Year

2013

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronic Engineering

Abstract

The zirconium alloy, Zircaloy-4 (Zr-4), has extensive applications in the nuclear industry as core structural components and fuel claddings in the nuclear reactors. The mechanical performance of these components can be strongly affected by the precipitation of brittle zirconium hydrides and the induced embrittlement. This work investigated the hydride precipitation behaviours in different Zr-4 materials with various thermo-mechanical states (i.e. the high-pressure torsion (HPT) processed disk, hot-rolled and annealed plate, and hot-extruded and annealed bar). The effect of hydride formation on the mechanical properties of Zr-4 materials was also studied.

A comparative study was conducted to clarify the effect of HPT processing on the hydride formation in Zr-4 materials. The Zr-4 materials before and after HPT processing were gaseously hydrided at 450 °C using different hydrogen pressures (10, 15 and 20 atm). Detailed quantitative characterisation of the microstructure of these hydrided samples showed that at the same hydriding conditions, more hydrides tended to form in the HPT samples compared with the counterparts without HPT processing. The enhanced potential for the hydride formation in the HPT samples was attributed to the generation of a large concentration of lattice defects by HPT, which could act as hydrogen traps, hydrogen fast diffusion channels and preferred hydride nucleation sites. Further, the gaseous hydriding-induced changes in the microstructure, texture and mechanical properties of Zr-4 processed by HPT were thoroughly assessed. Much δ-ZrH1.66 precipitation at 15 atm (volume fraction ~21%) incurred significant hardening of vacuum-annealed HPT samples, and pure ε-ZrH2 obtained at 20 atm showed a superior microhardness of 470 HV0.3 and a low fracture toughness of 0.63 MPa m1/2. The neutron diffraction measurements indicated that the δ-hydrides in HPT samples presented a weak (111) texture and followed the δ(111)//α(0001) orientation relationship (OR) with the α-Zr matrix.

Detailed microstructure and texture studies of the hydride precipitation in annealed Zr-4 materials were also performed by electron microscopy and neutron diffraction. Results showed that the precipitated δ-ZrH1.66 generally followed the δ(111)//α(0001) and δ[110]//α[11 2 0] OR with the α-Zr matrix. Texture analyses of these hydrided materials confirmed the result from the HPT samples that the δ-hydride precipitates displayed a weak texture, and further indicated that the hydride texture was determined by that of the α-Zr matrix. This texture dependence essentially originated from the observed OR between δ-hydride and α-Zr. The neutron diffraction line profile analysis and the high-resolution transmission electron microscopy observation revealed a significant amount of dislocations present in the δ-hydride with an estimated average density of one order of magnitude higher than that in the α-Zr matrix, which contributed to the accommodation of the substantial misfit strains associated with the hydride precipitation in the α-Zr matrix.

The hydride distribution in the bent and unbent Zr-4 plate samples subjected to gaseous hydriding was investigated by neutron tomography in conjunction with scanning electron microscopy and X-ray diffraction techniques. For the unbent plate sample hydrided at a hydrogen pressure of 20 atm, thin hydride layers on the surface and concentrated hydrides at the edges/corner of the sample were observed. This localised hydride accumulation was presumably resulted from the preferential hydrogen accumulation and saturation in these regions. For the bent plate hydrided at 10 atm, a thin hydride layer was present on the surface region with a significant compressive plastic deformation generated in the bending process. Besides, it was found that the hydride platelets in the hydrided bent plate tended to align parallel to the direction of the maximum tensile and compressive strain induced by the prior bending deformation. The electron backscatter diffraction (EBSD) observations on the hydrided bent sample showed that the primary OR between δ-hydride and α-Zr matrix transited from δ(111)//α(0001) in the tensile strain region to δ(111)//α(1010) in the compressive strain region of the bent sample.

Additionally, the influence of the hydride precipitation on the mechanical performance of hydrided Zr-4 plate materials containing different hydrogen contents were studied at room temperature. For the unnotched plate samples with the hydrogen contents ranging from 25–850 wt. ppm, the uniaxial tensile tests showed that the tensile ductility was severely degraded with increasing hydrogen content. Also, the degree of embrittlement and fracture mode were strongly related to the hydrogen content. When the hydrogen content reached a level of 850 wt. ppm, the plate exhibited negligible ductility, resulting in almost completely brittle fracture behaviour. For the hydrided notched plate, the tensile stress concentration associated with the notch tip facilitated the hydride accumulation at the region near the notch tip and the premature crack propagation through the hydride fracture during hydriding. The final brittle through-thickness failure for this notched sample was mainly ascribed to the formation of a continuous hydride network on the thickness section and the obtained very high hydrogen concentration (estimated to be 1965 wt. ppm).

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