Distinct He-induced damage evolution in nickel-based alloys irradiated at elevated temperatures

The nickel-based alloy Inconel 617 and Alloy 800H have been envisaged as the leading candidate for the higher-temperature Molten Salt Reactors (MSRs) due to their superior high-temperature properties. To evaluate their potential applications, both alloys were irradiated by He ion at 750 and 850 °C with corresponding He ion fluence. Same irradiation tests were performed on Hastelloy N alloy to study its microstructure evolution at elevated irradiation temperature, as well as being the reference alloy to reveal candidate alloys irradiation tolerance. Transmission electron microscopy (TEM) reveals the depth distribution of irradiation induced He bubbles and Frank loops. The TEM graphs and quantitative analysis shows that when irradiated at 850 °C, the formation of precipitated clusters lead to denser and boarder distribution of He bubbles in Inconel 617, while the bubble-loop complexes form and contribute to larger volume fraction of He bubbles in Hastelloy N alloy. The nanoindentation technique is used to measure their mechanical changes, and the results show that the Alloy 800H has the lowest degree of irradiation induced hardening when irradiated at 850 °C, whereas that of all alloys irradiated at 750 °C are basically similar. Further, the yield strength increment is calculated and deviations between experimental and calculated values are caused by the formation of precipitated clusters in Inconel 617 and the selection of smaller obstacle strength in Hastelloy N alloy. By comparison, the Alloy 800H demonstrates a greater potential to function as the structural material for higher-temperature MSRs due to smaller volume fraction and better resistance to irradiation induced hardening.