Zr cladding alloys have been used for many years as the first safety barrier layer of a nuclear reactor. However, the recent Fukushima accidents and industrial demands to increase the burnup of fuels have led to increasing interest in a detailed mechanistic understanding of aqueous corrosion and hydrogen pickup and the performance at high temperatures. As part of an international MUZIC-2 programme (Mechanistic Understanding of Zr Corrosion and Hydrogen pickup), I have used a range of advanced microscopy techniques to study the microstructure, the nanoscale chemistry and the porosity in a series of zirconium alloys at different stages of corrosion and hydrogen pickup. Samples from both autoclave and in-reactor conditions were available to compare, I have focussed on RXA (recrystallised 580°C) Zr-1.0Nb and annealed (720°C) Zr-1.0Nb alloys. A set of samples from different exposures times were chosen to represent early, pre-transition and post-transition samples in order to compare the microstructure and microchemistry of the oxides, the metal-oxide interface and the metal. (Scanning) Transmission Electron Microscopy ((S)TEM), Transmission Kikuchi Diffraction (TKD) and automated crystal orientation mapping with TEM (ASTAR mapping) were used to study the grain structure and phase distribution. Significant differences in grain morphology were observed between samples oxidised in the autoclave with different corrosion rates, with more uneven metal-oxide interface, more parallel cracks and less organised oxide grains in the fast corroding samples. Comparing with autoclave samples, the in-reactor samples have shorter, less well-aligned monoclinic grains and more tetragonal grains. The rapidly oxidising annealed Zr-1.0Nb alloy also have much higher tetragonal grain fraction comparing with the slow corrosion rate RXA Zr-1.0Nb alloys. Porosity in the oxide is predicted to have a major influence on the overall rate of corrosion and hydrogen pickup, and there is much more porosity in the annealed Zr-1.0Nb alloy than found in either the RXA alloy or the similar alloy exposed to neutron irradiation. A combination of Energy Dispersion X-ray (EDX) mapping in STEM and Electron Energy Loss Spectroscopy (EELS) analysis of second phase particles can reveal the main and the minor element distributions respectively. The annealed Zr-1.0Nb alloys have Î2-Zr SPPs with nano crystalline structure and much larger size. Although they does not relate with the higher density of cracks in the oxide, the large SPP size can connect together all the small cracks that are generated by the huge amount of tetragonal to monoclinic phase transformation during corrosion and provides pathway for corrosion and hydrogen pickup. Two kinds of SPPs are found in the RXA Zr-1.0Nb alloys, one is Î2-Nb and another one is Zr-Nb-Fe Laves phase. Neutron irradiation seems to have little effect on promoting fast oxidation or dissolution of Î2-Nb precipitates, but encourages dissolution of Fe from Laves phase precipitates. Electron Energy Loss Spectroscopy (EELS) analysis of the oxidation state of Nb in Î2-Nb SPPs in the oxide revealed the fully oxidised Nb⁵⁺ state in the SPPs deep into the oxide, but Nb²⁺ in the crystalline SPPs near the metal-oxide interface. EELS, TKD and ASTAR mapping have also revealed the presence of suboxide layers with the hexagonal ZrO structure predicted by ab initio modelling. The combined thickness of the ZrO suboxide and oxygen-saturated layers at the metal-oxide interface correlates well to the estimated instantaneous oxidation rate, suggesting that the presence of this oxygen- rich zone combining with the part where porosity is not interconnected is the protective oxide that is rate limiting in the key in the transport processes involved in corrosion and hydrogen pickup.

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Les effets du pressage à canaux égaux (ECAP), un procédé de déformation plastique sévère, ont été examinés dans un alliage Al-2.5Cu-1.5Mg (pourcentage en masse) prône à être durci par traitement thermique et précipitant dans la région α + S. Une multitude de techniques microscopiques, calorimétriques et analytiques ont été utilisés pour caractériser et quantifier les microstructures, incluant la diffraction Kikuchi, la microscopie électronique en transmission, la calorimétrie différentielle à balayage et la sonde atomique tomographique. Quatre différents traitements thermiques initiaux ont été réalisés pour créer quatre microstructures différentes, contenant soit aucun précipités, des clusters Cu-Mg ou/et des composés intermétalliques Al2CuMg. Chaque spécimen a été soumis au procédé ECAP à température ambiante et les effets correspondants sur la microstructure et les propriétés mécaniques ont été analysés. Des expériences en compression pour de petite déformation (inférieures à 7%) ont aussi été entreprises sur les échantillons trempés pour étudier les effets de la compression sur la formation des clusters. Après la trempe et la compression, des clusters Cu-Mg ont été trouvés dans la matrice et il a été élucidé que la formation des clusters était déclenchée par la compression. La fraction volumique des clusters est corrélée directement par la déformation appliquée : plus la déformation est importante, plus la fraction volumique des clusters est importante. Après ECAP, la microstructure est constituée de longues bandes nanocristallines séparée par de gros grains non-déformés pour les échantillons contenant seulement des clusters avant la déformation, tandis que la présence de phase S, avant ECAP, conduit à des microstructures constituées de zones à gros grains et de zones à grains raffinés, distribués d’une façon homogène à travers les échantillons. Bien que les spécimens présentaient clairement des microstructures différentes après ECAP, impliquant que différents mécanismes de renforcement entre en jeux, la limite élastique se situait au-delà de 500 MPa. La limite élastique des échantillons fabriqués par ECAP a été modélisée en superposant les différents mécanismes de renforcement et en saisissant les paramètres microstructurels venant de la caractérisation dans le modèle. Il a été démontré qu’une très bonne corrélation existait entre les limites élastiques provenant du modèle et celles expérimentales. / The effects of equal channel angular pressing (ECAP), a severe plastic deformation (SPD) technique, were investigated in an age hardenable Al-2.5Cu-1.5Mg (weight percent (wt.%)) alloy precipitating in the α + S phase field. A variety of microscopy, calorimetry and analytical techniques were employed to characterize and quantify the microstructure, including transmission kikuchi diffraction (TKD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and atom probe tomography (APT). Four different initial heat-treatments were conducted to achieve four different microstructures, containing either no precipitates, Cu-Mg clusters or/and Al2CuMg intermetallics. Each specimen was subjected to ECAP at room temperature and the related effects on the microstructure and mechanical properties were analysed. Compression experiments for small strains (less than 7%) were also undertaken on the as-quenched samples to investigate the effects of compression on the formation of clusters.After quenching and compression, Cu-Mg clusters were found in the matrix and it was elucidated that the formation of clusters was triggered by pressing. The volume fraction of clusters was found to be correlated to the strain applied: the higher the strain, the higher the volume fraction.After ECAP, the microstructure was constituted of long nanocrystalline bands separated by large undeformed grains for the samples containing only clusters before deformation, while the presence of S phase, prior to ECAP, lead to microstructures constituted of both coarse and refined zones distributed homogeneously throughout the sample. Although the samples presented clearly different microstructures after ECAP, implying that different strengthening mechanisms were active, the yield strength was found to lie above 500 MPa. The yield strength of the ECAP processed samples was modelled by superposing the different strengthening mechanisms altogether and by inputting the microstructural parameters coming from characterisation in the model. It was demonstrated that a very good correlation existed between the modelled and experimental yield strength values.