Microstructural Evolution and Hardening of Zr-2.5Nb-0.5Cu Spacer Material under Irradiation

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Dong, Qingshan
Zr-2.5Nb-0.5Cu , Irradiation , Microstructure , Hardness
Zr-2.5Nb-0.5Cu alloy is considered as a potential replacement of the current Inconel X-750 alloy for future spacer installations in CANDU® reactors. They are designed to be placed in the annular space between pressure and calandria tubes to support the pressure tube and prevent it from coming into contact with the calandria tube. During their service lifetime, they are exposed to a high flux of fast neutrons and multiple-directional loads. Therefore, it is important to study the effect of irradiation on the microstructure, microchemistry, and mechanical property of this alloy. In this dissertation, the effect of ion irradiation, heat treatment, alloying elements, pre-existing crystal defects on the microstructure and irradiation hardening was investigated to help us have a better understanding of this alloy before its application. Chapters 1 and 2 present a general introduction to the current investigated material and a literature review to previous related studies. Characterization of the microstructure and precipitates of the as-received material is presented in Chapter 3. Three types of microstructure and three types of second phase particles are detected in the as-received material. A comparison between annealing in TEM thin foils and bulk material is displayed in Chapter 4. The change of microstructure and precipitates is significantly slower in TEM thin foils than in bulk material, due to the presence of the foil surfaces. Chapter 5 reports the precipitate stability in the three types of microstructure under heavy ion irradiation. The starting microstructure shows a remarkable influence on the precipitate stability; the precipitates are more stable in the microstructure with a high density of point defect sinks. Chapters 6 and 7 cover the impact of alloying elements Nb and Cu on the loop formation and hardening in Zr alloys under self-ion irradiation. The existence of Nb and Cu exhibits a significant influence on the loop density and Burgers Vector of loops obtained, as well as ω precipitation and subsequent hardening behaviour. In Chapters 8 and 9, pure Mg was used as an analogue of Zr to provide additional insight into the influence that crystal internal defects, such as grain boundaries and gliding dislocations, have on the loop formation during irradiation. Noticeably different loop formation behaviour was observed in the vicinity of a grain boundary under electron irradiation. The pre-existing dislocations also displayed a noteworthy impact on the loop formation.
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