Roles of thematrix and of the primary carbides in the general high temperature oxidation behaviour of cobalt-based superalloys. part 1: the Co(10Ni,30Cr) matrix alone

Author(s): Alexandre Navet, Albert Leroy,Thierry Schweitzer, LionelAranda, Patrice Berthod

Many cast cobalt-based superalloys are composed of a dendritic matrix often of the Co(Ni,Cr) type and of primary carbides appeared in the interdendritic areas at the end of solidification. Since the carbide-forming elements (Cr, Ta…) are also easily oxidable these primary carbides bring a significant contribution to the nature and the kinetic phenomena taking place during oxidation at elevated temperature. What is occurs on surface is then often complex and to simplify the interpretations it may be useful to knowseparately howbehave matrix and carbides. The present work aims to try this uncoupling by characterizing first the high temperature oxidation behaviour of the matrix alone. For that three thermogravimetry tests were performed at 1000, 1100 and 1200°C during 46 hours and the whole mass variation files were analysed to precise how behaves this matrix-simulating alloy and the oxide scales resulting of its oxidation, not only during the isothermal part of exposure, but also during the heating and the cooling. The obtained resulted are devoted to be later compared to other ones determined in the same conditions for two other alloys having the same matrix but additionally containing a first type of carbides (CrxCy) and after a second type of carbides (TaC).

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