Effects of High Strain Rate on Properties and Microstructure Evolution of TWIP Steel Subjected to Impact Loading

The mechanical properties of the TWIP steel subjected to impact loading at various strain rates were analyzed by the split Hopkinson pressure bar. Meanwhile the microstructure of the TWIP steel fore-and-aft dynamic deformation was oberseved and analyzed by optical microscope (OM), X-ray diffraction (XRD), and transmission electron microscope (TEM). The results show that when the TWIP steel was deformed under dynamic condition, the stress, microhardness and work hardening rate increase with the increase of strain and strain rate; there is decline of work hardening rate for adiabatic temperature rising softening. There are many pin-like deformation twins in the microstructure of the TWIP steel subjected to impact loading, and the grain size after deformation is bigger than that before; the interaction of twins with dislocation and twins with twins, especially emergence of multiple deformation twins are the main strengthening mechanisms of the TWIP steel.The nucleation mechanism of deformation twins lbe "rebound mechanism"; the incomplete deformation twins can be observed when the strain rate is low; when strain rate increases, deformation twins unite together; and deformation twins become denser because the nucleation rate increases with increasing the strain rate.