Exploration of the high temperature oxidation of the M-30Cr alloys (M = Co and/or Ni) containing tantalum in absence of carbon
Résumé
Tantalum is often present in chromia–forming polycrystalline superalloys based on cobalt or on nickel. It may play a double role in their strengthening, as solid solution hardener and as a strong carbide–forming element (TaC carbides, generally). In carbon containing superalloys Ta is present in the matrix as well as in the primary carbides crystallized during solidification (and also in secondary carbides after having applied a precipitation heat treatment). Tantalum is also an element which is easy to oxidize and it generally takes part to the phenomena during the high temperature oxidation of the concerned alloys. The related phenomena may depend on the forms with which Ta is present in the alloy.
In this work, a series of {(66-x)Co–xNi–30Cr–4Ta; 0 ≤ x ≤ 66} alloys were elaborated by conventional foundry. First the obtained general chemical compositions were controlled by energy dispersive spectrometry and second the microstructures of the alloys in their as-cast states – logically carbide–free since the presence of carbon was avoided in these alloys – were explored by electron microscopy. Third, thermogravimetric tests at 1200°C were performed, followed by the exploitation of the mass gain files to access to the oxidation kinetics. Post–mortem characterization was carried out for knowing the natures and quantities of the oxides formed and the subsurface changes induced by oxidation.
Global kinetics of oxidation (chromia volatilization–corrected mass gain, chromia volatilization rate) of similar levels as for the same alloys but containing C to promote TaC
Same early oxide scale spallation starts and associated mass losses leading to negative total mass variations
Same differences between Co–based alloys and Ni–based alloys for the Ta stability in solid solution in the matrix (in the Co–base ones, here Ta–rich intermetallic compounds instead the TaC carbides in the carbon–containing quaternary and quinary corresponding alloys)
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