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Stabilité en composition et en température des phases η et δ dans les superalliages base nickel

Abstract : The use of polycrystalline nickel base superalloys for turbine discs application is limited to temperatures less than 700 °C because of the loss of the stability of the γ’ and γ’’ phases precipitate distributions at higher temperature. These phases could be partially replaced by other hardening phases known to be potentially stable at temperatures higher than 800 °C, like δ-Ni3Nb, δ-Ni3Ta, η-Ni3Ti or η-Ni3(Al,Nb). However, the crystallographic structure, the composition and the thermodynamic stability of these different phases are not well known. Therefore, the aim of the present study is to determine the composition and temperature stability domains of η and δ phases. Simplified compositions of nickel base superalloys with high Nb and Ta contents (> 4 at. %), inspired from literature data and thermodynamic calculations (Thermo-Calc+TCNI7), were produced by VIM. After homogenization and ageing treatments, the alloys were characterized by SEM and TEM to define the various precipitate morphologies and distributions and to determine the crystallographic structure of the different phases, their composition and their solvus temperature. Synchrotron high energy XRD, with in situ heating, was performed in order to follow the evolution of phase volume fractions in some alloys. First, the influence of the alloy composition on the crystallographic structure and the composition of η and δ phases is studied, and these results allow us to refine the composition criteria for the formation of η and δ phases. And secondly, the effect of the alloy composition on the temperature stability of η and δ phases and their precipitation kinetics are discussed. The experimental results are compared to the thermodynamic calculations and some large inconsistencies are highlighted for alloys with high Nb and Ta contents.
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Laurane Finet. Stabilité en composition et en température des phases η et δ dans les superalliages base nickel. Matériaux. PSL Research University, 2019. Français. ⟨NNT : 2019PSLEM062⟩. ⟨tel-02520895⟩

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