Mechanistic evolution of lamellar heterostructures in high-manganese TWIP steel: Annealing-driven recrystallization and bimodal grain formation

This study systematically investigates the annealing temperature-dependent microstructural evolution and strengthening mechanisms in a cold-rolled (50 % reduction) high-manganese TWIP steel (0.4C-23.8Mn-0.2Si-3.7Cr-0.5Cu). EBSD and TEM analyses reveal that annealing at 600 °C triggers preferential recrystallization nucleation along deformation twin interfaces, generating a bimodal lamellar heterostructure (1.32 ± 1.98 μm) characterized by fine recrystallized grains and recovered coarse grains. This bimodal distribution arises from divergent growth kinetics, where recrystallized grains nucleate while recovered grains coarsen, evidenced by a sharp decline in Σ3 boundary fraction (51.5 % → 38.6 %) with minimal change in grain orientation spread (7.08° → 5.98°). The heterostructure enables synergistic strengthening via hetero-deformation-induced (HDI) hardening at hetero-zone boundaries, which alleviates stress concentration and delays strain localization. Consequently, the 600 °C annealed sample achieves an optimal strength-ductility balance (yield strength: 1051 MPa, total elongation: 22 %), superior to samples annealed at other temperatures (400–800 °C). Nanoindentation analysis further quantifies substructure contributions, confirming dislocation-dominated hardening in nucleation-stage microstructures and validating the inadequacy of conventional Hall-Petch models for heterogeneous systems. The work establishes annealing at 600°C-700 °C as critical for activating bimodal lamellar heterostructures, providing a mechanistic framework to overcome strength-ductility trade-offs in TWIP steels.