| Abstract Scope |
Ferritic/martensitic (FM) steels are promising structural materials for advanced nuclear reactors. To reduce long-term radioactivity, reduced-activation ferritic/martensitic (RAFM) steels have been developed by substituting high-activation elements with low-activation alternatives. However, the impact of these alloying changes on helium bubble formation remains unclear. This study compares helium bubble formation in FM steel T91 and RAFM steel F82H, which contains lower concentrations of Ni and Si than T91. Both materials were sequentially irradiated with 100, 150, and 200 keV He ions to a total dose of 0.5 dpa and 9000 appm He at 500 °C. F82H exhibited larger helium bubbles with lower number density than T91. To investigate the effect of alloying elements, we characterized radiation-induced segregation near helium bubbles and grain boundaries. In T91, Ni-Mn-Si enriched clusters were observed near bubbles, whereas in F82H, only Mn-Si clusters formed. Cr enrichment was detected near grain boundaries in both steels but was absent near helium bubbles. These trends reflect differences in solute diffusion mechanisms and defect sink biases. Cr segregation at grain boundaries is primarily driven by interstitial-mediated diffusion. In contrast, helium bubble growth is governed by vacancy flux, and thus vacancy-mediated diffusion dominates around bubbles, explaining the lack of Cr enrichment there. Moreover, due to strong binding between vacancies and solute atoms, vacancy diffusivity can be reduced near these clusters. The more prominent Ni-Mn-Si clustering in T91 likely causes greater suppression of vacancy mobility and bubble growth compared to F82H. |