Reduced activation ferritic/martensitic (RAFM) steels have been developed as candidate structural materials for blanket modules of nuclear fusion reactors. Helium atoms are generated in RAFM steels through nuclear interactions of host atoms with the incident 14.1 MeV neutrons from the fusion reactions. The generated helium atoms diffuse and combine with vacancies. This eventually leads to helium bubble formation and causes volumetric changes (swelling) in RAFM steels. The mechanisms of helium bubble formation are still under investigation, and these need to be studied more if the effects of various external environmental conditions are to be explained. In this study, we present the effect of stress and post-irradiation annealing temperature on the surface swelling induced by helium ion implantation for various RAFM steels. 

The surface of pre-stressed RAFM steels was masked with a square mesh transmission electron microscopy grid during 160 keV helium ion implantation with a dose of 0.5{\times}10¹⁷ and 1{\times}10¹⁷ ions/cm² followed by post-implantation annealing for 2 h at 300 °C in an argon atmosphere heat treatment furnace. Aligned upheaved surface patterns corresponding to TEM-grid meshes are formed and measured by a contactless 3D surface profiler, which has a vertical resolution of 0.1 nm using white-light scanning and phase-shift interferometry. The surface swelling is found to increase as the stress magnitude increases. Moreover, tensile stress induces greater surface swelling compared to compressive stress. The difference in the microstructures of the RAFM steels affects the helium bubble formation and the surface swelling as manifested by TEM observation. The effect of post-implantation annealing temperature is also studied.