Statistical mechanics of nucleation in solids: a kinetics driven morphological transition

We examine in detail, through extensive molecular dynamics simulations, the process of nucleation of a triangular phase in a model two-dimensional square crystal consisting of particles interacting via a pair and three-body potentials. We find that at high temperatures, the critical nucleus of the triangular phase is isotropic with local triangular order. At low temperatures, the critical nucleus consists of a twinned bicrystal which is anisotropic and breaks the symmetry of the underlying square phase. We show that this morphological transition is caused by the dynamics of non-elastic degrees of freedom, viz. interfacial defect fields. This transition is similar to that between 'ferrite' and 'martensite' as observed in real materials when a solid such as iron is cooled across a structural transition. Based on our simulation results, we arrive at a single theoretical formalism which qualitatively describes the transformation to both a ferrite and a martensite