Mechanistic insights into the dual role of NbC in strength-toughness synergy of nuclear grade Fe-Ni-based alloys

The safe operation of Generation ⅠⅤ (Gen-ⅠⅤ) nuclear reactors demands stringent dynamic impact properties in Fe-Ni-based alloys. Their solution-treated microstructure consists of γ matrix and NbC, with the latter critically influencing toughness. However, the role of NbC under impact loading remains poorly understood. In this work, an innovative strategy is proposed to regulate the NbC size and amount by adjusting the product of Nb and C mass fractions, with the resulting NbC characteristics systematically investigated for their effects on impact response and deformation mechanisms. NbC size strongly governs the crack initiation energy (Wi), through the competition among Orowan strengthening, matrix softening, and the crack-promoting effect of coarse NbC, giving rise to an optimal intermediate size that maximizes toughness. Furthermore, under optimized NbC size, a moderate increase in NbC amount significantly enhances the crack propagation energy (Wp). High-resolution observations illustrate that NbC promotes the formation of abundant deformation substructures in its vicinity, including stacking faults, Lomer-Cottrell locks, and deformation twins. These planar structures effectively impede dislocation motion, provide ample room for dislocation multiplication and storage, and sustain strain hardening, while also activating multiple slip systems to improve local plasticity near the V-notch. The synergistic effects resulting from the optimized NbC characteristics enable the alloy to achieve an excellent strength-toughness balance, with a yield strength of 226 MPa, ultimate tensile strength of 585 MPa, and impact toughness of 456 J/cm2. These findings provide important guidance for designing strong yet tough Fe-Ni-based alloys via NbC regulation for applications in Gen-IV nuclear reactors.