Phase transformations in metallic materials have a major impact on vital engineering aspects of the material behavior such as ductility, strength and formability. Some phase transformations, such as the formation of pearlite and bainite, occur through diffusion-based processes where the constituents in the microstructure are redistributed. Being based on diffusion, these kinds of phase transformations tend to be relatively slow. On the other hand, phase transformations can also proceed by pure displacements in the crystal lattice structure. This is typical for the very rapid and diffusionless formation of martensite in austenitic steels by martensitic phase transformation.
A phenomenological model of martensitic phase transformation
Taking a continuum-mechanical perspective, a phenomenological finite strain plasticity model is established, treating the volume fraction of martensite as an internal variable. Along with a transformation condition, dependent on the state of deformation and on temperature, this allows the evolution of the martensitic phase to be traced. The presence of a transformation condition allows establishment of a transformation potential surface, much like the yield condition and yield surface found in plasticity theory.
Depending on which one is active, the yield and transformation conditions determine the response of the material. The relative influence of austenite and martensite on mechanical material properties is considered through a homogenization procedure, based on the phase fractions.
Extensions and applications of the phase transformation model
The original model has been extended in several steps, for example taking full thermo-mechanical coupling and high strain rate deformation into consideration. The model is suitable for large-scale simulations of metal forming processes involving materials exposed to martensitic phase transformation. It has been employed in studies of, for example, metal forming, surface treatment by laser peening and fracture.