Advanced Characterization of Hydrides in Zirconium
Zirconium and its alloys are of great importance to the nuclear power industry due to their unique combination of mechanical properties, low neutron-capture cross-section, and sufficient corrosion resistance under reducing water conditions. Zirconium alloys constitute most nuclear fuel claddings and structural components in CANDU and light water reactors. Despite many desirable material properties, zirconium is susceptible to a corrosion reaction that results in the gradual pick-up of hydrogen during service in nuclear power plants. Once the local hydrogen concentration exceeds the Terminal Solid Solubility of Precipitation, brittle hydrides can form. The precipitated hydride phase has been shown to degrade the bulk mechanical properties of zirconium cladding and introduce mechanisms for failure through delayed hydride cracking. The behavior of Zr-H system at low hydrogen concentrations continues to be the subject of research due to both its wide engineering application and the high complexity of the system. An idea to consider not only each single phase participating in the hydride-matrix interaction but also different hydride orientations (relative to parent Zr texture) opens new horizons for investigation of the Zr-H system. The current study is the first attempt to provide insight regarding the micro-scale direction of possible hydrogen migration within the α-Zr matrix and to evaluate changes in local strains on the grain length scale in the Zr-H system undergoing hydride precipitation. Finally, the research presented in this thesis aimed to improve our understanding of the stability of the Zr-H system. Phase changes occurring in a sample immediately after electrolytic hydriding, was investigated along with a reverse conversion occurring in the as-hydrided sample upon prolonged ageing. Surface hydride formation and its transformation during ageing was inspected using advanced characterization techniques such as SEM and AFM imaging, EBSD, and X-ray diffraction.