STATIC AND DYNAMIC PRECURSORS OF DISPLACIVE TRANSFORMATIONS NEAR CRYSTALLINE DEFECTS

An exact nucleation energy formula has been derived for the fluctuation energy necessary to initiate a coherent martensitic transformation in the strain field of a defect of arbitrary potency within the framework of a standard Ginzburg-Landau strain free energy model of the form: f(eta) = g(eta) + K1(deleta)2. The formula is limited to planar defects and planar interface propagation geometries, and precursor metastable strain profiles are calculated exactly. The defect induced strain field expands as the driving force is increased until the athermal instability limit is reached. This limit can be simply expressed in terms of the defect potency, or alternatively in terms of the strain spinodal, but the latter condition proves to be the weaker of the two in most cases tested. Once the nucleation condition is achieved, the equations of growth become identical to those previously derived by Chan. Some considerations are also given to the extended free energy form: f(ets) = g(eta) + K1(deleta)2 + K2(del2eta)2 where it is found that for some values of K2 and K1, a defect will be decorated by an oscillatory strain field, in others a monotonicaily decaying field, and in a third range, no metastable defect strain field appears to be possible at all.