Phase-field simulations of pearlitic divergence in Fe-C-Mn steels

Anomalous divergence of the pearlitic lamellae is typically observed during isothermal eutectoid transformation when the steel composition falls in the three-phase (austenite+ferrite+cementite) region. Here, the transformation progresses such that the composition of austenite changes continuously in the region ahead of the growing pearlite thus necessitating volume diffusion of Mn and resulting in a temporal increase in the interlamellar spacing as the growth kinetics becomes sluggish. In the present work, we develop a CALPHAD-informed multicomponent multiphase-field model to simulate the morphological evolution of pearlite in ternary Fe-2.46at.%C-3.50at.%Mn and Fe-2.51at.%C-5.40at.%Mn steels. Our phase-field simulations of lamellar growth capture the complex physics of multicomponent diffusion while provides in-depth insights into the mechanism of pearlitic divergence. Numerical investigations suggest that a temporal decrease in the driving force which is otherwise necessary to support manganese diffusion from the ferrite to cementite leads to lamellar divergence. Present investigations while showcasing the capabilities of our numerical approach enable the prediction of divergent pearlitic microstructure for a range of compositions and heat treatment cycles.