The non-steady-state growth of divergent pearlite in Fe–C–Mn steels: a phase-field investigation

The eutectoid decomposition of austenite is generally analysed as a steady-state transformation. Although such a time-invariant framework is appropriate for binary systems, in ternary Fe–C–Mn alloys, particularly in the three-phase regime, a characteristic non-stationary equilibrium condition results in the formation of a unique microstructure, called 'divergent pearlite'. In the present work, the isothermal growth of the divergent pearlite, under different transformation temperatures; $605{}^{\circ }\text{C}$, $625{}^{\circ }\text{C}$ and $650{}^{\circ }\text{C}$, is investigated by adopting a phase-field approach which establishes local-equilibrium (LE) condition across the interface. Though most theoretical approaches intend to setup such condition, the current numerical technique elegantly recovers the non-stationary partitioning equilibrium (P-LE). The thermodynamical framework, which dictates this unique equilibrium condition, is introduced by incorporating the CALPHAD-based data. In addition to rendering the microstructure which is consistent with the observed divergent pearlite, the factors governing the characteristic kinetics and phase distributions are analysed. In complete agreement with the existing studies, it is recognised that the non-steady-state growth is induced by a proportional decrease in the matrix carbon-content, which reduces the transformation kinetics, by influencing the Mn partitioning driving force. The characteristic proportionality which exists between these governing factors is unravelled in the current investigation. Moreover, it is also identified that the transition from the non-steady-state evolution in three-phase regime to predominantly steady state in two-phase regime is continuous. In other words, at higher undercooling, a resolvable segment of time-invariant growth is observed in the initial stages, which is subsequently followed by the divergent evolution.