Formation and regulation mechanisms of nano-oxides in cast 9Cr-ODS steel containing ternary rare earth elements

To address the challenge of oxide precipitation regulation in the large-scale fabrication of oxide dispersion strengthened (ODS) steels for the first wall of fusion reactors, the introduction of multiple rare earth (RE) elements is considered a promising strategy. In this study, we proposed a novel ternary RE composite strategy, and systematically investigated its effects on the formation of nano-oxides in cast ODS steels, as well as its regulatory mechanisms. It was found that the Y-Pr-Sc and Y–Ce-Sc additions promoted the formation of binary composite RE oxides such as (Y, Sc)₂TiO₅, (Y, Pr)₂TiO₅ and (Y, Ce)₂TiO₅, while a ternary composite RE oxide (Y, Ce, Pr)₂TiO₅ formed in the Y–Ce-Pr sample. Calculation analysis revealed that the electronic structure among atoms was the key factors determining the formation of binary or ternary composite RE oxides. Notably, the nano-oxides in the Y-Pr-Sc and Y–Ce-Sc samples were 30-50% smaller and 8-9 times more numerous than those in the Y-only sample. Their average sizes were 6.7 and 8.3 nm, and the corresponding number densities were 3.7 × 10²² and 1.5 × 10²² m⁻³. These enhancements were attributed to reduced nucleation energy barriers and smaller critical nucleation radius, which increased nucleation sites and suppressed coarsening. Additionally, size of (Zr, Ti)-rich carbides, Cr-rich carbides, and RE-rich oxides decreased by approximately half compared with those in the Y-only sample. The findings in this study provide important theoretical guidance for the steel composition and structural design in large-scale production of 9Cr-ODS steels.