Analyse des hétérogénéités de microstructure et de microtexture héritées par transformation de phase β→α dans des pièces massives en alliage Ti-10V-2Fe-3Al : influence sur la dispersion des propriétés mécaniques

Abstract : The β-metastable titanium alloys such as Ti-10V-2Fe-3Al are gradually replacing α/β alloys in aeronautical applications thanks to their improved specific strength. However, their microstructures are complex and multi-scale, consisting of a β matrix (of millimetric grains) partially transformed into primary αp nodules (micrometric) and secondary αs lamellae (sub-micrometric). The final mechanical properties are very sensitive to local variations of the microstructure, which are not always fully controlled during forging of massive parts. Moreover, the β matrix, which represent 40% of the volume and whose elastic and plastic behavior is strongly anisotropic (like the α phase) complicates the understanding of the mechanisms of deformation. The first objective of this thesis was to efficiently characterize the microstructure/texture of the different constituents (β/αp/αs) and their heterogeneities within half-finished products and forged parts by using techniques of multi-scale characterization (neutron diffraction, electronic imaging coupled with image analysis and EBSD, reconstruction of high temperature microtextures β/αp). As a result the fragmentation of the β grains into subgrains, the αp macrozones, the destruction of the orientation relation between β/αp and the organization of the αs lamellae in colonies or basket weave was quantified and the differences in size of domains revealed by crystallography and by standard imaging were pointed out. The second objective is to apply this methodology to the analysis of fracture surfaces of samples exhibiting singular behavior (in tension or in fatigue) in order to characterize the microstructural configurations leading to early cracking. This analysis was mainly performed by manual polishing coupled with EBSD acquisitions but also by using 3D imaging by SEM-FIB (Focus Ion Beam) and TKD (Transmission Kikuchi Diffraction) technique on a thin foil FIB-extracted from the crack initiation site. Finally, this experimental study was completed by a micromechanical simulation on a 100% β model microstructure. The objective was to evaluate the influence of the elastic anisotropy of the β phase on the genesis of incompatibility stresses in the elastic and elasto-plastic regimes. The overall results contribute to a better understanding of the variations of mechanical properties related to the local microstructure.
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https://hal.univ-lorraine.fr/tel-02331875
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Submitted on : Thursday, October 24, 2019 - 3:16:32 PM
Last modification on : Saturday, October 26, 2019 - 1:55:46 AM

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  • HAL Id : tel-02331875, version 1

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Maria Rita Chini. Analyse des hétérogénéités de microstructure et de microtexture héritées par transformation de phase β→α dans des pièces massives en alliage Ti-10V-2Fe-3Al : influence sur la dispersion des propriétés mécaniques. Mécanique des matériaux [physics.class-ph]. Université de Lorraine, 2018. Français. ⟨tel-02331875⟩

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