THE OXIDATION OF A DIRECTIONALLY SOLIDIFIED NI-AL-CR3C2 ALLOY AT 1100-DEGREES-C AND 1200-DEGREES-C IN OXYGEN

The oxidation behavior of a directionally solidified Ni-Al-Cr3C2 alloy of overall composition Ni-12.3Cr-6.9Al-1.8C (wt. %) has been investig ated at 1100 and 1200 degrees C under I atm oxygen. Experiments have a lso been carried out on specimens having the same composition but with a nonaligned structure. At 1100 degrees C, in both cases and unlike c onventional nickel-base superalloys with the same chromium and aluminu m contents, aluminium was found to oxidize internally beneath an exter nal Cr2O3 scale. Although the volume fraction of the internal precipit ates was significant, they showed no tendency to coalesce into a compa ct subsurface layer, but formed randomly distributed clusters in the a lloy matrix. The kinetics of oxidation and morphologies of the oxide s cales were not substantially affected either by thermal cycling or by the alloy microstructure. At the higher temperature, continuous Al2O3 scales formed beneath thick layers of transient nickel and nickel-chro mium-rich oxides; no internal precipitation of aluminum-rich oxides wa s observed. However, internal degradation of the directionally solidif ied specimens at 1200 degrees C was quite significant, due to in situ oxidation of primary carbides. The multilayered scales formed at 1200 degrees C spalled extensively on cooling as a consequence of loss of c ontact, starting preferentially at the intersections of the Cr3C2 fibe rs with the alloy-scale interface. The observed behavior can be attrib uted to a reduction in the availability of chromium because of the mul tiphase structure of the alloy; this, in turn, resulted in an increase in the flux of oxygen inward, leading to internal oxidation of alumin um at 1100 degrees C. The almost exclusive external oxidation of alumi num becomes possible at 1200 degrees C, probably because of an increas e in the diffusivity of aluminum in the alloy matrix.