PHONON SOFTENING AND MARTENSITIC-TRANSFORMATION IN ALPHA-FE

The phonon dispersion of the low-temperature BCC-phase of pure iron (a lpha-Fe) was investigated as a function of temperature. A strong softe ning of the entire T-1[xi xi 0] and T-1[xi xi 2 xi] branch is observed on approaching the martensitic alpha-gamma-phase transition temperatu re. The eigenvectors of these phonons are in the direction of displace ments needed for the transformation to the FCC-(gamma-) phase. This in dicates low potential barriers for displacements towards the closed-pa cked structure and can be interpreted as a dynamical precursor for the martensitic phase transition.