Background: Hastelloy-N alloy is selected as the structural material for molten salt reactor (MSR)

however

it is well-known that helium atoms play an important role in nickel-based materials after severe neutron irradiation

since they can drastically alter mechanical properties

potentially embrittle materials even at low concentration. Purpose: This study aims to understand the hardening of Hastelloy-N alloy material caused by helium irradiation. Methods: Samples were irradiated by helium ions of various energies and intensities at room temperature. After annealed at temperature of 773 K

873 K and 973 K separately

and some typical samples annealed at 873 K were chosen as objects of investigation. Nano-indentation technique was used to obtain the hardness; elastic recoil detection (ERD) was used to detect the helium concentration; and positron annihilation spectroscopy (PAS) was used to investigate S parameter. The microstructure was characterized by transmission electron microscopy (TEM). Results: The hardness of the Hastelloy-N alloys irradiated by He+ increased with the dose whilst the S parameter first increased with irradiation dose

but decresed after reach certain incident depth. After annealing

both the hardness and the S parameters decreased. The TEM image showed lots of radiation-induced defects after irradiation. The small defects were annihilated and 1-nm small helium bubble was observed after annealing. Conclusion: The large radiation-induced defects were believed to be the causes of the hardening. While the recovery of hardening after 873 K annealing was ascribed to the annihilation of the small defects

the desorption of helium and the decreasing number of HenVm. Abstract