Microstructural evolution and strength–ductility optimisation in austenitic low-density steel through repeated annealing

This study introduces a repeated annealing (RA) strategy to optimise the strength–ductility balance in an austenitic low-density steel with a composition of Fe-18Mn-6.5Al-0.75C (wt.%). This alloy was synthesised under a controlled atmosphere and cast into a copper mould. Subsequent hot-rolling and solution treatment (ST) yielded a coarse-grained austenitic matrix with low yield strength (YS) (435 MPa) but exceptional ductility (59%). Cold rolling (CR) of the ST condition induced significant grain refinement and elevated dislocation density, resulting in a remarkable increase in YS to 1397 MPa and the highest ultimate tensile strength (UTS) of 1438 MPa. However, this came at the expense of ductility, which dropped sharply to 4%. To mitigate this limitation, a novel RA strategy was employed on the CR material. This approach effectively reduced dislocation density and localised strain accumulation while promoting the development of a refined bimodal grain structure on the micron scale. The RA-treated alloy achieved a balanced combination of strength and ductility, with a YS of 635 MPa and elongation of 51%, significantly surpassing the tensile toughness (product of strength and elongation (PSE)) of both ST and CR conditions. The PSE values improved from 43.3 (ST) to 48.1 GPa% (RA) at a strain rate of 10 ⁻³ s ⁻¹ , underscoring the efficacy of the RA process in enhancing mechanical performance. Strengthening mechanisms were primarily governed by dislocation interactions in the CR state and grain boundary effects in the RA condition. Furthermore, the RA sample exhibited superior work-hardening capacity, achieving a UTS of 958 MPa. Fractographic analysis revealed ductile fracture characteristics at low strain rates, transitioning to mixed-mode fracture with a declining dimple fraction at higher strain rates. These findings offer a promising pathway for designing lightweight steels with superior mechanical performance through microstructure engineering, without relying on complex alloying or severe deformation routes.