Dynamical Precursors of Martensitic Transitions

Abstract
The phonon dispersion of the high temperature bcc phase of the group 3 and 4 metals is characterised by a valley of low energy and strongly damped transverse phonons. The large fluctuations in amplitude related to these phonons probe the lattice potential for displacements towards the close packed product phases into which these materials tend to transform. At the same time they stabilize the open bcc structure via their dominating contribution to the lattice entropy. In pure monoatomic metals these precursors are of purely dynamical nature, only the introduction of defects leads to a static condensation of the displacements. Because the trajectory for the displacive transformation and the elementary jump for self diffusion in these metals point in the same direction, the height of the migration barrier and the tendency of the bcc metals to transform are related to each other. Whereas, for the bcc→close packed transition shuffles and homogenous strains involved in the transition could be identified, such an assignment was not possible for the inverse transition, nor for transitions from close packed→close packed structures.