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High temperature oxidation of cobalt-based alloys strengthened by hafnium carbides Part 2: Metallography results

Patrice Berthod, Pascal Villeger

The three cast HfC-strengthened cobalt based alloys previously tested in oxidation at 1200°C during 50 hours by recording mass gain were, in this second part of this work, metallographiocally characterized, as the two ternary chromium carbides – strengthened alloys introduced in the study for comparisons. X-Ray Diffraction runs and Energy Dispersion Spectrometry were performed to bring information concerning the constitution of the external oxides while SEM examinations allowed characterizing the microstructure changes in the bulk. None of the five alloys showed a chromia-forming behaviour as already suggested by the rather fastmass gains. The oxides are multi-constituted and notably contain cobalt oxides, spinel cobalt-chromiumoxide, and sometimes hafniumoxide in the case of the HfC-strengthened alloys. In contrast with the ternary alloys that loosed all their carbides, the interdendritic HfC carbide network of the three Hf-containing alloys remained unchanged during the high temperature exposure. This demonstrates the interesting mechanical potential of such alloys for high temperatures application, but maybe also the necessity to improve their oxidation resistance, for example by increasing their chromium content or by applying a chromium rich coating.