Fei Long

During operation in nuclear reactors, zirconium core components undergo a slow process of hydrogen pickup, followed by the onset of the precipitation of zirconium hydrides. These brittle precipitates lead to degradation in the mechanical properties of the core components of the nuclear reactor, which is of importance to the industry because this can affect the life span of components in the reactor or during subsequent storage. There are still significant uncertainties as to the mechanical properties of the zirconium hydrides due to their complex characteristics: a wide range of possible precipitate sizes and geometries, variations of the hydride-matrix orientation relationship, and changes in mechanical properties with temperature, including an observed ductile-to-brittle transition of zirconium, including some hydride. In this study, using a novel approach, we address how the properties of δ-Zr hydrides themselves vary with both changes of temperature and irradiation damage. Mechanical properties were obtained using nanoindentation testing for both zirconium hydride and—as a comparison—for a Zr2.5Nb pressure tube. Proton irradiation was used to emulate the effects of neutron irradiation. After proton irradiation, the influence of temperature on hardness was observed by carrying out indentation tests from room temperature up to 300°C, collecting data at 50°C intervals. The influence of proton irradiation was analyzed using five different damage levels, including nonirradiated, from 0.05 to 0.8 dpa. An increase in temperature correlated with a decrease of the δ-Zr hydride hardness, with a more pronounced decrease with temperature for hydrides than for Zr2.5Nb.

The dissolution/precipitation behavior of hydrides in Zircaloy-2 was investigated by electron microscopy. The in-situ S/TEM heating/cooling experiments were carried out on hydrides observed in the TEM foil at different locations. For a hydride in a thin area, where dislocations can be imaged, a distinct memory effect was seen with reprecipitation occurring at the same location where the hydride existed before dissolution. Dissolved hydrides left behind dislocation nests, and precipitation-induced local plasticity was observed. For hydrides in a thicker part of the TEM foil clear dissolution/precipitation hysteresis was seen with a temperature difference of approximately 50°C. When reprecipitating, hydrides in thick parts of the foil also formed at the same position as before, displaying a memory effect. The existence of a memory effect for hydride reprecipitation was also observed during ex situ SEM imaging. After one cycle of heating/cooling, hydrides were observed to predominantly precipitate at their original positions. Electron backscatter diffraction analysis revealed that the precipitated hydride obeyed the conventional orientation relationship with the parental zirconium, with a habit plane close to the zirconium (0002) plane. The plasticity induced by hydride precipitation was evaluated by geometrically necessary dislocation analysis, demonstrating localized deformation in the zirconium matrix around hydrides. The precipitation-induced dislocations were found to be mainly <a>-type dislocations by TEM. The effect of high-temperature annealing on hydride precipitation was investigated. A 3-day annealing at 800°C caused significant grain growth of the zirconium that resulted in the subsequent hydride distribution being relatively uniform, and predominantly along grain boundaries. Last, a small number of hydrides were found to precipitate with the atypical (0001)Zr//(001)hydride orientation relationship. These observations suggest two contributory causes to the memory effect: at the grain level dislocation nests act as preferential nucleation sites for hydrides, and intergranular stress acts as a driver moving hydrogen to preferred grain locations/interfaces.

The effect of heavy-ion irradiation on deformation mechanisms of a Zr-2.5Nb alloy was investigated by using the in situ transmission electron microscopy deformation technique. The gliding behavior of prismatic 〈a〉 dislocations has been dynamically observed before and after irradiation at room temperature and 300 °C. Irradiation induced loops were shown to strongly pin the gliding dislocations. Unpinning occurred while loops were incorporated into or eliminated by 〈a〉 dislocations. In the irradiated sample, loop depleted areas with a boundary parallel to the basal plane trace were found by post-mortem observation after room temperature deformation, supporting the possibility of basal channel formation in bulk neutron irradiated samples. Strong activity of pyramidal slip was also observed at both temperatures, which might be another important mechanism to induce plastic instability in irradiated zirconium alloys. Finally, {011¯1}⟨01¯12⟩ twinning was identified in the irradiated sample deformed at 300 °C.

Thin foil dog bone samples prepared from a hot rolled Zr-2.5Nb alloy have been deformed by tensile deformation to different plastic strains. The development of slip traces during loading was observed in situ through SEM, revealing that deformation starts preferentially in certain sets of grains during the elastic-plastic transition region. TEM characterization showed that sub-grain boundaries formed during hot rolling consisted of screw ⟨a⟩ dislocations or screw ⟨c⟩ and ⟨a⟩ dislocations. Prismatic ⟨a⟩ dislocations with large screw or edge components have been identified from the sample with 0.5% plastic strain. Basal ⟨a⟩ and pyramidal ⟨c + a⟩ dislocations were found in the sample that had been deformed with 1.5% plastic strain, implying that these dislocations require larger stresses to be activated.