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Influence de la microstructure initiale sur les évolutions microstructurales au cours du revenu et du vieillissement dans l'alliage de titane ß-métastable Ti-17

Abstract : This PhD work is about the study of the formation of bimodal microstructures by solid-solid phase transformations and their evolutions during subsequent heat treatments, especially the one that simulates a long-term ageing in use, in the β-metastable titanium alloy Ti-17. Various initial dual-phase αprimary + βmetastable microstructures were formed from the β phase field, with significant differences in the phase microstructural parameters (fraction, size, density, spatial distribution) and chemical compositions. The influence of these initial microstructures and the heat treatment parameters (heating rate, duration and temperature of isothermal holding) on the kinetics and sequences of phase transformations during short-term ageing was then studied by means of in-situ characterisation techniques (electrical resistivity, synchrotron X-ray diffraction). Multi-scale microstructural characterisations (XRD, SEM, TEM-EDX) were coupled with quantification tools developed to precise the evolutions of the microstructural parameters and the chemical compositions during isothermal holding, in order to establish links with the mechanical properties and hardness. The complementarity of these means used eventually enabled to determine the transformation temperature domains of metastable phases (ωisothermal and α''isothermal), of which the kinetics and quantity vary with the heating rate and the initial microstructure. These differences have consequences on the resulting microstructure, in particular concerning the size and the density of the αsecondary grains. The phase chemical compositions were determined and compared to those calculated at thermodynamic equilibrium. Finally, the subsequent microstructural (increase in the α phase fraction, precipitation of the ordered α2 phase) and chemical (compositions that tend towards the ones at equilibrium) evolutions during a long-term thermal ageing were investigated and correlated to the evolutions of the mechanical properties
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https://hal.univ-lorraine.fr/tel-02499543
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Submitted on : Thursday, March 5, 2020 - 12:55:27 PM
Last modification on : Thursday, February 25, 2021 - 11:05:02 AM

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  • HAL Id : tel-02499543, version 1

Citation

Nicolas Maury. Influence de la microstructure initiale sur les évolutions microstructurales au cours du revenu et du vieillissement dans l'alliage de titane ß-métastable Ti-17. Génie des procédés. Université de Lorraine, 2018. Français. ⟨NNT : 2018LORR0125⟩. ⟨tel-02499543⟩

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