The mechanism of dynamic strain aging for type A serrations in tensile curves of a medium-Mn steel

Abstract

The objective of the present study is to clarify the mechanism of dynamic strain aging (DSA) causing serrations in tensile flow curves of Fe-5.15Mn-0.15C-0.37Si-0.0039N (wt%) medium-Mn steel specimens with triple phases of retained austenite (γR), ferrite (α) and tempered martensite (α'T). For the purpose, tensile tests were performed at various conditions of deformation temperature (Td = 273 - 333 K) and initial strain rate (έini = 5 × 10−4 - 1 × 10−2 s−1). The medium-Mn steel specimens revealed type A serrations after the propagation of the Lüders band in their tensile curves. The serrations were not related to both α and α'T; they were not caused by strain-induced martensitic transformation, but by DSA in γR. The DSA was not explained by the short-range diffusion model based on the interaction between partial dislocations and C-Mn complexes due to the absence of intersection between staying time and reorientation time. In a viewpoint of the dislocation arrest model involving long-range diffusion, critical strains of γR for serrations (ecγ) were measured. They showed the normal Portevine-Le Châtelier behavior that the ecγ value decreases with increasing Td and with decreasing έini. The activation energy measured using the ecγ values was similar to the activation energy for the dislocation pipe diffusion of C atoms. This result indicates that the DSA occurring in the present medium-Mn steel is explained by the dislocation arrest model involving the long-range pipe diffusion of C atoms, not by the short-range diffusion model involving the reorientation of C-Mn complexes.