Enhancing elevated-temperature strain hardening in heterostructured lightweight steel: Suppressing thermal instability of Lüders bands

Lightweight steels exhibit strength-ductility trade-off under elevated temperatures. Although fine-grained heterostructure designs offer a promising solution, the implementation is compromised by the emergence of unstable Lüders bands. In this study, we propose a synergistic strategy that integrates heterostructure architecture with pre-straining to enhance elevated-temperature performance and elucidate its underlying mechanisms. A fine-grained heterostructure (containing large austenite, small austenite, B2 bands, and B2 particles) was prepared via thermal-mechanical processing, exhibiting initial Lüders bands deformation. A pre-strained ( ε ≈5%) counterpart was subsequently fabricated with the elimination of Lüders bands, simultaneously introducing initial dislocation configurations. Tensile tests reveal that the pre-strained structure exhibits exceptional elevated-temperature tensile strengths of 1311 MPa (773 K) and 1107 MPa (873 K), coupled with substantially enhanced strain hardening. The improved hardening originates from: ( i ) elimination of the Lüders bands, avoiding plastic instability induced by damage accumulation; ( ii ) activation of strain bands facilitated by the notable hetero-deformation. In crystallography, the enhanced hardening stems from fine slip bands in austenite, whose formation is driven by high initial dislocation density and sustained back stress strengthening. This work provides a valuable strategy to design high-performance lightweight steels suited for elevated-temperature applications.