Nucleation of a solid in solid is initiated by the appearance of distinct dynamical heterogeneities, consisting of `active particles whose trajectories show an abrupt transition from ballistic to diffusive, coincident with the discontinuous transitio
n in microstructure from a {it twinned martensite} to {it ferrite}. The active particles exhibit intermittent jamming and flow. The nature of active particle trajectories decides the fate of the transforming solid -- on suppressing single particle diffusion, the transformation proceeds via rare string-like correlated excitations, giving rise to twinned martensitic nuclei. These string-like excitations flow along ridges in the potential energy topography set up by inactive particles. We characterize this transition using a thermodynamics in the space of trajectories in terms of a dynamical action for the active particles confined by the inactive particles. Our study brings together the physics of glass, jamming, plasticity and solid nucleation.
We study the nucleation dynamics of a model solid state transformation and the criterion for microstructure selection using a molecular dynamics (MD) simulation. Our simulations show a range of microstructures depending on the depth of quench. We clo
sely follow the dynamics of the solid and find that transient {em non-affine zones} (NAZ) are created at and evolve with the rapidly moving transformation front. The dynamics of these plastic regions determines the selection of microstructure. We formulate an {it elastoplastic model} which couples the elastic strain to the non-affine deformation, and recover all the qualitative features of the MD simulation. Using this model, we construct a dynamical phase diagram for microstructure selection, in addition to making definite testable predictions.
