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. [zeta zeta 0]TA(2) 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. (C) 2007 Elsevier B.V. All rights reserved.