Soft-phonon feature, site defects, and a frustrated phase transition in Ni50 Ti47 Fe3: Experiments and first-principles calculations

A soft-phonon feature associated with the shape-memory transition in NiTi is observed in the phonon density of states (DOS) of the B2 phase of both NiTi and Ni50 Ti47 Fe3 (with Fe substituted for Ti) using inelastic neutron scattering. In both alloys, the feature softens with decreasing temperature, but the softening occurs about 100 K lower in the Fe-substituted alloy, indicating a decreased transition temperature. Electrical resistivity and magnetic susceptibility verify the decreased transition temperature but also show that the transition develops second-order-like behavior similar to that observed by others in Ni44 Ti50 Fe6 (with Fe substituted for Ni). First-principles calculations supported by Mössbauer spectroscopy and neutron diffraction indicate a double-defect scenario, where Fe occupies Ni sites and the displaced Ni occupies the empty Ti sites in the Ti-substituted alloys. A comparison between the current results for Ti-substituted alloys, and related experimental data for alloys featuring Fe substitution for Ni, indicates that the instability temperature is controlled by the number of Fe atoms occupying the Ni sites, while the second-order-like behavior is caused by the addition of the Ni antisite defects. We argue that this latter behavior results from percolated networks of interacting defects acting to frustrate the symmetry-breaking strains.