Phase Field Modelling of Bainitic Transformation

A phase field model was developed to study the interaction between the displacive transformation, diffusion process and carbide formation during the bainitic type transformation. The thermodynamic data for the chemical free energy were obtained using the Thermo-Calc software. Also, the transformation strains reproducing the γ → α transformation with three transformation variants of ferrite were used. In the case of carbide free bainite, the bainitic transformation develops in two stages: the first, fast stage, wherein the displacive transformation is dominant at the onset of the transformation, and the second stage, which begins when the diffusion-controlled decomposition takes control over the transformation kinetics and ferrite morphology, and results in the formation of a carbon enriched layer around ferrite grains. Both plate-like and rod-like shapes of the ferrite grains can be obtained depending on the thermodynamic conditions and diffusion mobility. The carbide nucleation was modeled as the formation of carbon sinks either in retained austenite or in supersaturated ferrite with different nucleation sequences. The results indicated that in the case of carbide formation in retained austenite, the carbide nucleation might play a secondary role controlling the transformation kinetics and microstructure evolution after the completion of the fast transformation stage; however, elastic interactions could control the ferrite morphology even at a later stage, thus leading to the change of the ferrite grain shape from a rod-like to plate-like. In the case of carbide formation in supersaturated ferrite, the results demonstrated that initially, due to a slow nucleation of carbides and fast carbon partitioning, carbon diffusion to austenite was the dominant process leading to the decrease in the transformation rate. However, after some period of time, the effect of carbon depletion by carbides in the solid solution became more significant, which led to the reduction of the average concentration of carbon in the whole system and to the acceleration of the structural transformation. In both cases, the rate of carbide nucleation and the number of nucleated carbides controlled both the transformation rate and the period of time before the transformation acceleration.