Abstract
Beryllium, irradiated at around 50 °C for 15 years up to a fast neutron fluence (>1 MeV) of 4.67 × 10 22 n cm −2, was annealed at temperatures of 500, 750, 825 and 900 °C. In a previous study, the influence of the annealing temperature and time on the He content and the microstructure was investigated with optical and scanning electron microscopy. To complement the observations made, transmission electron microscopy measurements on the same set of specimens were conducted to determine the helium behaviour at nanometer scale. It was found that the as-irradiated material is heavily stressed and contains many dislocation loops. In the samples annealed at 500 and 750 °C, these dislocation loops disappeared and a large number of small faceted bubbles have formed inside the grains. Close to grain boundaries, a bubble-free area was observed. Annealing at 825 and 900 °C results in the complete removal of all small bubbles, and the local defect structure reverts back to the one of the un-irradiated material.
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