Evolution of oxides and their microstructures at 800 degrees C in a gamma-gamma ` stabilised Co-Ni-Al-Mo-Ta superalloy

The paper reports mechanism of high temperature oxidation of the new gamma'-strengthened Co-30Ni-10Al-5Mo-2Ta (at%) alloy. The plots of mass gain vs. time show a parabolic growth behaviour at a temperature range of 650-800 degrees C. The cross-sections of the oxidised samples reveal formation of a complex multilayer of oxides. The top layer comprises of Co3O4 followed by a solid solution of (Co1-xNix)O (x varying from 0-0.2). A middle layer evolves with time comprising hexagonal molybdate Co2Mo3O8, metastable TaO and amorphous Al2O3. The gamma' dissolve near the interface exposing the gamma solid solution with growth of amorphous Al2O3 along the grain boundary.