• Cross-sectional TEM quantifies He-bubble size, density, and bubble-volume-fraction swelling, enabling calibration of surface-step-based swelling in RAFM steels. • Bubble nucleation is most pronounced at 300–400 °C, while coarsening at 500 °C drives a sharp swelling increase; step height tracks relative swelling trends, whereas nanoindentation hardening mainly reflects bubble density and obstacle spacing. • T730 exhibits the highest swelling resistance due to its higher M 23 C 6 /MX precipitate fraction (enhanced He trapping), and sequential implantation reveals order-dependent He–H synergy, stronger for He + H than for H + He. In fusion-relevant environments where transmutation helium accumulates, He bubble formation can become a critical damage mechanism in reduced-activation ferritic/martensitic (RAFM) steels because it drives swelling and can degrade mechanical performance. Swelling is often assessed indirectly from surface-step height measurements and nanoindentation mapping, yet their quantitative relationship to the underlying bubble population remains unclear. Here, He bubbles in K-RAFM steels and a Eurofer97-like reference steel were quantified by cross-sectional TEM after 160 keV He implantation and post-irradiation annealing (PIA) at 300–500 °C. Bubble size distributions, number densities, and bubble-volume-fraction-based swelling were evaluated and directly compared with step-height- and hardness-based metrics. Bubble nucleation was most evident at 300–400 °C through an increase in number density, whereas pronounced coarsening at 500 °C led to a sharp increase in swelling. Step height reproduced the temperature-dependent swelling trend, while the nanoindentation hardening response was more closely linked to bubble number density than to bubble size. In addition, sequential H/He implantation revealed a clear He–H synergy, with a stronger enhancement of bubble density and swelling when H was implanted after He (He + H) than in the reverse order (H + He).
Kim et al. (Sun,) studied this question.