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Influence de la microstructure sur les micromécanismes de déformation en fluage dans le superalliage AD730TM

Abstract : The study carried out during this thesis consists in analyzing the micromechanisms that control the creep deformation at 700 ° C, in the AD730(tm) superalloy developed by Aubert & Duval for the manufacture of turbine disks of the future turbojet engines for civil applications. The microstructure of the superalloy (size, distribution and chemical composition of the matrix and precipitates) has been characterized by conventional transmission electron microscopy (TEM) and by chemical analysis techniques (EDX and EELS spectroscopies). Deformation micromechanisms were identified at TEM after creep tests at 700°C at 500 MPa, 600 MPa and 850 MPa for different microstructural states. The study of the microstructures showed that thermal treatments can modify the local chemical composition of the different phases: variation of the Cr content in the matrix g, germination of g phase in the primary precipitates and variation of the Ti content in the secondary precipitates. These chemical fluctuations modify the antiphase boundary energy and the stacking fault energy that control the dislocation propagation. The quantification of these parameters allowed us to interpret the macroscopic behavior of the superalloy. Whatever the applied stress and the microstructural state, the observed intragranular deformation mechanisms correspond to conventional mechanisms reported in the literature for polycrystalline superalloys. Our results were validated by evaluating the stress levels associated with these mechanisms. The increase in the creep deformation rate at 700°C in the case of the fine-grained microstructure (in comparison with the coarse-grained microstructure), could be attributed to the primary precipitates where plasticity grows and is emitted. The increase of the creep strain rate at 850 MPa could be associated with the role of the grain boundaries which initiates the emission of several slip systems. In conclusion, this work allowed us to identify the parameters at microscopic scale that control the creep behavior at 700°C in the analyzed superalloy.
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Winnie Vultos. Influence de la microstructure sur les micromécanismes de déformation en fluage dans le superalliage AD730TM. Matériaux. Université Paul Sabatier - Toulouse III, 2019. Français. ⟨NNT : 2019TOU30166⟩. ⟨tel-02935098⟩

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