Thermodynamic calculations used for characterizing the sub-surface changes induced by high temperature oxidation for carbides-reinforced alloys
Résumé
To achieve sufficient mechanical properties for high temperature applications many refractory cast alloys are strengthened by carbides. When these ones are oxidized at high temperature their subsurface microstructures evolve. There is generally a disappearance of carbides from surface over a depth which increases with temperature and time while, in some conditions, curious phenomena occur for carbides in an area separating carbidefree zone and bulk (coarsening, stoichiometry change, precipitation of additional carbides). In addition, iron alloys may also know modification of matrix nature in the zone affected by oxidation, due to local chemical changes. The aim of this work is to list the types of subsurface modifications (concerning carbides only) induced by high temperature oxidation, and to interpret these local microstructure phenomena by using thermodynamic calculations.
Thus, in addition to the development of oxides layer over the external surface and sometimes internal oxides growth, oxidation at high temperature promotes several subsurface microstructure modifications. Among them there are sometimes curious phenomena concerning deeper carbides, obviously induced by a not outward but inward diffusion of carbon released by disappearing carbides. The growing carbidefree zone, which always acts, may be considered as not desirable for the alloy surface because of a decrease in hardness and probably in wear resistance; but it can be also considered as an especially refractory skin which can prevent emergence of molten alloy as in case of sudden significant increase of temperature in service.
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