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Etude des mécanismes de déformation viscoplastiques du dioxyde d’uranium polycristallin au voisinage de la stoechiométrie : influence de l’activité d’oxygène et de la microstructure

Abstract : This study contributes to our understanding of high temperature viscoplastic deformation processes in polycrystalline uranium dioxide, with particular attention paid to the influence of the initial material microstructure and the equilibrium partial pressure of oxygen. The material response to stress was characterised through high temperature mechanical tests, carried out in a controlled gaseous atmosphere, combined with numerical modelling of these tests and microstructural characterisations of deformed samples using SEM/EBSD and TEM. Three polycrystalline microstructures were examined in compression and three and four point bending. Logarithmic strains were implemented in a finite strain framework to model all three types of tests. A Frederick-Armstrong material behaviour law was further assumed. When applied to compression tests, the modelling enabled us to determine the conditions in which tests can be considered uniaxial. The interpretation of the stationary strain rates with a phenomenological behaviour law for samples from the first set with an 11 μm average grain size (manufactured from a powder metallurgy route), indicates a stress exponent of 5, which is characteristic of restauration creep, and an oxygen partial pressure exponent of 1/6. The destructive characterisation of samples subsequent to testing reveals a microstructure typical of restauration creep, in which the original grains fragment into subgrains separated by low angle boundaries. However, using the laws of mass action which assume an infinitely diluted, non-interacting population of point defects, the dependence of stationary strain rates to the partial pressure of oxygen does not enable us, at this stage, to conclude to a restauration mechanism controlled by cation volume diffusion. Compression tests carried out on the second set of samples produced by SPS at JRC-Karlsruhe, reveal a hardening effect attributed to their smaller grain size (3 μm average grain size), and the concomitance of both dislocation creep and diffusional creep deformation mechanisms. Bending tests carried out on the last set of samples, produced via a powder metallurgy route, reveal a compression and tensile response which is asymmetric. Post-test microstructural analyses indicate that grain fragmentation, characteristic of dislocation creep, occurs to a greater extent in regions of the specimens subjected to tensile loading. Further, three point bending response curves systematically go through a maximum prior to the establishment of a stationary regime. This phenomenon is attributed to the stable propagation of intergranular cracks in tensile loaded regions. Finally, an original study was carried out of the material response under the combined influence of temperature, stress and external proton irradiations. The experiments did not enable us to induce any measurable viscoplastic strain, but did reveal substantial swelling and embrittlement of the material as a result of the irradiation.
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Submitted on : Wednesday, January 12, 2022 - 11:29:09 AM
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Jean-Baptiste Parise. Etude des mécanismes de déformation viscoplastiques du dioxyde d’uranium polycristallin au voisinage de la stoechiométrie : influence de l’activité d’oxygène et de la microstructure. Matériaux. Université de Limoges, 2021. Français. ⟨NNT : 2021LIMO0076⟩. ⟨tel-03522685⟩



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