Spallation and transient oxide growth on PWA 1484 superalloy

A multi-layered oxide forms on single crystal PWA 1484 alloy when oxidized at high temperatures, consisting of a spinel ((Ni(Cr,Al,Co)2O4) at the air/oxide interface, interspersed with tantalum rich oxide particles (CrTaO4/NiTa2O6), and an inner-most layer of α-alumina (α-Al2O3) in contact with the alloy. Observations on rapid cooling after oxidation show that the spallation behavior of this multi-layer oxide depends on time in room temperature laboratory air, the sulfur content of the alloy, and the alloy surface preparation. Time-lapse optical microscopy reveals that spallation of the multi-layer oxide occurs differently on polished surfaces in comparison to ground surfaces. On ground surfaces spallation occurs solely at the metal/alumina interface in both alloys, and follows the grinding scratches’ long axes. The higher S alloy (1.7 ppmw S) almost completely spalls to bare metal (∼95% oxide spalled), whereas very little spallation (∼5%) occurs on the lower S alloy (0.2 ppmw S). On polished surfaces spallation occurs along both the alumina/spinel and alumina/alloy interfaces of the higher sulfur containing alloy but only along the alumina/spinel interface of the oxide on the low-sulfur alloy, leaving the alumina in contact with the metal. The lack of significant difference in measured residual stress in the α-alumina combined with the fact that spallation occurred over periods of several hours at room temperature suggests that failure is a time-dependent process likely associated with the presence of moisture, once a critical oxide thickness has been exceeded. The role of sulfur is likely associated with a reduction in fracture toughness of the oxide and oxide/metal interfaces through formation of voids or reduction in bond energy. ting of the alumina-covered, low-sulfur alloy (0.2 ppmw) indicates that outward grain boundary diffusion of aluminum and tantalum occurs through the α-alumina but not nickel. Also, no new Ni-rich oxides reform confirming that they are indeed transient oxides formed during the initial stages of oxidation of the bare alloy.