Modelling of dislocation-induced martensitic transformation in anisotropic crystals

The structural transformation caused by dislocation-induced heterogeneous nucleation in the fcc?→?bcc martensitic transformation in elastically anisotropic crystals is investigated by using the phase field microelasticity model. The three-dimensional microstructure of the dislocation-induced martensitic embryos is obtained. It is found that the embryos are not single-domain particles as is usually assumed but rather a complex self-organized assemblage of stress-accommodating twin-related microdomains. Sessile metastable martensitic embryos around the dislocation loops form in the prototype Fe–Ni alloy system above the temperatures of the martensitic transformation. A possibility that the presence of these pre-existing embryos could be responsible, at least, for a part of the elastic modulus softening with the temperature decrease observed in many martensitic systems is discussed. The effects of elastic anisotropy, the “chemical” energy barrier and structural anisotropy of the Landau free energy on the formation and growth of martensitic embryos are investigated. The assumptions of elastic isotropy and a choice of the anisotropic term in Landau polynomial do not significantly affect the microstructure of martensitic embryos but may appreciably change the undercooling that is necessary to eliminate the total nucleation barrier and start the athermal martensitic transformation.