Quasi-in-situ analysis of TRIP behaviors in shear zones of high-manganese steel specimen under dynamic compression

Owing to martensitic transformation during deformation, high-manganese transformation-induced plasticity (TRIP) steels show an excellent combination of strength and ductility. They are considered as second-generation automobile steels. Because of the influence of strain rate, the TRIP behaviors of high-manganese steels may be different during dynamic and static compressions. Therefore, it is necessary to study the TRIP behaviors during dynamic deformation. Based on the research on the TRIP behaviors of high-manganese steel at low strain rates, in this study, the TRIP behaviors were evaluated at high strain rates. Given the special shape of hat-shaped specimen and fixed position of shear zone, the grains present in the shear zone of high-manganese steel before and after dynamic compression were quasi-in-situ characterized using the electron backscattering diffraction (EBSD) technique. Besides, the phase distribution of grains in different locations of shear zone was analyzed. In addition, finite-element simulations and stress calculations were conducted using the ANSYS/LS-DYNA and MATLAB softwares, respectively, to further analyze the differences in the phase transformation of each grain. The results show that the combined action of stress and strain, orientation of austenite, and the interactions among grains influences the TRIP behaviors. The higher the stress and strain the easier the phase transformation. Because of the existence of shear stress in hat-shaped specimens, phase transformation is more likely to occur in austenite with orientation along〈100〉 and〈110〉than austenite with orientation along〈111〉, and phase transformation is most likely to occur in austenite with orientation along〈110〉. Moreover, the phase transformation behavior of austenite with a favorable orientation and large grain size will be less affected by neighboring grains and easier to achieve a complete phase transformation. However, the phase transformation of striped grains with a beneficial orientation will be constrained when the phase transformation of neighboring grains is difficult. Grains with sharp corners easily undergo phase transformation because of stress concentration. If the shear stress of twinning is larger than that of slip, but the largest and second largest stresses are almost equal, both the twin systems may compete with each other and phase transformation becomes difficult. Martensitic transformation often occurs near the grain boundary where the stress concentration is severe during dynamic compression but rarely in grains. α'-M has a shape of thin sheet, and its variant selection is obvious.