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Traitements thermomécaniques des colonies de lamelles parallèles du Zircaloy-4 trempé-β.

Abstract : Zircaloy-4 used for fuel cladding in nuclear plants is quenched from the β range and then extruded and rolled in the upper α range. At the start of this mechanical process, the alloy possesses a lamellar, Widmanstätten microstructure. This one, which is critical for the subsequent forming process, appears under two forms: basket weave and colonies of parallel lamellas. These are difficult to break when they are normal to the compressive load. The thesis studies three aspects of this phenomenon. The first concerns the quenching conditions: homogenization time in the β range and cooling rate. An adaptation of the Jominy test to Zircaloy-4 shows that these two parameters have a decisive influence on the size of the colonies (via the β grain size) and the thickness of the lamellas. The second presents compression tests under three orthogonal directions. Results show that the third pass breaks the colonies that resisted to the previous attempts and refine noticeably the microstructure. In particular at 750°C, three passes are sufficient to obtain grains of 30 µm, but the best results are obtained at 650°C (grains of 17 µm) and at high strain rate (grains of 10 µm). Thirdly, a three-dimensional crystal plasticity model is implemented in the finite elements code ABAQUS to simulate the behaviour of lamellas under stress. It takes into account their crystallographic orientation in addition to their morphology. In most cases, the lamellas bend at the onset of the macroscopic deformation, which induces localization phenomena.
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Yamen Ben Ammar. Traitements thermomécaniques des colonies de lamelles parallèles du Zircaloy-4 trempé-β.. Autre. Ecole Nationale Supérieure des Mines de Saint-Etienne, 2012. Français. ⟨NNT : 2012EMSE0676⟩. ⟨tel-00849217⟩

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