Effect of NbC Addition on the High-Temperature Oxidation Resistance of Co- and Ni-Based Chromium-Rich Alloys

Two {M-25Cr}-based alloys, with M = Co or Ni, containing 0.5C and 3.9Nb (wt%) to favor the crystallization of NbC carbides during solidification were prepared by casting and then subjected to oxidation in air at 1000, 1100 and 1200 A degrees C for 46 h. The results were studied by comparison with recent similar ones obtained with Nb-free ternary alloys issued from the same preparation protocol and tested in oxidation in the same conditions. In both cases interdendritic script-like-shaped NbC carbides were obtained as expected, however, not as single carbides in the nickel alloy which also contain chromium carbides. Preliminary exposure to 1200 A degrees C having shown a morphological evolution of the NbC, as-cast and preliminarily aged versions of both alloys were tested in oxidation. It appeared first that, whatever the aged state, the cobalt alloy behaved much worse than the nickel one, that the presence of Nb deteriorated the behavior in both cases and that aging the alloys, which resulted in NbC fragmentation, was also detrimental for the high-temperature oxidation behavior.