Combining Crystal Plasticity and Electron Microscopy to Elucidate Texture Dependent Micro-Mechanisms of Tensile Deformation in Lath Martensitic Steel

Abstract

A modified 9Cr-1Mo steel having lath martensitic microstructure has been subjected to the hot-rolling at three different temperatures followed by a normalization at 1025 °C to form different crystallographic textures after thermomechanical processing. The samples hot-rolled at 875 °C, 1000 °C and 1050 °C showed major texture components as Goss (i.e. {110}<001>), Cube (i.e. {001}<0‾10>) and Gamma (i.e. {111}<‾1‾12>), respectively. Next, these samples have been uniaxial tensile tested at quasi-static strain rate at room temperature, and tensile properties are evaluated. The results indicated almost similar strength levels for Goss and Cube oriented specimens, and significantly reduced strength for Gamma oriented samples. However, the Cube and Goss oriented samples showed different strain hardening rates owing to the occurrence of deformation induced twinning and anti-twinning phenomenon as revealed by the Visco-plastic self-consistent polycrystal plasticity simulations. Simulation results were validated with experimental observations using high-resolution transmission electron microscopy. Anisotropic parameters have also been simulated considering the difference in initial crystallographic orientations. Study of deformation micro-mechanism at different length scale of martensitic units (e.g., prior-austenite grain, martensitic packets, block, sub-block, and laths) revealed negligible rotations at the prior-austenite grain level, whilst the lattice rotations were found to be significant at martensitic sub-block length scale. The investigation indicated that some specific types of martensitic variants generally participated in large lattice rotation during deformation for differently textured samples.