Genesis of plasticity-induced serrated metal flow in medium-Mn steel

Serration is a complex issue in medium-Mn steel, associated with a jerky metal flow and deformation banding. The problem has been well debated over the decades. In order to investigate the reason, a medium-Mn steel is prepared by hot-rolling in the austenitic domain, followed by air cooling. This results in martensitic microstructure due to high hardenability provided by adequate austenite stabilizers (C, Mn, Ni) in the steel. The intercritical annealing was performed for martensite to austenite reversion at 650 °C for different durations. From solute partitioning kinetics estimated by Dictra simulation, it appears that martensite released C-atoms, but the product austenite could not occupy them into octahedral under thermodynamic and kinetic constraints. This leads to an undissolved state of C, disowned by both martensite and austenite. With an enhanced diffusivity by intercritical annealing at 650 °C, those undissolved C-atoms without a bonding restriction repeatedly interact with sample dislocations, acquired from the rolling process and martensite. The interaction minimizes elastic energy by forming numerous C-dislocation aggregates into clusters, as the building block of Cottrell atmospheres. In metal plasticity theory, the Cottrell atmosphere usually dictates at room temperature. This work suggests it at an elevated temperature to enlighten hitherto unexplored mechanism of the dynamical C-dislocation interaction behind serrations. Additionally, austenite inhomogeneity by a lack of redistribution of Ni and Mn at 650 °C is found to be responsible for a staircase-wise propagation of the jerky metal flow.