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.