Droplet Fluctuations in the Morphology and Kinetics of Martensites

We present a mode-coupling theory which describes the morphology and kinetics of the nucleation of twinned martensite droplets within a parent crystal. This involves a ``slow'' vacancy field that lives at the parent-product interface. Our theory suggests that a slow quench from the parent phase produces an equilibrium product, while a fast quench produces a metastable martensite. In two dimensions, the martensite nuclei grow as strips having alternating twin domains, with well-defined front velocities. Several empirically known structural and kinetic relations drop out naturally from our theory.