HETEROGENEOUS NUCLEATION AT A SINGLE DEFECT AND SUBSEQUENT GROWTH OF MARTENSITIC PHASE TRANSITIONS

Abstract
A description of heterogeneous nucleation at a single defect, followed by the evolution of the nucleus as it grows in the parent phase background, is given. The theory is based on a nonlinear and nonlocal elastic free energy expansion, and the main assumption in our treatment is that of the interface separating parent and martensitic phases being completely coherent. The nucleating event is simply the response of the parent phase to the stress field of a defect that couples to the order parameter strain describing the transition. At the first-order transition temperature, around a weak defect, a small strain is produced. Then, after supercooling the system through the bulk transition temperature, a martensitic nucleus may appear, and we follow its growth as described by the deterministic dynamics of the elastic field. We show that the growth is going to lead to a twinned product phase due to dynamical energy reasons, viz. minimizing the kinetic energy, in contrast to the static energetics arguments of nonlinear elastic theories. Thus, this evolution of a martensitic embryo provides a simple and natural explanation of the phenomenon of autocatalytic twin formation.