Microstructure evolution and fracture behaviour of TWIP steel under dynamic loading

Exploring the dynamic mechanical behaviours of twin-induced plasticity steels supports the safety design of which some critical parts and structures are made. In this work, the microstructure evolution and fracture behaviours of TWIP steel at 700–3000 s⁻¹ were investigated by split Hopkinson tension bar and SEM-EBSD characterization. The results reveal that the positive strain rate sensitivity at the early stage of strain is attributed to the promotion of twinning and dislocation multiplication. With the increasing strain rate, the adiabatic temperature rise (∼110 K) increases the stacking fault energy to more than 50 mJ m⁻², so that the critical shear stress for twinning is higher than the critical shear stress for slip at fracture, resulting in the suppression of twinning and the rapid decrease of the strain hardening rate. The accumulation of dislocations near grain boundaries or twin boundaries facilitates the strain localization to further form the high dislocation density structures and the interlaced twin structures which may be the onset sources for nucleating microcracks. The transgranular cracking was shown from the side of the specimen by optical microscopy. The frontal observation of the fracture morphologies by SEM indicates that the transformation of typical ductile fractures with dimples into quasi-cleavage fractures with river patterns may be caused by the enhanced local stress. The present work elucidates the deformation mechanisms and fracture behaviours at high strain rates, providing a new perspective for further understanding the competition between the twinning and dislocation slip and the intrinsic mechanism of ductility loss under dynamic conditions of TWIP steels.