On the origin of the barrier in the bainite phase transformation

The experimental lengthening kinetics of bainitic ferrite in steels have been consistently shown in the literature to be slower than those predicted using the diffusional Zener-Hillert model. Reconciliation of the experimental and calculated kinetics has required the introduction of a ‘barrier energy’ which until now has not had a clear physical meaning. A modified diffusional growth model based on the Zener-Hillert model for plate-like ferrite growth is introduced with a physics-based barrier energy. The model assumes diffusion controlled growth and takes into account a barrier energy arising from interfacial disconnections motion and their interaction with defects in the matrix. The model is able to successfully describe the C-curve growth rate of Widmanstätten ferrite and bainitic ferrite in a wide range of steels.