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

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 (.R), ferrite (alpha) and tempered martensite (a'(T)). For the purpose, tensile tests were performed at various conditions of deformation temperature (T-d = 273 - 333 K) and initial strain rate (epsilon(ini) = 5 x10(-4) - 1 x10(-2) s(-1)). The medium-Mn steel specimens revealed type A serrations after the propagation of the Luders band in their tensile curves. The serrations were not related to both alpha and alpha'(T); they were not caused by strain-induced martensitic transformation, but by DSA in gamma(R). 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 gamma(R) for serrations (e c.) were measured. They showed the normal Portevine-Le Chatelier behavior that the e(c)(gamma) value decreases with increasing T-d and with decreasing epsilon(ini). The activation energy measured using the e c.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. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

The study aims to clarify the mechanism of dynamic strain aging (DSA) causing serrations in Fe-5.15Mn-0.15C-0.37Si-0.0039N medium-Mn steel specimens. Tensile tests revealed that the serrations were caused by DSA in γ_R and not related to other phases. The results suggest that DSA in the medium-Mn steel can be explained by the dislocation arrest model involving long-range pipe diffusion.