Analysis of strain hardening behavior of a high-Mn TWIP steel using electron microscopy and cyclic stress relaxation

Abstract

Electron microscopy and cyclic stress relaxation experiments were employed to study the dislocation-obstacle interactions and activated hardening mechanisms, during tensile straining of a high-Mn TWIP steel specimen, and to estimate the contributions of hardening mechanisms to the strain hardening behavior. Electron contrast channeling and high-resolution electron backscattered diffraction images showed that deformation twins and their twin boundaries affect the dislocation evolutions inside both the matrix and twin lamellae. High-resolution transmission electron microscopy suggested that twin boundaries (TBs) can be passed by dislocations through the dislocation multiplication mechanism resulting in the nucleation of sessile dislocations and alternated stacking faults at TBs. Consequently, high dislocation density was developed behind the TBs resulting in enhancement of the strain hardening rate through the composite hardening effect. Various thermally activated parameters determined through cyclic stress relaxation tests indicated that dislocation evolution, developed by TBs in terms of the composite effect, was the prevalent rate-controlling mechanism, imparting higher strengthening than TBs in terms of the dynamic Hall–Petch effect. Finally, the high nucleation tendency of Shockley partial dislocations at TBs was interpreted from the variation of the stress exponent and independent plastic strain rate, and also from the general stacking fault energy point of view.