Inelastic neutron scattering of a Ti-44Â at.%Ni-6Â at.%Fe alloy exhibiting an incommensurate-commensurate transition

Inelastic neutron scattering has been made for a Ti-44 at.%Ni-6 at.%Fe alloy which exhibits a second-order like incommensurate-commensurate (IC-C) transformation [M.-S. Choi, T. Fukuda, T. Kakeshita, Philos. Mag. 86 (2006) 67] in order to understand the nature of the transformation. The phonon frequencies of the [ζ ζ 0]-TA2 phonon branch near ζ = 1/3 decrease as temperature decreases, exhibit the lowest value at about 200 K and then increase on further cooling. This temperature (200 K) is near the temperature below which diffuse incommensurate scattering can be noticed, and it is by 20 K higher than the IC-C transition temperature. Presumably, the phonon softening is responsible for the formation of the incommensurate state, but not directly connected to the formation of the commensurate state.