Combining Nickel Alloying and Heterostructure to Overcome the Strength‐Ductility Tradeoff in High Nitrogen Austenitic Stainless Steel

High nitrogen austenitic stainless steels often exhibit excellent ductility but are accompanied by low strength. The intrinsic tradeoff between achieving high strength and retaining substantial plastic deformability has long constituted a persistent challenge. In this study, nickel (Ni) alloying in synergy with a multilevel grain heterostructure was exploited to resolve the strength‐ductility tradeoff. The annealed microstructure of 0Ni steel was predominantly austenite but interspersed with δ‐ferrite and σ phase, which rendered the steel prone to localized stress concentration and markedly reduced ductility. Ni alloying effectively stabilized the austenite and suppressed the formation of δ‐ferrite in 4Ni steel, thereby mitigating localized stress concentration and enhancing ductility. The anticipated Ni‐induced strength degradation in 4Ni steel was offset through the enhanced dislocation strengthening. The designed multilevel grain heterostructure within the 4Ni steel introduces a high density of grain boundaries along with a significant population of geometrically necessary dislocations (GNDs). This microstructure not only provides substantial grain boundary strengthening but also induces a pronounced heterodeformation induced (HDI) hardening effect. These synergistic mechanisms achieve an exceptional strength‐ductility combination.