Diffuse-interface approach to predicting morphologies of critical nucleus and equilibrium structure for cubic to tetragonal transformations

A cubic to tetragonal transformation leads to simultaneous nucleation and growth of several orientation variants. The size and morphology of a critical nucleus, comprising of one or more variants, are determined by the competition among bulk thermodynamic driving force, elastic strain energy, and interfacial energy. In this work, a diffuse-interface model is developed to predict both the critical nucleus morphology of a single variant tetragonal precipitate described by a conserved field and the critical morphology of a two-variant nucleus and equilibrium multivariant twinning microstructure resulted from a structural transformation described by non-conserved fields. A constrained string method was employed to compute the probable minimum energy paths and the microstructures along each path. Numerical experiments indicate that our approach works effectively for predicting the critical nucleus morphology both the precipitation processes and structural transformations.