Oxidation behaviours at 1,000, 1,100 and 1,200°C of a 32.5wt.%Cr -containing Co-based alloy reinforced by hafnium carbides
Résumé
Interdendritic carbides used to strengthen cast superalloys working in hot conditions have to be extremely stable at high temperature. The volume fractions and geometrical shapes of the common chromium carbides evolve too rapidly in such conditions with consequently a lack in mechanical resistance. More stable carbides, as the TaC ones, are themselves not sufficient since they also undergo fraction reduction and fragmentation phenomena. The HfC ones, which present the same script-like morphology as the eutectic TaC, appeared much more resistant to such phenomena, and they consequently lead to exceptional creep resistance.
However it was recently seen that a significant interdendritic network of eutectic HfC carbides, able to bring high resistance to creep deformation, may also threaten the resistance of the concerned cobalt-based alloys in oxidation at high temperature, by leading to a not chromia-forming behaviour. To try improving their oxidation resistance, the chromium content in these alloys was increased from 25wt.% to 32.5wt.%. The new alloys, with always 0.25 to 0.50 wt.%C and 3.7 to 7.4 wt.%Hf but with 7.5wt.%Cr more, were tested in oxidation between 1,000 and 1,200°C for 50 hours in dry synthetic air [5], and the mass gain files characterized according to the {m x dm/dt = Kp – Kv x m} method). The oxidized samples are here metallographically characterized to allow interpreting the differences previously observed in the mass gain kinetics.
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