Microstructure of Ni and X-750 irradiated at low and high temperatures

Nickel-based alloys have been widely used in nuclear applications because of high-temperature strength, toughness and corrosion properties. Inconel X-750 is a nickel-chromium alloy precipitation-hardened by additions of aluminum and titanium to form gamma prime (γ’) precipitates. It is used as spacer material in the fuel channels of CANDU^®1 (Canada Deuterium Uranium) reactors, to separate hot pressure tubes (250-300 ^oC) and cold calandria tubes (60-80 ^oC). Understanding the microstructural stability at both low and high operating temperatures are important for evaluating material integrity and predicting in-reactor performance. In this paper, the microstructures of pure Ni and Inconel X-750 irradiated in the National Research Universal (NRU) reactor at 60 ^oC and 300 ^oC were studied. The results showed that dislocation loops, small dot defects and cavities were induced at both temperatures; their sizes increased with temperature whereas their densities decreased with temperature. Frank loops were created at both temperatures, but the pure Ni contained a much lower density of loops compared to Inconel X-750 under the same irradiation conditions. The higher density of Frank loops in Inconel X-750 was because of its lower stacking fault energy due to the addition of alloying elements such as Ti. The γ’ precipitates in Inconel X-750 were disordered at 60 ^oC but ordered at 300 ^oC. Nano-size carbide precipitates were observed at 300 ^oC but not at 60 ^oC. In summary, a clear temperature effect on the microstructural evolution, due to neutron irradiation at 60 and 300 ^oC, was observed, which is expected to lead to differences in performance for the materials at low and high temperatures.