Helium bubble formation in 800 MeV proton-irradiated 304L stainless steel and alloy 718 during post-irradiation annealing

The bubble formation in AISI 304L and Alloy 718 irradiated with 800 MeV protons and annealed at temperatures up to 1100 °C has been studied by TEM. The specimens were obtained from spent target components of the LANSCE facility at the Los Alamos National Laboratory. In this and other spallation sources high concentrations of helium are generated concurrent with the displacement damage. In the specimens irradiated with around 3×1025 p/m2 (≈8.4 dpa), first visible bubbles appeared at 700 °C in Alloy 718 and at 800 °C in 304L stainless steel, respectively. Two temperature regions with different coarsening mechanisms could be identified and interpreted as bubble migration and coalescence at lower temperatures and Ostwald ripening at higher temperatures. From the measured bubble densities and size distributions, He concentrations were determined and compared to values obtained by release and nuclear reaction measurements, respectively.