Effect of strain rate and low temperature on mechanical behaviour and microstructure evolution in twinning-induced plasticity steel

The mechanical behaviour and microstructure evolution of twinning-induced plasticity (TWIP) steel subjected to tensile tests at high strain rate and low temperature were investigated. Under extreme conditions (3 x 103 s-1 and liquid nitrogen temperature), the TWIP steel exhibits high strain hardening capability and high strength, which are intimately related to deformation twinning. Here we investigate the plasticity and failure processes of a TWIP steel under extreme conditions, such as shock and/or cryogenic loading, and identify that deformation twinning no longer serves the fine plasticity. Obvious ductility loss has been found in TWIP steel even with strong strain hardening capability, which always leads to large uniform tensile ductility against local necking. In this study, the microstructure observations suggest that both of the second twin systems and nanotwin structures are highly active under extreme conditions. Twin-twin and dislocation-twin interactions accelerate the crack nucleation at and propagation along the twin boundaries. The conversion from twinning-induced plasticity to twinning-assisted cracking is controlled by twin structure and its spatio-temporal evolution, which are most susceptible to strain rate and temperature. These results provide an underlying micromechanism that controls the plasticity and failure in TWIP materials under extreme conditions and contributes to the understanding of twin structure-related processes to a broad community.