Mechanical properties and deformation behaviour of TWIP steel at different strain rates

The mechanical properties of TWIP at different strain rates ranging from 5 s(-1) to 500 s(-1) were investigated by a VHS 160/100-20 high-speed tensile testing machine. The Johnson-Cook (J-C) dynamic constitutive model of TWIP steel was established based on the test results and verified by numerical simulation. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to analyse the microstructures of TWIP steel before and after tensile deformation. The findings demonstrate that under the tested strain rate, both the tensile strength and yield strength of TWIP steel increase with increasing strain rate, whereas the elongation decreases first and then increases. The strain rate of 100 s(-1) is the turning point of the mechanical properties. The stack fault energy (SFE) is 26 mJ m(-2) at room temperature, and the microstructure of TWIP steel before and after deformation is composed of single austenite, which demonstrates that only TWIP effect occurs. Dislocations accumulate along grain boundaries, forming structures such as dislocation cells and dislocation walls, causing stress concentration and promoting the formation of microcracks. With increasing strain rate, the dislocation cells are more obvious, and the thickness of the deformation twins decreases, resulting in an increase in the stress required for dislocations to cross the twin boundaries and material strengthening. The formation of nanoscale twins can not only effectively hinder the dislocation motion, but also provide space for dislocation slip and change the grain orientation, so that the slip system which is not conducive to deformation has a new orientation, resulting in a synergistic effect on strength and plasticity of TWIP steel. This work elucidates the relationship between the microscopic deformation mechanism and the macroscopic mechanical properties of TWIP steel at a specific strain rate, and a dynamic constitutive model is established, which can provide a reference for the application of TWIP steel in the design of anti-explosion and anti-impact structures.