The influence of lattice softening of the parent phase on the martensitic] transformation in FeNi and FePt alloys

Studies of the interactions between the magnetic transition and the first-order displacive martensitic transformation in FeNi and ordered and disordered FePt alloys are reviewed in an effort to find answers to two questions. Is lattice softening a necessary pre-condition for transformation? How does pre-transformation softening, when it occurs, affect the nucleation and growth of a martensitic plate? The clear demonstration that the low temperature transformation which produces martensite with the well-known acicular morphology occurs only in the ferromagnetic f.c.c. phase suggests that softening of the parent phase is necessary. However, the fact that a transformation to lath martensite occurs, at temperatures above room temperature, in dilute alloys in which the parent phase is paramagnetic, casts doubt on the validity of this generalization. There are a number of indications that the lattice softening which accompanies the magnetic transition influences the kinetics of the displacive transformation but the exact mechanisms by which nucleation is affected is still unclear. Single-crystal measurements of the soft modes in disordered FePt clearly show that they are not coupled directly to the transformation lattice strains and, consequently, it is unlikely that there is a direct connection between the soft mode lattice displacements and the displacements which produce the Bain strain. It is probable, however, that there are important indirect effects of softening on nucleation in these alloys. The influence of softening on growth, on the contrary, is clearer. Pre-transformation lattice softening is a necessary but not sufficient condition for thermoelastic reversible growth and shape memory properties.