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Déformation intense d'alliages d'aluminium à durcissement structural : mécanismes de précipitation et comportement mécanique

Abstract : The combination of two mechanisms to increase mechanical strength, namely precipitation and grain size reduction, has been explored in this thesis in the aim of increasing the properties of age hardenable aluminum alloy from the 7### series.Manufacturing by severe plastic deformation makes it possible to obtain nanostructured alloys with high density of grain boundaries, which allows increasing the yield strength according to the Hall-Petch law. However, the high density of defects (dislocations, vacancies, grain boundaries ...) and the internal stresses generated by this deformation results in inherently unstable nanostructures when precipitation heat treatment is performed. These nanostructures experience rapid grain growth and drastic changes in precipitation mechanisms (heterogeneous precipitation, accelerated kinetics).In this work we have studied nanostructures obtained by severe plastic deformation using HPT and HPS (High pressure torsion / sliding) on a model alloy, Al-2% Fe and a commercial alloy AA7449 enriched with iron. The strategy was to stabilize the ultra-fine grain structure by intermetallic iron-rich nanoparticles (Zener pinning) to allow homogeneous precipitation hardening and thus combine the two mechanisms to increase the yield strength. In this context, we have particularly investigated: 1) the influence of solutes on the physical mechanisms leading to dynamic recrystallization nanostructuring; 2) specific mechanisms involved in co-deforming phases with very different mechanical behaviors; 3) the phase transformations that may lead either to the formation of a supersaturated solid solution or, on the contrary, to the decomposition of a solid solution by deformation-induced precipitation; 4) the relationship between the nanostructures thus generated, their thermal stability and related mechanical properties.The observation of the microstructures and understanding of the mechanisms induced by the deformation and relations with the mechanical behavior has been undertaken with many techniques: scanning and transmission electron microscopy (SEM/TEM), ASTAR (orientation mapping by TEM), and atom probe tomography. The study of precipitation was carried out by DSC (differential scanning calorimetry), SAXS (small angle X-ray scattering) and in-situ TEM. Finally, the relationship with the mechanical behavior has been established on the basis of tensile tests and micro-hardness measurements.
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Amandine Duchaussoy. Déformation intense d'alliages d'aluminium à durcissement structural : mécanismes de précipitation et comportement mécanique. Mécanique des matériaux [physics.class-ph]. Normandie Université, 2019. Français. ⟨NNT : 2019NORMR135⟩. ⟨tel-02893363⟩

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