The interaction between the displacive transformation and the diffusion process in the bainitic type transformation

The interaction between the displacive transformation and the diffusion process during the bainitic type transformation was studied using a phase field model incorporating both processes. A chemical free energy density function was derived to represent thermodynamic properties of low-alloy steels at high supercooling conditions at which the direct transformation from austenite to ferrite at a constant carbon concentration is possible. The results demonstrate that in such conditions both the displacive transformation and diffusional decomposition process may play important roles controlling the transformation kinetics and microstructure. A fast displacive transformation is dominant at the onset of the transformation; however, the diffusion-controlled decomposition can take control over the transformation kinetics and even the ferrite morphology at later stages as a result of the formation of a carbon enriched layer around ferrite grains. Both plate-like and rod-like shapes of ferrite grains can be obtained depending on the thermodynamic conditions and diffusion mobility.