Oxidation behavior and microstructure degeneration of cast Ni-based superalloy M951 at 900 °C

Abstract

Oxidation behavior and microstructure degeneration of Ni-based superalloy M951 were investigated at 900 °C for 200 h. Oxide layer formed on the surface of the superalloy included external NiO layer, intermediate Cr2O3 layer, and inner Al2O3 layer. The front of oxidation layer progressed in the sequences: intrusion of Al2O3 spots in matrix, connection of Al2O3 oxide channel in surficial layer, and the formation of continuous Al2O3 layer. Continuous Cr2O3 scale and Al2O3 scale can be protective to the superalloy at 900 °C. The oxide layers at grain boundary were similar compositionally to the surface oxide layers, while the rapid diffusion of oxygen along grain boundary promoted the formation of large oxidation channel. A γ′-denuded region which formed between matrix and oxide layer indicated the degeneration of strengthener after long-term exposure at high temperature. Moreover, secondary carbide M23C6 was observed in the superalloy.