• Vilasi, M.
• Berthod, P.
• Mathieu, Stéphane
• Michel, G.

Abstract

Superalloys, which are mainly based on cobalt, nickel and/or iron, are usually exposed to high temperature oxidation. To resist this phenomenon they are designed to contain high amounts in Cr, Al or Si to develop external protective oxide scales. These elements must be in quantities high enough in the bulk, or only in the subsurface where they are really useful to combat hot oxidation. In the later case a surface enrichment can be achieved by using pack-cementation, a CVD technique used since many years ago to superficially enrich cobalt alloys with chromium, nickel alloys with aluminium or iron alloys with silicon, for example. The success of such operation depends on thermodynamic and kinetic conditions, but also on some microstructure characteristics of the alloy. The aim of this work is to explore the possible influence of the nature of the carbides which are in some cases used to mechanically reinforce some refractory cast alloys (notably against creep deformation).It appears that the Cr-enrichment failed in the case of all alloys which initially contained interdendritic chromium carbides as single reinforcing phase or present toget her with tantalum carbides. Whatever the base element, this was due to the formation of a chromium carbide involving the carbon atoms diffusing from deeper, released by the dissolution of the bulk chromium carbides. In contrast, when TaC were the single carbides present in a Co-based or Fe-based alloy, the inward chromium diffusion was never inhibited and a sub-surface really enriched in chromium (up to 30wt.% in some cases) was observed. This difference results from the higher stability of the TaC at high temperature. In contrast, the lack of stability of tantalum carbides earlier observed in Ni-based alloys led again to the development of a carbide layer along the surface of the TaC-containing nickel alloy. In the cases where the Cr-enrichment of the sub-surface was successful, the incorporation in the sub-surface of the cement grains still sticked to surface and a deeper inward diffusion of the added chromium can be achieved by applying an additional long high-temperature heat treatment. To conclude, one can say that t he success of the sub-surface enrichment in chromium by pack-cementation of carbides-reinforced alloys thus supposes that the bulk's carbides are very stable, even in presence of a neighbour high Cr activity. It appeared here that TaC are to be prefered to Cr7C3 or Cr23C6, at least in cobalt-based or iron-based alloys.

Topics

• impedance spectroscopy
• surface
• Carbon
• grain
• nickel
• chromium
• phase
• aluminium
• carbide
• cement
• Silicon
• cobalt
• iron
• creep
• chemical vapor deposition
• superalloy
• tantalum
• nickel alloy
• iron alloy
• cobalt alloy