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Etude des mécanismes de formation des carbures obtenus par cémentation basse pression d'un alliage de tantale, application à la pyrochimie.

Abstract : Tantalum is a metal with a good corrosion resistance. However, tantalum pyrochemical crucibles are damaged by the molten metals they contain, especially by diffusion at the tantalum grain boundaries. A solution is to carburize the crucibles. During carburizing, two carbides are formed: TaCx and Ta2Cx in the form of two homogeneous surface layers. During cooling, a layer of Ta2Cx precipitates is also formed.In this study, the low pressure carburizing process is applied to the TaW2.5% alloy (Ta97.5wt.%-W2.5wt.%). The aim is to benefit from the advantages of tungsten addition: limitation of the grain growth during the treatment and an increase of the carbide precipitation at the grain boundaries. However, these carbides differ from those of pure tantalum by their nature and their morphology (lamellar layer).A CALPHAD study was performed to obtain a thermodynamic database and thus improve the understanding of MC and M2C carbide formation (with M=Ta,W). In addition, EBSD analyzes have shown the crystallographic relationships {110}Ta//{0001}Ta2Cx and {111}TaCx//{0001}Ta2Cx.Finally, the layer growth kinetics were studied according to the different parameters (duration of carburizing and diffusion, temperature, surface state and microstructural state). This study thus made possible to obtain different phases on the surface of crucibles: MC, M2C or TaW + M2C precipitates. Then, these surface microstructures were evaluated against attack by liquid metals (Al, Bi) and salts (CaCl2).
Keywords : Carbides Tantalum TaW Alloy
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Laurent Carette. Etude des mécanismes de formation des carbures obtenus par cémentation basse pression d'un alliage de tantale, application à la pyrochimie.. Autre. Ecole nationale supérieure d'arts et métiers - ENSAM, 2019. Français. ⟨NNT : 2019ENAM0029⟩. ⟨tel-02924657⟩

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