Microstructural characterization and mechanical properties of Excel (Zr-3.5%Sn-0.8%Mo-0.8%Nb), a dual phase αZr-hcp and βZr-bcc pressure tube material, is discussed in the current study which is presented in manuscript format. Chapter 3 discusses phase transformation temperatures using different techniques such as quantitative metallography, differential scanning calorimetry (DSC), and electrical resistivity. It was found that the αZr → αZr+βZr and αZr+βZr → βZr transformation temperatures are in the range of 600-690°C and 960-970°C respectively. Also it was observed that upon quenching from temperatures below ~860°C the martensitic transformation of βZr to –hcp is halted and instead the microstructure transforms into retained βZr with ω hexagonal precipitates inside βZr grains. Chapter 4 deals with aging response of Excel alloy. Precipitation hardening was observed in samples water-quenched from high in the αZr+βZr or βZr regions followed by aging. The optimum aging conditions were found to be 450°C for 1 hour. Transmission electron microscopy (TEM) showed dispersion of fine precipitates (~10nm) inside the martensitic phase. Energy dispersive X-ray spectroscopy (EDS) showed the chemical composition of precipitates to be Zr-30wt%Mo-25wt%Nb-2wt%Fe. Electron crystallography using whole pattern symmetry of the convergent beam electron diffraction (CBED) patterns together with selected area diffraction (SAD) polycrystalline ring patterns, suggests the -6m2 point group for the precipitates belonging to hexagonal crystal structure, with a= 2.936 Å and c=4.481 Å, i.e. c/a =1.526. Crystallographic texture and high temperature tensile properties as well as creep-rupture properties of different microstructures are discussed in Chapter 5. Texture analysis showed that solution treatment high in the αZr+βZr or βZr regions followed by water quenching or air cooling results in a more random texture compared to typical pressure tube texture. Variant selection was observed upon water quenching while partial memory effect and some transformation texture with variant selection was observed in the air-cooled sample. The results of creep-rupture tests suggest that fully martensitic and aged microstructure has better creep properties at high stress levels (>700 MPa) while the microstructure from air cooling from high in the αZr+βZr region is less sensitive to stress and shows better creep properties compared to the as-received annealed microstructure at lower stresses (<560 MPa). / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2012-08-23 16:22:45.395

Zirconium and its alloys have been used extensively in both light and heavy water reactors where neutron irradiation is known to cause microstructural evolution, leading to degradation of mechanical properties and dimensional instabilities. Dimensional instabilities due to irradiation growth are particularly crucial for Zr alloy Excel which is the proposed candidate material for the conceptual CANDU-Super Critical Water Cooled Reactors (SCWR) pressure tube. This study employs the in-situ ion irradiation technique and transmission electron microscopy to investigate the irradiation induced microstructural evolution in Zr and Zr alloy Excel. The current study is presented as a manuscript format dissertation comprised of five manuscript chapters. Chapter 3 reports the formation of irradiation induced prismatic defects directly from cascade collapse in pure Zr at low dose (0.008 dpa) in a temperature range of 300oC-500oC. The morphology and yield of these defects are found to be temperature and dose dependent. In Chapter 4, irradiating Zircaloy-2 under similar conditions to pure Zr, reveals that nucleation rate of small prismatic loops increases, whereas their growth is suppressed which indicates that these defect clusters are not only temperature dependent but also impurity dependent. Chapters 5, 6 and 7 report the irradiation induced microstructural changes at various temperatures up to a dose of 10 dpa, in several microstructures of Zr alloy Excel, achieved by different solution treatments. The major focus of these experimental studies was the formation of <c>-component loops in α-phase; decomposition of β-phase; and irradiation induced microchemical changes. It was found that nucleation and growth of <c>-component loops is strongly dependent on irradiation temperature, parent microstructure, and presence of alloying elements. <c>-component loops nucleate above a threshold incubation dose which decreases with an increase of irradiation temperature. Energy dispersive X-ray spectroscopy (EDS) mapping on irradiated microstructures revealed the formation of small Sn clusters in α-phase which have a significant effect on the morphology of <c>-component loops. Fe plays an important role in the nucleation of <c>-component loops, as it distributes itself during irradiation either from β-phase or from pre-existing secondary phase precipitates in α-phase. Furthermore irradiation induced decomposition of β-phase was observed in the form of ω-phase precipitation and irradiation induced clustering of alloying elements. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2013-12-31 23:50:30.352