Transient Oxidation of a γ-Ni-28Cr-11Al Alloy

gamma-NiCrAl alloys with relatively low Al contents tend to form a layered oxide scale during the early stages of oxidation, rather than an exclusive alpha-Al2O3 scale, the so-called "thermally grown oxide" (TGO). A layered oxide scale was established on a model gamma-Ni-28Cr-11Al (at.%) alloy after isothermal oxidation for several minutes at 1100A degrees C. The layered scale consisted of an NiO layer at the oxide/gas interface, an inner Cr2O3 layer, and an alpha-Al2O3 layer at the oxide/alloy interface. The evolution of such an NiO/Cr2O3/Al2O3 layered structure on this alloy differs from that proposed in earlier work. During heating, a Cr2O3 outer layer and a discontinuous inner layer of Al2O3 initially formed, with metallic Ni particles dispersed between the two layers. A rapid transformation occurred in the scale shortly after the sample reached maximum temperature (1100A degrees C), when two (possibly coupled) phenomena occurred: (i) the inner transition alumina transformed to alpha-Al2O3, and (ii) Ni particles oxidized to form the outer NiO layer. Subsequently, NiO reacted with Cr2O3 and Al2O3 to form spinel. Continued growth of the oxide scale and development of the TGO was dominated by growth of the inner alpha-Al2O3 layer.