Phenomenological Model for Deformation-Induced Ferrite Transformation

A phenomenological model is proposed to describe the conditions under which an ultrafine ferrite (UFF) microstructure forms in low-alloyed steels as a result of deformation-induced ferrite transformation (DIFT). Assuming that corner points in the dislocation cell substructure of microshear bands provide suitable nucleation sites for ferrite, the model predicts that UFF forms in low-carbon steels independent of the steel chemistry and is promoted by increasing the strain rate. Analyzing ferrite growth rates suggests that the temperature range for UFF formation depends primarily on a minimum interfacial velocity criterion that must be fulfilled. In addition to these general trends, a more quantitative comparison of the model with experimental results for a Mo-containing steel is provided. This comparison suggests ferrite growth rates during DIFT that are somewhat larger than during conventional cooling transformation. These enhanced growth rates can be attributed to the presence of fast diffusion paths.