Soft-phonon feature, site defects, and a frustrated phase transition inNi50Ti47Fe3: Experiments and first-principles calculations

Abstract

A soft-phonon feature associated with the shape-memory transition in NiTi is observed in the phonon density of states ͑DOS͒ of the B2 phase of both NiTi and Ni 50 Ti 47 Fe 3 ͑with Fe substituted for Ti͒ 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 Ni 44 Ti 50 Fe 6 ͑with Fe substituted for Ni͒. 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.

Strain glass in Fe-doped Ti–Ni

“…The change of transformation behavior with Fe doping has been studied previously [14,15,19,20,27] and the appearance of a strange state (with a negative temperature dependence of resistivity) at high Fe doping level has also been reported [14,15,21,22]. Our results above are in agreement with these previous studies [14,15,22].…”

“…It should be noted that different views on the nature of these effects exist [21,22], which consider that such resistivity anomaly is caused by nesting effect of Fermi surface. But our strain glass scenario seems to provide a consistent explanation for all the properties observed, including thermal, mechanical, and electrical properties.…”

“…However, Ti-Ni-Fe and Ni-rich Ti-Ni systems exhibit NTD in martensitic alloys prior to the martensitic transformation and in the abnormal non-transforming alloys [13][14][15]21,22] (now proven to be strain glasses). Such abnormal behavior has been studied through the first principle simulation in the frame of electronic structure recently [21,22]. Now with the relationship between the resistivity and the microscopic picture of the strain glass established in this study (Fig.…”

“…By contrast, the precursory state prior to a martensitic transformation does not show these "freezing" characteristics, as evidenced by the absence of the above-mentioned anomalies in AC and DC mechanical measurements [18]. Distinguishing clearly the strain glass state from the pre-martensitic state may help solve two long-standing puzzles associated with the pre-martensitic state and abnormal non-transforming compositions of Ti-Ni and Ti-Ni-Fe systems [13][14][15][19][20][21][22]; one is the origin of nano-domains prior to the normal martensitic transformation and the other is the abnormal "negative temperature dependence" of the electrical resistivity in the abnormal non-transforming alloys.…”

“…The phase diagram demonstrates clearly the relationship among the wellobserved precursory nano-domains state, the martensitic state and the strain glass state. It was also found that the long-standing puzzle -"negative temperature dependence" of electrical resistivity in the pre-martensitic state and in the abnormal non-transforming compositions of Ti-Ni and Ti-Ni-Fe systems [13][14][15]21,22] -can be easily understood by the existence of the static R nano-domains.…”