THE SOFT MODE CRITERION FOR MARTENSITIC TRANSFORMATIONS

Abstract
Complete or almost complete softening of C' occurs at T_m in various second order transformations of ferroelectries, magnetics and superconductors, but not in most first order transformations of metals and alloys, suggesting that either the soft mode criterion is not applicable to the latter, or the behavior is complicated by other effects. Since the crystallographic theory defines the sequence of events and atomic motions between states it offers an opportunity to rationalize this behavior. We describe the crystallographic theory and the dynamical model for soft modes whereby dislocation sites are preferred for correlated ionic shifts, which manifest themselves as an "internal tensile stress" and a resolved shear stress on the primary shear system for a localized softening which is greater than that indicated by the measured elastic constants. These effects can be swamped by the latent heat and hardened by the incipient volume change relaxation and habit plane shear in first order transformations. A review of experimental data overwhelmingly supports the "localized soft mode model" for nucleation by dislocation dissociations.