High temperature oxidation behavior and mechanism of a new Ni-Co-based superalloy

Abstract

The oxidation behavior and mechanism of a new Ni-Co-based superalloy developed for advanced engine turbine disks, were investigated at various temperature for different exposure times. The isothermal oxidation experiments, scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and electron probe microanalysis (EPMA) were employed to analyze the oxidation characteristics of the new Ni-Co-based superalloy. The results show that the oxidation kinetics can be described by a parabolic law at each temperature and the oxidation rate constants K at temperature of 800 °C, 900 °C and 1000 °C are 0.015, 0.033 and 0.138 mg2 cm−4 h−1, respectively. The oxidation process is controlled by out-diffusion of metallic elements and in-diffusion of oxygen. Multifarious oxides are formed on the surface of the alloy due to the complex alloying elements. The oxide scale is consisted of three parts: the external oxide layer containing TiO2 and a small amount of Cr2O3; the middle oxide layer consisted of Cr2O3, TiO2, Al2O3 and the spinel phase; the internal oxide layer comprised of Al2O3. With further oxidation, TiO2 accumulates gradually on the external oxide layer because of the large diffusion distance of Ti, while O penetrated into the matrix can be captured by Al to prevent the further diffusion into alloy interior.