Observation of phonon softening in a Ti–Ni–Fe alloy by inelastic X-ray scattering

Phonon softening of the martensitic transformation, which is a typical first-order transformation, is an interesting concept in connection with the transformation mechanism. Recently, Kakeshita and his group studied a wide range of Fe-substituted Ti–Ni alloys [Ti–(50 − x)Ni–xFe, at.%] and reported anomalies in electrical resistivity, magnetic susceptibility and specific heat. Furthermore, they observed electron diffraction patterns and reported second-order-like features in a Ti–44Ni–6Fe alloy with an incommensurate–commensurate phase transformation prior to the transformation to the R phase. Inelastic X-ray scattering experiments were performed in order to observe the phonon behavior in Ti–44Ni–6Fe. Since the energy of phonons is about six orders of magnitude lower than that of X-rays, precise experiments are necessary to observe phonons by X-ray inelastic scattering experiments. The beam-line BL35XU at the synchrotron radiation facility, SPring-8 was utilized for the experiments. [ζ ζ 0]TA2 phonon modes were measured at various temperatures. The temperature at which the softest phonon is observed is at approximately 180 K, corresponding to the incommensurate–commensurate transformation temperature. Possible differences between results of neutron inelastic scattering and X-ray inelastic scattering experiments will be discussed in the text.