PNNL

Abstract

Defect accumulation and microstructural evolution during ion irradiation at elevated temperatures are governed by competing processes of defect production, driven by dose rate, and defect recovery, controlled by diffusion, interaction, and annihilation. This study investigates the effects of irradiation temperature and dose rate on microstructural evolution, deuterium retention, and lithium volatilization in ?-LiAlO2 pellets subjected to sequential He+ and D+ ion irradiation. Experiments were performed to a total fluence of 3E17 (He++D+)/cm2 at 623–773 K with an average dose rate of 7.7E-4 dpa/s, and to 2E17 (He++D+)/cm2 at 773 K with dose rates ranging from 6.8E-5 to 7.3E-4 dpa/s. At 623 K, the microstructure consisted primarily of cavities and fractures without precipitate formation, while small precipitates emerged at 673 K. Increasing the irradiation temperature to 723–773 K promoted the formation of larger, faceted LiAl5O8 precipitates and surface amorphization, accompanied by pronounced lithium depletion and H-D isotopic exchange. At 773 K, microstructural evolution exhibited a strong dose-rate dependence. Medium and high dose rates produced an amorphized surface layer over a crystalline subsurface containing LiAl5O8 precipitates and gas blisters at the crystalline-amorphous interface, whereas low dose-rate irradiation preserved surface crystallinity with cavities distributed in the matrix, around precipitates, and along grain boundaries. Precipitate morphology was anisotropic with limited size dependence on dose rate. These results elucidate coupled effects of temperature and dose rate and demonstrate that sequential He+ and D+ irradiation at 773 K effectively reproduces microstructural features and H isotope behavior observed in neutron-irradiated ?-LiAlO2 at 573 K.