In situ TEM observation of the evolution of helium bubbles in Hastelloy N alloy during annealing

In situ transmission electron microscopy (TEM) was used to capture the dynamic evolution of helium bubbles in Hastelloy N alloy during annealing at 400 °C, 500 °C and 600 °C. The TEM results showed that intragranular helium bubbles hardly coarsened after annealing at 400 °C and 500 °C for 60 min. Although the formation of a new helium bubble was observed at the grain boundary for the sample annealed at 500 °C, these intergranular helium bubbles did not coalesce due to the low surface diffusion, the distance between helium bubbles and the misorientation of two grains. The high internal pressure of the helium bubbles calculated using the Carnahan–Starling hard sphere model illustrated that the migration and coalescence (MC) mechanism was more apparent in helium bubbles coarsening for the sample annealed at 600 °C. However, the MC mechanism was restricted by chromium atoms and dislocation lines, which caused the emergence of the suppressed MC mechanism.