STRUCTURAL PHASE-TRANSFORMATION AND PHONON SOFTENING IN IRON-BASED ALLOYS

The interrelation of Invar and Martensite in iron-based alloys like Fe 3Ni is discussed on the basis of ab initio calculations for different crystal structures. Inelastic neutron scattering and ultrasonic experi ments have shown that a soft transverse phonon mode in [110] direction acts as a precursor in the close packed fee phase of these systems. U nfortunately there are no experimental results for phonon modes in the transformed bet phase. We have examined in detail the changes in the phonon dispersions of these systems by varying the ratio of the lattic e constants along the Bain path. In recent calculations we have used t he method of Varma and Weber in combination with the full potential LA PW (FLAPW) method to calculate frozen phonons for the tetragonal phase . Our new numerical results reproduce the experimental findings for th e fee phase and give an overview of the changes of the phonon energies when varying the c/a ratio towards the bet structure. In particular w e observe that phonon softening is caused by the long-range electron-p honon interaction in systems with magnetovolume instabilities, and van ishes after the martensitic transformation.