TRANSFORMATION PATH DEPENDENCE OF THE BAIN STRAIN RELAXATION DURING DECOMPOSITION OF A HYPEREUTECTOID CUBE ALLOY

A transmission electron microscopy (TEM) investigation is conducted to study the relaxation of the Bain transformation strain during decomposition of a Be-rich hypereutectoid CuBe alloy. The relaxation is found to occur mainly through two basic mechanisms; habit plane rotation and the formation of a polytwin morphology. Depending on the path of the initial stages of the decomposition, these two mechanisms interplay in different ways to give rise to two different kinds of precipitate morphologies. For quenched and aged alloys, the relaxation process involves global habit plane rotation of single-domain plates along with a reconfiguration into a macroscopic polytwin morphology. For air-cooled alloys, the relaxation process involves the formation of sawtooth type polytwin plates characterized by a segment-wise habit plane rotation. A few questions which resulted from the experiments are considered by analysis of the elastic strain energy of a polytwin plate. It is suggested that the sawtooth morphology is a metastable morphology which does not satisfy the invariant plane strain condition and which comes about through certain kinetic effects under the influence of the elastic instability of the intermediate alpha' face-centered tetragonal (fct) state.