Microstructure and mechanical properties of heat-treated and neutron irradiated TRISO-ZrC coatings

Six developmental sets of as-fabricated and heat-treated, near- and hyper-stoichiometric ZrC coated TRISO particles were subject to fast neutron (E > 0.1 MeV) fluences of 2 and 6 × 1025 neutrons/m2 at 800 and 1250 °C to assess the effects of irradiation on the coating microstructure and mechanical properties. Pre-irradiation microstructural analysis showed that the all but one of the near-stoichiometric samples fabricated by CVD had a homogenous grain structure while others including the hyper-stoichiometric sample had a distinct tiered band pattern with alternating carbon rich interlayers. The band structure in the near-stoichiometric samples became prominent following the heat treatment and the homogenous grained sample underwent severe grain growth. Post-irradiation observations indicated that neutron irradiation did not have any significant effects on the bulk microstructure of any of the samples regardless of the stoichiometry. Post-irradiation softening and reduction in modulus at the highest dose (6 dpa) were observed in all samples regardless of the composition and structure but were less significant in specimens with a banded microstructure. It was concluded that the carbon interlayers which contributed to the formation of the band structure had played a role in preserving the microstructure and the mechanical properties following both heat treatment and irradiation.