Phase field microelasticity modeling of heterogeneous nucleation and growth in martensitic alloys

The phase field model is employed to study the structural mechanism of heterogeneous nucleation in the face-centered cubic to body-centered cubic martensitic transformation. The transformation is triggered in the undercooled parent phase by defects of varying potency, a dislocation loop or a group of loops. It is found that the metastable homogeneous state of the parent phase in this case is not necessarily stable even if the undercooling is small and the potency of defects is low – the parent phase transforms around the defects, forming a sessile metastable martensitic embryo. When the undercooling and the defect potency reach a critical value, the embryo loses its metastability with respect to the barrierless growth until the transformation is complete. This growth corresponds to an athermal martensitic transformation. It is shown that both sessile and growing martensitic embryos are not a single-domain particle as is usually assumed but rather an assemblage of twin-related domains. The modeling does not impose any a priori constraint on the possible microstructure of a martensitic nucleus.