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

Abstract

γ-NiCrAl alloys with relatively low Al contents tend to form a layered oxide scale during the early stages of oxidation, rather than an exclusive α-Al₂O₃ scale, the so-called “thermally grown oxide” (TGO). A layered oxide scale was established on a model γ-Ni–28Cr–11Al (at.%) alloy after isothermal oxidation for several minutes at 1100°C. The layered scale consisted of an NiO layer at the oxide/gas interface, an inner Cr₂O₃ layer, and an α-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 (1100°C), when two (possibly coupled) phenomena occurred: (i) the inner transition alumina transformed to α-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 α-Al₂O₃ layer.