Influence of carbides morphologies changes on the behaviour of preliminary aged alloys in high temperature oxidation
Résumé
A sufficient long exposure to very high temperature may induce significant changes for the interdendritic carbides present in some alloys and superalloys. It is well known that such microstructural modifications may promote decreases in the high temperature mechanical properties but it is also possible that they influence the behaviour of the alloy in oxidation at high temperature. It was already seen in previous works that a more or less long aging treatment initially applied to a simple ternary nickel-based alloy and to a more complex cobalt-based superalloy is able to influence their oxidation rates. The aim of this work is to complete these preliminary kinetic studies by an extensive metallography characterization, especially in the case of the cobalt-based superalloy which is the most complex and the closest (out of the two ones) to real refractory alloys used in high temperature applications. A more or less long preliminary exposure to high temperature can influence the behaviour in high temperature oxidation of equiaxed carbidesstrengthened alloys and superalloys. This was previously seen for the kinetic constants of oxidation (Kl , Kp and Kv increase with aging duration), and in the present work for the thickness of external oxide for example. Further works on this subject can be applying an exposure either at temperature higher than 1200°C or during a time longer than 100 hours, to more efficiently modify the morphologies and/or volume fractions of the interdendritic carbides, and then to enhance the changes in the surface and sub-surface affected by oxidation.
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