Structural Phase Transformation and Phonon Softening in Iron-Based Alloys

Abstract
The interrelation of Invar and Martensite in iron-based alloys like Fe₃Ni is discussed on the basis of ab initio calculations for different crystal structures. Inelastic neutron scattering and ultrasonic experiments have shown that a soft transverse phonon mode in [110] direction acts as a precursor in the close packed fcc phase of these systems. Unfortunately there are no experimental results for phonon modes in the transformed bct phase. We have examined in detail the changes in the phonon dispersions of these systems by varying the ratio of the lattice constants along the Bain path. In recent calculations we have used the method of Varma and Weber in combination with the full potential LAPW (FLAPW) method to calculate frozen phonons for the tetragonal phase. Our new numerical results reproduce the experimental findings for the fcc phase and give an overview of the changes of the phonon energies when varying the c/a ratio towards the bct structure. In particular we observe that phonon softening is caused by the long-range electron-phonon interaction in systems with magnetovolume instabilities, and vanishes after the martensitic transformation.