Phase decomposition and bubble evolution in Xe implanted U₃Si₂ at 450^∘C

The microstructure investigations of a U₃Si₂ specimen implanted by 84 MeV Xe ions at 450^∘C are reported here. In the region that corresponds to the highest irradiation dose, U₃Si₂ was found to decompose into a Si-enriched nanocrystalline USi matrix phase and a U-enriched amorphous inclusion phase. Density functional theory (DFT) calculations were used to help understand the decomposition mechanism. Xe bubbles of different morphologies were observed in USi nano-grains, on USi grain boundaries, and phase boundaries. Original U₃Si₂ micro-grains were preserved on both sides of the irradiation damage peak region within the sample, implying a radiation dose threshold for decomposition of approximately 150 dpa at 450^∘C. In the preserved U₃Si₂ region beneath the irradiation damage peak, where significant amount of Xe ions were deposited, a monomodal size distribution of intragranular Xe bubbles with an average size of 2.68 nm formed. In both decomposed and preserved U₃Si₂, the size of Xe bubbles was found to be lower than 50 nm. Based on this observation, the fission gas behavior of U₃Si₂ is controllable and free of run-away swelling despite the occurrence of decomposition at this irradiation temperature.