Roles of the matrix and of the primary carbides in the general high temperature oxidation behaviour of cobalt-based superalloys. part 3: the Co(10Ni,30Cr) matrix with tantalum carbides

摘要

In this third and final part of this study dealing with the specific role of interdendritic carbides on the global high temperature oxidation behaviour of conventionally cast cobalt-based alloys, this was the one of a second type of carbides among the most commonly used for creep-resistance which was examined. Tantalum carbides present a better stability at high temperature than chromium carbides but their presence induces the one of a particularly oxidable element in high quantity and for some of them emerging from surface directly exposed to oxidation. The same tests of 46 hours at 1000, 1100 and 1200℃ were applied to a Co-10Ni-30Cr-0.5C-7.5Ta alloy. They revealed that detectable oxidation during heating occurred sooner than for the two first studied alloys (Co-10Ni-30Cr and Co-10Ni-30Cr-0.5C). In addition, the isothermal mass gain - purely parabolic for this tantalum-containing alloy in contrast with the two others - was a little faster. And finally the scale spallation started at lower temperature than the ternary carbide-free alloy, as this was observed for the alloy containing chromium carbides.