Defect-induced heterogeneous transformations and thermal growth in athermal martensite

A continuum model is proposed to explain defect-mediated heterogeneous martensitic transformations. As an example, the square-rectangular proper ferroelastic phase transition is considered. The role of a defect is considered to provide an inhomogeneous stress field acting on the pure system. This stress field modifies the free-energy density and has the strongest effect around (at) the defect. Certain external stresses of the correct symmetry can increase the transformation temperature, and the portion of the system around the defect, and only this portion, will therefore have a higher transition temperature. Thus, these regions can be thermally triggered first to become nuclei of martensite upon cooling. The temperature dependence of the equilibrium size of martensite in the matrix of the high-temperature phase is modeled and calculated for a coherent interface under free-boundary conditions, in which a simple functional form is assumed to simulate the stress field produced by a slab inclusion and/or defect. The results qualitatively agree with experimental observations.