Interpretation of cryogenic-temperature Charpy impact toughness by microstructural evolution of dynamically compressed specimens in austenitic 0.4C-(22-26)Mn steels

In this study, the Charpy impact toughness of three austenitic high-Mn steels was evaluated at room and cryogenic temperatures, and interpreted by deformation mechanisms in relation to the microstructural evolution of dynamically compressed specimens. Under dynamic compressive loading, nanocell structures composed of subgrains were formed by the reaction with twins and dislocations, and resulted in a high-strain-rate deformation mechanism that enhanced the strength, ductility and toughness within the stacking fault energy (SFE) range of the twinning-induced plasticity (TWIP) mechanism at room temperature. At cryogenic temperature, the formation of nanocell structures was activated with increasing Mn content, which showed the opposite trend to the room-temperature case. Since the cryogenic-temperature SFEs were lower by ∼30% than the room-temperature SFEs, a considerable amount of ε-martensite was formed in the 0.4C-22Mn steel by the transformation-induced plasticity (TRIP) mechanism, while the TWIP mechanism was working, thereby leading to increased Charpy toughness compared to the 0.4C-24Mn and 0.4C-26Mn steels. The Charpy impact toughness results were discussed using a new schematic diagram of deformation mechanisms based on SFE, loading condition and test temperature.