Mechanistic exploration of high strain-hardening and TWIP effects in Fe-15.5Mn-0.6C-1.4Al steel under compression-tensile loading

Abstract

This study investigates the effects of large pre-compression deformation on strain-hardening and the twinning-induced plasticity (TWIP) effect in high-manganese steel, addressing a critical limitation of traditional TWIP steels, i.e., relatively low yield strength. Using advanced ex-situ electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), we reveal the roles of nanotwins, high-density dislocations, and substructure evolution in enhancing the mechanical response under complex strain paths. Results indicate that large pre-compression significantly elevates yield and ultimate tensile strength while preserving elongation, a unique strength-ductility synergy rarely achieved in pre-strained steels. The intricate coexistence of high-density dislocation and nanotwin microstructural networks under significant pre-deformation complicates dislocation slip pathways, contributing to a unique strength-ductility balance and enhancing work-hardening capability. Changes in strain paths activate new deformation twins, which, being dynamically nucleated, introduce new interfaces and alter the crystallographic orientations, thereby enhancing the material's dislocation storage capacity and maintaining a high work-hardening rate. Pre-compression-induced heterogeneous microstructure exhibits significant hetero-deformation-induced (HDI) hardening during tensile loading, enhancing tensile strength, delaying necking, and improving deformation stability. Cross-slip in fine-grained regions (FGs) promotes dislocation interaction and the formation of robust dislocation networks, further improving the strain-hardening capability of the steel. Finally, a parametric model is proposed to quantify the synergetic contributions of twins, grain boundaries (GBs), dislocations, and HDI-hardening in optimizing the properties of pre-strained steel, providing a foundational understanding of TWIP steel behavior under varying strain path loading conditions. These insights advance the fundamental principles governing TWIP steel deformation, supporting the development of high-performance Fe-Mn-C-Al alloys for automotive applications.