Alumina Failure and Post-failure Oxidation in the NiCoCrAlY Alloy System at High Temperature

This paper examines the oxidation behavior of thin specimens of cast NiCoCrAlY alloys at 1150 ^∘ C through successive stages, from Al_2 O_3 growth to complete alloy conversion to oxide. Five alloy compositions were used, with varying fractions and compositions of γ and β . The time evolution of the alloy composition during Al_2 O_3 growth was simulated using the DICTRA module of Thermo-Calc and calculated analytically in the approximation of flat profiles. Simulated and experimental profiles were found to be in good agreement, indicating that the phase equilibrium and mass balance were correctly reproduced in the simulations. Local variations of alloy composition were observed in thinner specimens and found to be comparable with the variations expected from the uncertainty on the initial specimen thickness. The variations observed in the time-to- Al_2 O_3 failure were greater than expected on this basis, suggesting that additional sources of variability were in effect. Alumina failure was followed by the growth of a Cr_2 O_3 layer at the alloy–scale interface. Similarly, Cr consumption eventually led to Cr_2 O_3 failure, and Ni- and Co-containing spinel oxide formed, converting the Cr_2 O_3 at the alloy–scale interface and the Al_2 O_3 at the scale–gas interface. The remaining NiCo alloy was then converted to (Ni,Co)O. This sequence occurred without abrupt increase in the mass gain, due to the continued presence of the remnant Al_2 O_3 layer, and to the small amount of metal left to oxidize when the (Ni,Co)O eventually broke through the scale. The evolution of the scale composition throughout the oxidation stages is discussed based on an analysis of the thermodynamic conditions at the alloy–scale interface.