Real-time Observation of Stress-strain Behavior beyond Necking in Martensitic Steel by in-situ Synchrotron X-ray Diffraction

In general, the stress-strain relationship of materials obtained by standard uniaxial tensile test, which can identify the hardening behavior only up to necking. Beyond necking, the material behavior is usually estimated by extrapolating or numerical modelling based on hardening behavior prior to the uniform elongation. This study investigated the post-necking hardening behavior of a fully martensitic steel by in-situ synchrotron X-ray diffraction during tensile deformation. From the in-situ results, the dislocation density, lattice strain and phase stress were calculated within the necked region and outside the necked region. A near steady-state flow with some hardening was observed within the necked region of a martensitic steel. However, beyond uniform elongation, outside the necked region the dislocation density and phase stress decreased slightly, suggesting stress relaxation. Steady-state flow and dislocation densities at large strains suggest dynamic recovery occurs in the martensitic steel at room temperature. © 2024 The Iron and Steel Institute of Japan.