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Two alloys based on nickel and designed to be reinforced by MC carbides thanks to the presence of Hf and Ta were produced by casting. They were subjected to 50 h-long isothermal exposure at 1250 °C in synthetic air with thermogravimetric monitoring of the oxidation progress. In the as-cast state, they contain both significant quantities of (Hf,Ta)C carbides. Their verified melting start temperatures, close to 1300 °C, allowed performing the planned oxidation test. The two alloys demonstrated a chromia-forming behavior with limited mass gain rates. However, they also showed a rather low resistance to oxide spallation at cooling, which is in proportion with the Ta/Hf ratio. After 50 h at 1250 °C, the morphology of the carbides had significantly evolved, from their initial script-like shape to a fragmented and coalesced state. The results are promising, but the use of these alloys at 1250 °C needs further improvements on the mechanical level.

The metallurgical principle constituted by a chromium–rich metallic matrix associated with eutectic script monocarbides represents a promising way in terms of high temperature performance. In this work, focus was done on polycrystalline nickel–based alloys reinforced by monocarbides involving both tantalum and hafnium, with as objectives exploring the microstructures that can be obtained and the metallurgical, chemical and mechanical behaviors of what they can be able. Two alloys were synthesized by foundry under inert atmosphere. Their as–cast microstructures were visualized by electron microscopy and their behavior at high temperature were explored, for the following temperature and time parameters: 1100°C and 50 hours for all tests. The obtained results show that the carbide population obtained after solidification is mainly composed of MC carbides in both cases but it is composed of MC carbides only for the alloy with more hafnium than tantalum. The morphological resistance of the MC carbides is good for the test conditions for both alloys. Concerning oxidation, a chromia–forming behavior was observed in the two cases, with furthermore an obvious good resistance against scale spallation at cooling. The oxidation–induced subsurface alloy degradations and the heat–induced bulk microstructure deteriorations were very limited. The bending tests under constant load allow the sample demonstrating good creep resistance. To summarize both alloys appeared as good bases for developing quite usable superalloys for this temperature where the high performance ’ nickel–based single crystals start to encounter serious problems as consequences of the loss of their reinforcing particles.