Analytical study of surface and bulk oxidation of Astroloy

Nickel-base alloys, such as Astroloy, used for aeronautical turbine disks, are sensitive to time-dependent cracking in environments containing oxygen. The mosaic structure of the alloy consisting islands (200 nm average size) surrounded by the -phase (100 nm thick) induces complex oxidation phenomena. Various analytical approaches allow the delineation of all the steps from segregation to oxidation occurring on the surface of such a duplex structure. The protection of Astroloy by its outer oxide layer against oxygen penetration was studied also, using alternative¹⁶O₂then¹⁸O₂oxidation. In association with STEM studies, it is shown that the outer oxide scale is not a real barrier against oxygen penetration and that inner precipitation of chronium (+ aluminium and titanium)-enriched oxides, takes place especially in the structure.