Investigation on microstructure of Hastelloy N after irradiation by 30 keV He ions

Background: Hastelloy N alloys are selected as the structural materials of molten salt reactor (MSR). Few results have been reported about their microstructures after irradiation. Purpose: To investigate the microstructural evolution of Hastelloy N induced by irradiation. Methods: Samples were irradiated at 500℃ with 30 keV He ions to different doses. Then the microstructures were characterized by scanning electron microscope (SEM) and transmission electron microscopy (TEM). Results: Surface blisters were observed in all bulk samples. Black dot defects emerged in TEM sample irradiated at low dose (0.124 dpa). As the dose increased, dislocation loops and He bubbles were observed and the density of black dots decreased. When the dose got to 1.24 dpa, both the sizes and density of dislocation loops and He bubbles increased remarkably. Conclusions: Black dot defects, dislocation loops and He bubbles are induced by He⁺ irradiation at high temperature, the densities and sizes of which are related to the irradiation doses. Accumulation of He bubbles will result in blisters of alloys. (authors)