Carbide transformations occurring between 1000 and 1100°C in the sub-surface of a Ni-25Cr-0.50C alloy induced by hot oxidation
Résumé
Alloys and superalloys elaborated by foundry practice are often strengthened by carbides. They constitute an important family of refractory alloys for applications at high temperature. The metallic carbide-forming elements present in the reinforcing carbides are easily oxidable. When they are used in hot conditions in aggressive environments, such alloys are subjected to changes of the carbides initially present in the sub-surface. These changes are induced by high temperature oxidation occurring on surface. Such microstructure modifications may possibly modify the local refractoriness as well as the local mechanical properties in sub-surface.
These phenomena can be of different types: (eventually) development of a carbide-free zone (case of alloys reinforced by chromium carbides and/or TaC carbides) or not (case of HfC-reinforced alloys), local coarsening/nature change of the carbides initially present, precipitation of new ones ...
This study focuses on the oxidation−induced phenomena encountered in the sub-surfaces by ternary Ni-Cr-C alloys, with additionally the study of the effect of the temperature and duration of oxidation. The phenomena are interpreted using thermodynamic calculations.
Oxidation thus induces curious microstructure phenomena in the sub-surface of the carbide-strengthened alloys. Some of them (new carbides zone) are much more difficult to interpret/explain than the other (carbide-free zone notably). Obviously, in addition to temperature-dependent, these later phenomena are also time-dependent, as suggested by the additional tests with longer durations. Further work may consist in new duration increasing to follow further the evolution with time of this particular intermediate zone. In parallel one can also wonder how the oxidizing atmosphere may influence these phenomena (e.g. presence of water vapour in air).
Origine : Fichiers produits par l'(les) auteur(s)