Effect of Ion Implantation on the Mechanical Properties of the Grain and Grain Boundary Regions of Inconel X750
Abstract
Annulus gas spacers in CANada Deuterium Uranium (CANDU) nuclear reactors are made from the heat-treated Inconel X750 Nickel-based alloy. This alloy is designed to have high strength and creep resistance at elevated temperature. Unlike other reactor designs, the CANDU reactor has a high thermal neutron fluence, which results in an enhancement of the radiation damage and the internal production of helium and hydrogen. They are thus susceptible to microstructural instability and mechanical property degradation with time. Studies of ex-service spacers have indicated that they display intergranular embrittlement and lower ultimate tensile strength compared to nonirradiated Inconel X750. The primary degradation mechanism remains unclear, and thus provides the focus of the current investigation. Sole and sequential Ni+ and He+ ion irradiation, up to different doses and irradiation temperature, was used to simulate neutron irradiation and to explore the microstructural evolution and mechanical property degradation in X750. The discussion of the microstructural evolution is focused on the irradiation-induced defect clusters, formation of helium bubbles and the stability of strengthening precipitates γ'. Utilizing of focused ion beam (FIB) and transmission electron microscopy (TEM) to perform high resolution microstructure characterization was a major contribution of this work. The microstructure is correlated to the mechanical properties measured through nanoindentation hardness test on irradiated and nonirradiated material. Established theories were applied to assess the contribution of ion-induced defect clustering, γ' precipitate disordering state, and helium bubble accumulation to the hardness of the X750 alloy and was compared to the nanoindentation hardness results. This approach is unique to the literature since it demonstrates both the individual and the combined effects of the microstructural features on mechanical behavior. Furthermore, a novel approach was curried out to determine the effect of the misorientation angle, irradiation-induced crystallographic damage, and accumulated helium on the strength of a grain boundary in Inconel X750. Bending of notched cantilever micro-beams X750 in the nonirradiated condition and after irradiation with high energy Ni+ and He+ ions was implemented. Bending test results suggests that the grain boundary strength of X750 alloy is related to the grain boundary energy and decreases as the energy increases. Dispersed barrier hardening model along with Finite Element (FE) model were applied to analyze the bending test finding.