Synchrotron X-ray diffraction investigations into the micromechanics of hydrides in zircaloy-4
Creators: name: Weekes, Hannah
Publisher: name: Imperial College London
Year: 2015

description: Due to their low thermal neutron capture cross-section, adequate mechanical strength and high corrosion resistance, zirconium alloys are used as structural components (i.e. fuel cladding, fuel assemblies and internal components) within a nuclear reactor. During reactor operation, however, they are prone to hydrogen pick-up via numerous routes, including corrosion, radiolysis of water and fuel oxidation. In situations where the concentration of hydrogen exceeds the solubility of the host material, i.e. reactor shut-down/post-operation storage, the precipitation of brittle hydride precipitates can jeopardise the structural integrity of the component. This study explores the behaviour of these brittle hydrides under extreme conditions using high energy synchrotron X-ray diffraction combined with post-mortem analytical techniques. The deformation response of hydride packets embedded in a zirconium matrix has been investigated during in situ micro-Laue diffraction and in situ SEM studies. The hydride packets' ability to act as a both a barrier and provider for slip band propagation has been revealed as well as its potential to plastically deform under certain conditions. Microbeam Laue diffraction has provided insight into the strains associated with hydride precipitation and, when coupled with post-mortem EBSD and TEM analysis, has concluded that hydrides can act as shearable precipitates, under at least some conditions. The phenomenon of hydride stress-reorientation, where macroscopic packets are seen to reprecipitate perpendicular to an applied stress on cooling, has been monitored during in situ thermomechanical cycling. The strains associated with hydride precipitation have been found to differ depending on whether they precipitate radially or circumferentially in the zirconium matrix and is strongly dependent on axis of load. The redistribution of hydrogen to α-Zr grains in a more favourable orientation when under an applied stress was observed during cycling and was also found to be dependent on the axis of load; TD-aligned samples redistributed hydrides 90° from the tensile axis while RD-aligned samples redistributed it 50-60°. Thermomechanical loading over the course of four cycles has also shown to reduce the difference between the terminal solid solubility of hydrogen during dissolution (TSSD) and precipitation (TSSP). The final chapter addresses the question of how stress affects the solubility of hydrogen in zirconium. Considered with respect to two well-document models behind delayed hydride cracking - (i) `diffusion first' and (ii) `precipitation first' - Zircaloy-4 powder, both with and without hydrides, was loaded under pressure and its behaviour investigated using X-ray diffraction. A change in hydrogen solubility with stress has been suggested. Hysteresis in both axial ratio of α-Zr and hydrogen solubility with load has been noted, with the latter found to be dependent on whether the hydride is dissolving into or precipitating out of the zirconium matrix.