Nucleation of solids in solids: ferrites and martensites

When a solid such as iron is cooled across a structural transition, its final microstructure depends sensitively on the cooling rate and the depth of quench. For instance, an infinitesimally slow cooling or a shallow quench results in an equilibrium "ferrite," while a rapid cooling or a deep quench gives rise to a metastable twinned "martensite." In this paper, we arrive at a single formalism which qualitatively describes the transformation to both a ferrite and a martensite. Fundamental to this understanding is our identification of the crucial dynamical role played by nonelastic degrees of freedom in determining the final microstructure of the product solid.