Phase-field model prediction of nucleation and coarsening during austenite/ferrite transformation in steels

A phase-field simulation is performed to study the kinetics of austenite to ferrite (γ → α) transformation in a low-carbon steel during continuous cooling. Emphasis is placed on the influence of nucleation, along with ferrite grain coarsening behind the transformation front, on microstructural evolution. Results show that grain coarsening is significant even before all nucleation has been completed and occurs via two different coarsening mechanisms, grain boundary migration and ferrite grain crystallographic rotation, both of which can be clearly observed occurring as the simulated microstructure evolves. For some grains, sudden growth jumps are predicted by the model—a phenomenon that has been observed before by synchrotron X-ray diffraction. This study quantitatively demonstrates the phenomenon that increasing cooling rate leads to nucleation off initial austenite grain boundaries, which is also verified by studying the morphology of ferrite grains as predicted using different nucleation mode assumptions. A relationship between nucleation site distribution and the nucleation rate is demonstrated by computer simulation.