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The Mechanism of Dynamic Strain Aging for Type a Serrations in Tensile Curves of a Medium-Mn Steel

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Document pages: 49 pages

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-5.1Mn-0.15C (wt ) medium-Mn steel specimens with triple phases of retained austenite (γR ), ferrite (α) and tempered martensite (α T ). For the purpose, many 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.

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