Mitigation of helium bubble formation in 12Cr-ODS steels: Comparative insights with non-ODS alloys

The effect of Y-Ti-O nano-oxides on helium bubble formation in 12Cr oxide dispersion strengthened (ODS) steels was investigated through direct comparison with a composition-matched Fe-12Cr model alloy. Both materials were recrystallized to produce nearly defect-free initial microstructures, enabling assessment of interactions between nano-oxides and helium bubbles with minimal influence from pre-existing dislocations or heterogeneous sink structures. Specimens were irradiated with 160 keV He⁺ ions at 400°C, yielding a peak displacement damage of 1.6 dpa and a peak helium implantation concentration of 4.0 at.%. Cross-sectional TEM revealed that nano-oxides suppressed helium bubble nucleation and growth, leading to lower bubble number density and smaller mean bubble size, and reducing helium-bubble swelling by ∼70% relative to the Fe-12Cr alloy. Helium irradiation also induced nano-oxide evolution, characterized by partial dissolution of larger particles and reprecipitation of finer (<3 nm) oxides, with the most pronounced dissolution occurring in regions with a steep dose-rate gradient. The depth-dependent nano-oxide sink strength showed a strong correlation with the measured swelling reduction, indicating a controlling role of nano-oxides in trapping helium and vacancies and delaying bubble development. Nano-indentation further confirmed reduced irradiation-induced hardening in the 12Cr-ODS steels. Overall, these results demonstrate the key contribution of Y-Ti-O nano-oxides to enhanced helium tolerance and swelling resistance in ferritic steels under helium irradiation.