Microstructure of low-dose neutron-irradiated Si3N4 and SiAlON ceramics after thermal annealing

• This is the first HREM observation for silicon nitride-based ceramics polymorphs.
• Microstructure change of irradiated Si3N4 and SiAlON polymorphs were observed.
• No dislocation loops in grains after thermal annealing were observed.
• There was no voids/bubbles formation along grain boundaries.
• HREM micrographs confirmed that defects were annihilated by thermal annealing.

Four kinds of silicon nitride-related ceramics were neutron irradiated up to 8.5 × 1024 n/m2 (E > 0.1 MeV) at 563 K. Induced swelling were almost recovered by thermal annealing up to 1473 K. Microstructure of the as-irradiated silicon nitride-based ceramics and those of after thermal annealing up to 1123, 1223 and 1473 K were observed with a high-resolution transmission electron microscope (HREM) in order to clarify their feature and thermal stability. HREM microstructures of the as-irradiated specimens revealed that the atomic configuration projected onto the basal plane was similar as a perfect crystalline arrangement without lattice distortion. It was confirmed that only point defects or point-like defects were generated during the neutron irradiation of the present condition. In addition, microstructure analysis of thermally annealed specimens did not reveal any voids along grain boundaries and triple junctions. Furthermore, formations of dislocation loops were not confirmed after thermal annealing up to 1473 K, in all Si3N4 and SiAlON polymorphs.