Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Prof. Robert J. Klassen

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.

Summary for Lay Audience

Inconel X750 Nickel based super alloy is used in the core of CANDU (Canadian Deuterium Uranium) reactors as a fuel channel spacer (garter springs) to separate the hot pressure tube and the cold calandria tube. This alloy is mainly strengthened by a coherent precipitate know as γ′. Recently, it has been found that the spacers became very brittle after long exposure to neutron radiation and the embrittlement is characterized as intergranular failure. The current research involves nanoindentation hardness testing and micro-cantilever bending testing to assess the effects of irradiation on the hardening and onset of grain boundary embrittlement in the Inconel X750 spring spacers. The experiments are designed in such way to enable us to simulate the working condition of a CANDU reactor. The samples were irradiated solely and sequentially with heavy ions (Ni+) to emulate neutron irradiation, and with helium ions (He+) to simulate the helium produced from the transmutation reaction within the X750. The implantation was carried out at two temperatures 25°C and 200°C, to study the effect of implantation temperature on the mechanical properties of X750. Results show that low dose of sole implanted Ni+ soften the alloy because of disordering the strengthening phase γ′, as observed by Transmission Electron Microscopy (TEM). In contrast, He+ implantation harden the material because of Helium babble formation. This observation was true for both implantation temperature. Sequential He+/Ni+ implantation results show that the softening and hardening mechanisms are operating in parallel and their effects are additive. To measure the grain boundary strength in irradiated and non-irradiated condition, a novel notched micro-cantilever beam bending test was performed. The cantilever micro-beams were notched along grain boundaries. Results show that He+ implanted samples have the highest yield strength in compare to Ni+ implanted and nonimplanted counterparts. Therefore, the intergranular embrittlement observed in the spacer alloy was attributed to the accumulation of helium bubbles in the grain boundaries. The obtained data will be beneficial to the operators of CANDU reactors since they will allow better life predictions to be made for existing spacers and will also be helpful for the design of the future nuclear reactors.

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