Unveiling the mechanism of yttrium-related microstructure inhibiting or promoting high-temperature oxidation based on Ni-Al-Y alloys

Yttrium is an attractive component for nickel base alloys to improve high-temperature oxidation resistance. However, the application suffers from microstructure related problem, due to less unconscious of Y-related oxidation behavior, especially interacting with other key components, such as aluminium. This study introduces the discovery of microstructure depended external and internal oxidation behaviors based on four simplified Ni-Al-Y alloys during isothermally exposing at 800°C/1000°C in air. Using multiscale microstructural characterizations and thermodynamic calculations, different oxidation layer thicknesses have been observed in the four alloys, especially varying with Y and Al concentrations. In the three high-Y-content alloys, Y-related microstructures are found to strongly drive the oxidation process. Original-Ni5Y compound with strong tendency to precipitate secondary γ-Ni strip is favorable to the formation of inner oxidation layer containing mixed oxide particles, whereas original-γ-Ni phase drives the formation of outer scale of NiO. Dense and well-configured lamella-like phase boundaries (PBs) are beneficial to inhibiting the development of the inner layer, compared to coarse and irregular PBs. The resistance to inner oxidation of minor-Y-addition alloy has been significantly improved at 1000°C, benefiting from the mixed oxides along grain boundaries (GBs) and front of inner oxidation layers. From the thermodynamic basis, a type of modified diagram retrieved from ternary isothermal section is utilized to exhibit the oxide evolution, which is consistent with experimental results, especially for those complicated products.