Abstract: Dynamics of Portevin-Le Chatelier (PLC) banding were thoroughly revealed through grain refinement in high-Mn austenitic steel. Fully recrystallized 22Mn-0.6C steels (wt%), with mean grain sizes ranging from 8.4 µm (coarse grain, CG) to 0.82 µm (ultrafine grain, UFG), were successfully fabricated through repeated cold rolling and subsequent annealing. Serrations were observed on the global stress-strain curves of all specimens, while the UFG specimen showed notably weak serrations in the early stages of deformation. Using the digital image correlation (DIC) technique, the serration behavior on the stress-strain curves of all specimens was found to perfectly correspond to the local heterogeneous deformation characterized by the formation, propagation, and annihilation of PLC bands. With grain refinement, localized deformation within the PLC band decreased, while the deformation beyond it became more pronounced, resulting in more homogeneous deformation across the entire gage part. This study presents the first experimental evaluation of the local strain-hardening rate through the simultaneous application of in-situ synchrotron XRD and DIC measurements during the tensile test. It was found that necking was periodically triggered but was soon overcome through the high strain hardening within the propagating PLC bands, which was achieved through the localized multiplication of dislocations and the formation of stacking faults. The UFG specimen exhibited greater local strain hardening than the CG specimen, induced by the formation of more dislocations and stacking faults. Such an increase in local strain hardening with grain refinement resulted in more homogeneous deformation behavior and consequently weaker serrations in the early stages of deformation.

Key words: Serration, Portevin-Le Chatelier (PLC) band, Digital image correlation (DIC), In-situ synchrotron XRD diffraction, Grain refinement, Local strain hardening