Micromechanics, microstructure and formation of zirconium hydrides in nuclear fission applications

Thesis

Hydride formation in zirconium based nuclear fuel cladding can lead to reduction of ductility and fracture toughness. This detrimental effect of hydrogen embrittlement is critically dependent on the morphology of hydrides in microscale and the Zr/hydride dual-phase microstructure. In this work Zr hydride thin film samples were prepared for microstructural and micromechanical characterisation. The microstructure near the Zr/hydride interface was examined with the use of scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). A new model was developed to account for the effect of concentration-dependent diffusion on the formation of hydride precipitates. Microcantilever tests and nanoindentation were carried out to compare the deformation properties of the δ-hydride with those of the α Zr. Pre-notched microcantilevers were fabricated in the individual phases and across the phase boundary with the use of focused ion beam (FIB). The experimental results show that the δ-hydride exhibits isotropic micromechanical properties. All the 2-4 μm wide δ-hydride microcantilevers underwent brittle fracture and the conditional fracture toughness is 1.8-2.0 MPam^1/2 in a variety of crystallographic directions. The Young’s modulus of the δ-hydride is 88-94 GPa and its proportional limit in microscale (1016-1189 MPa) is approximately double that of the α-Zr (401-600 MPa). The indentation size effect (ISE) was experimentally investigated and the hardness increased by 65% to 5.1 GPa when the indenter penetration depth was reduced from 5 µm to 0.5 µm. Despite interfacial microvoids and secondary hydride microcracks were observed after applying the bending load, the interface-notched dual-phase microcantilevers predominantly underwent plastic deformation without primary crack propagation.

Authors: Chan, KHH

• DOI: 10.5287/ora-koqzeyqbb
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: micromechanical testing; SEM; EBSD; micromechanics; XRD; indentation size effect; zirconium hydride; microstructure characterisation; hydrogen embrittlement; microcantilever; fracture toughness; FIB; nanoindentation
• Subjects: Micromechanics; Nuclear fuel rods; Hydrogen embrittlement