (B19′) and the hypothetical (B33) martensite structures, the monoclinic angle, and the lattice constants of the martensite structure have been thoroughly investigated. The results show that the vibrational effect at finite temperatures plays a decisive role in resolving the reported discrepancies between experimental and theoretical results.