Probing the impact of grain interior and grain boundaries on the mechanical behavior of a high-Mn austenitic steel

Abstract

We elucidate here the grain orientation/grain boundary dependence on the deformation behavior of high-Mn steel via tensile tests. The ultra-fine grained/fine grained (UFG/FG) specimen was characterized by superior strength-ductility combination as compared to the as-received specimen. In order to establish the relationship between macro and micro/nano, nanoscale deformation experiments and post-mortem microscopy of the deformed region were conducted on near-defect-free (NDF) specimen, avoiding the influence of defects. Weak anisotropy in elastic modulus and hardness was observed for NDF specimen. The modulus and hardness of grain boundary were 163 GPa and 4.32 GPa, respectively, which were relatively greater as compared to the grain interior, revealing significant grain boundary effect. Indentations in the “grain interior” had highest frequency of ratio of c/d (c - the plastic zone radius, d - distance of the indent from the grain boundary) of ∼1.2, irrespective of the orientation the indents located, implying that the ratio is a property related to the grain boundary. The higher strengthening effect at the high angle grain boundary could be attributed to higher strain and geometry necessary dislocation density surrounded the grain boundary. The strain rate sensitivity (m) and activation volume (v) obtained from nanoindentation had weak dependence on grain orientation and v was ∼5–15 b3, indicating that neither diffusional creep processes nor conventional dislocation by-pass governs plastic deformation independently. With the increase of v, the deformation mechanism transfers from dislocation slip dominant to SFs dominant, and finally twining dominant.