Structural damage and its annealing response in neutron irradiated magnesium†

Transmission electron microscopy and immersion density techniques were used to characterize the nature and extent of radiation damage in zone refined and commercial purity magnesium irradiated to 1.7 × 1021 neutrons/cm2 (>0.1 MeV) at about 75°C. Swelling from radiation-induced voids was 1.9 and 0.7 per cent, respectively, for the high purity and commercial magnesium. This difference was caused by larger voids in the zone refined material. The voids had a non-equiaxed shape, being bounded by {0001}, {1010}, and {1011} and were flattened in ⟨0001⟩, but the larger voids were more equiaxed. There was pronounced clustering of voids on {0001} planes. The irradiation-induced dislocation structure was also clustered on the basal planes. This dislocation structure is deduced to consist of a complex tangle of 1/2 ⟨2023⟩ partial dislocations and their associated stacking faults. During postirradiation annealing, voids began to anneal out in 1 hr at 150°C and were eliminated in 1 hr at 320°C. The smaller voids as well as voids near grain boundaries disappeared first. Slow annealing of voids near some grain boundaries suggested that not all grain boundaries are equally good sinks for vacancies. Voids also became more equiaxed during annealing. Stacking faults were eliminated before the voids disappeared, and the dislocations farmed networks. Annealing treatments at 500 °C or higher caused the appearance of gas bubbles.