Comparative analysis of helium bubble formation in 9Cr and 14Cr ODS steels implanted at different temperatures

Helium (He) accumulation and bubble formation pose significant challenges for the structural integrity of fusion materials. Oxide dispersion-strengthened (ODS) steels mitigate these effects by their finely distributed nano-oxides. This study investigates He bubble formation in 9Cr and 14Cr ODS steels implanted at 25 °C, 400 °C, and 600 °C. High-angle annular dark field (HAADF) imaging and electron energy-loss spectroscopy (EELS) were employed to analyze bubble nucleation sites and He retention mechanisms. Distinct He trapping behaviors are observed between two alloys. In 9Cr ODS, He bubbles consistently form at incoherent nano-oxide interfaces across all temperatures, also at grain boundaries (GBs). In contrast, 14Cr ODS shows temperature-dependent behavior: at 25 °C, coherent nano-oxides are inactive, and bubbles form mainly at GBs and within matrix; at 400 °C, coherent oxides become active traps, significantly reducing GB bubble density; at 600 °C, bubbles reappear at GBs alongside continued trapping at nano-oxides. EELS analysis of He atom density inside bubbles, compared with theoretical equilibrium models, reveals that in 14Cr ODS, most He bubbles in matrix at 25 °C and along GBs at 600 °C are supersaturated. In contrast, bubbles in 9Cr ODS remain near equilibrium at all temperatures. These findings highlight the impact of temperature and microstructural differences on He behavior. Incoherent nano-oxides are efficient traps but low in density, while coherent oxides are thermally activated and only effective over a narrow temperature range. These results provide new insights into the mechanisms of He trapping and their implications for the development of radiation-resistant ODS steels.