Three cast cobalt-based alloys containing or not chromium carbides and tantalum carbides were exposed at 900°C to 100% water vapour for 46 or 100 h. The oxidised samples were characterised by XRD, SEM and EDS. Chromia was the main oxide formed in all cases. Spinel oxide appeared on the carbide-free alloy and NiO was also detected on the TaC-containing alloy. Chromia was the single oxide formed on the alloy containing chromium carbides. The scales were thin and the subsurfaces and the bulks were slightly affected by oxidation or thermal exposure. All alloys demonstrated remarkable oxidation resistance but oxide spallation, enhanced maybe by the low thickness of the scales, by the porosities induced by chromia volatilisation, as well as by the CrTaO 4 complex oxide formed at the interface in the case of the TaC-containing alloy, is a critical problem to solve. The Co-10Ni-30Cr-0.5C was the best of the three alloys.

Four alloys based on niobium and containing about 33wt.%Cr, 0.4wt.C and, in atomic content equivalent to the carbon one, Ta, Ti, Hf or Zr, were elaborated by classical foundry under inert atmosphere. Their as-cast microstructures were characterized by X-ray diffraction, electron microscopy, energy dispersion spectrometry and while their room temperature hardness was specified by Vickers indentation. The microstructures are in the four cases composed of a dendritic Nb-based solid solution and of an interdendritic NbCr2 Laves phase. Despite the MC-former behavior of Ta, Ti, Hf and Zr usually observed in nickel or cobalt-based alloys, none of the four alloys contain MC carbides. Carbon is essentially visible as graphite flakes. These alloys are brittle at room temperature and hard to machine. Indentation shows that the Vickers hardness is very high, close to 1000HV10kg. Indentation lead to crack propagation through the niobium phase and the Laves areas. Obviously no niobium-based alloys microstructurally similar to high performance MC-strengthened nickel-based and cobalt-based can be expected. However the high temperature mechanical and chemical properties of these alloys remain to be investigated.

Many of the cast superalloys chosen for specific applications at high temperatures, such as tools devoted for shaping molten glass, are based on nickel or cobalt. They contain chromium and carbon to achieve good resistances against both mechanical stresses and hot oxidation/corrosion, by favoring the formation of reinforcing carbides at solidification and the development of a protective oxide scale of chromia during service. In presence of tantalum or titanium in the chemical composition of the alloys, high performance MC carbides may be obtained but this depends on the base element. One recently observed how the respective proportions of nickel, cobalt and chromium may promote the formation of TiC or TaC at the expense of chromium carbides. The ratings chosen for No, Co and Cr may have high influence on the oxidation of the alloys at high temperature this is what was studied in this work.