Abstract: A novel oxide dispersion strengthened (ODS) steel with 9 wt. % Cr is designed to improve the performance of materials for applications in the advanced nuclear energy systems. After heat treatment, coherent core-shell nanoparticles consisting of Y₃TaO₇ as cores and VN as shells are formed. It contributes to an excellent balance of tensile strength of 1028 MPa and impact toughness of 87.5 J/cm² at room temperature, superior to most ODS steels fabricated by powder metallurgy. Even after exposure at an elevated temperature of 700 °C up to 15,000 h, the structure of core-shell, average size and number density of nanoparticles remain excellent stability, benefiting from the better inhibition of VN shells to element diffusion and lower interfacial energies between the coherent nanoparticles and matrix. The nanoparticles effectively impede the microstructural recovery by strongly pinning the movement of grain boundaries and dislocations. Thus, 9Cr-ODS steel exhibits outstanding stability of mechanical properties during long-term high temperature. These findings provide important insights into alloy design strategies for next-generation ODS steels intended for extreme environments, including high temperatures and intense neutron irradiation.

Key words: ODS steel, Nano-scale oxides, Core-shell structure, Strength and toughness, Thermal stability