Oxidation at high temperature of two commercial cobalt-based superalloys elaborated by casting and obtained with a fine microstructure
Résumé
Numerous superalloys exist for meeting high temperature uses' needs , with among them the cast cobalt-based ones. Many of these Co-based superalloys present the double particularity to resist corrosion by m olten aggressive s ubstances thanks to rather high c hromium quantities and high temperature mechanical stresses (occasioning creep deformation) thanks to primary carbides present in the interdendritic boundaries and to secondary fine carbides precipitated inside the matrix. The commercial Co-based superalloys are the most ofte n elaborated by foundr y in great quantities and their solidification is achieved in moulds with rather great s ize to obtain various pieces for aero-engines for instance. The microstructures which are obtained pr esent conseque ntly c oarse grains, but smaller grain sizes ma y be obtaine d in case of fast solidification; it can be interesting to investigate the high temperature properties of alloys with the same chemical compos ition as wellknown commercial alloys but with a finer microstructure, high temperature oxidation resistance for instance. In this study two cobalt-based superalloys were elaborated by targeting the c hemical compositions of two commercial ones. The y were obtained by foundry way in conditions of rather fast cooling favoring fine microstructures. They were thereafter subjected to more or less long exposures at two high temperatures in oxidizing atmosphere, principally to get some data concerning their beha vior in oxidation at high temperature, but als o to see how their microstructures are changing during exposure to high temperature, since these behaviors can be different by com parison to the microstructurally coarser commercial alloys. Thus, despite that these two cobalt-based alloys are not advantaged by first a cobalt matrix and second by rather low chromium contents by comparison with 30wt.%Crcontaining nickel-based alloys, the resistances against high temperature oxidation of these fine microstructure versions of commercial superalloys remain correct, notably for the second 24wt.%Cr-containing one.
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