Atomic-scale mechanisms of Ni-modulated strength–ductility–toughness balance in 30CrMnSi martensitic steel

Achieving a favorable balance among strength, ductility, and toughness remains a major challenge for high-strength martensitic steels. Despite extensive studies on Ni-induced toughening in martensitic steels, atomic-scale investigations linking Ni–C interactions, C migration, and local shear response remain limited. In this study, 30CrMnSi martensitic steel and its Ni-alloyed counterpart were investigated by combining mechanical testing, fracture characterization, and density functional theory (DFT) calculations based on multicomponent special quasirandom structures. Solution-enthalpy and electronic-structure analyses reveal a pronounced repulsive tendency between Ni and neighboring interstitial C, with C becoming energetically more favorable in Fe-dominated local environments. Climbing-image nudged elastic band (CI-NEB) calculations further show that Ni modifies the local C migration-energy landscape and lowers the characteristic high-energy states along all three representative migration paths. Interplanar sliding calculations along the (110) [ path demonstrate that Ni reduces the maximum sliding energy by approximately 6.4%–10.4% across three representative C configurations, indicating reduced local shear resistance. Experimentally, Ni alloying results in a 6.9% reduction in tensile strength, whereas elongation and reduction in area increase by approximately 51% and 58%, respectively. The Charpy impact absorbed energy increases by approximately 81% at room temperature and 95% at −40 °C, accompanied by a more ductile fracture response. Microstructural characterization reveals finer prior-austenite grains and martensite laths, together with a lower apparent dislocation density, in the Ni-alloyed steel. These results suggest that Ni enhances plastic accommodation through complementary regulation of interstitial C behavior and local interplanar sliding, thereby improving the strength–ductility–toughness balance of martensitic steel.