氦离子辐照对镍基合金硬度的影响

Investigation of helium irradiation induced hardening in nickel-based alloy

在室温下对镍基合金进行了氦离子辐照,利用纳米压痕仪测试了微观硬度,利用慢正电子多普勒展宽谱(Doppler Broadening Spectrum,DBS)和透射电子显微镜(Transmission Electron Microscope,TEM)分析了微观缺陷,利用离子束分析弹性反冲探测(Elastic Recoil Detection,ERD)技术测量了氦的浓度深度分布。结果显示合金样品的硬度随剂量而增大,退火后合金样品硬度增量有所减小,并观测到氦泡生成。合金硬化的主要原因是由于氦离子辐照产生了1-7 nm的缺陷团簇,而退火后不稳定缺陷的回复及氦-空位复合体数量的减少造成了硬化强度减弱。

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.