Tailoring critical Al concentration to form external Al₂O₃ scale on Ni–Al alloys by computational approach

Nickel (Ni)-based superalloys for high-temperature applications are often designed to form a continuous and slow-growing oxide scale by adding Al and Cr and other beneficial elements. In the present work, the critical Al concentration in Ni–Al alloys needed to establish an α-Al₂O₃ scale in contrast to internal oxide formation is predicted as a function of temperature by means of the CALPHAD approach coupled with models in the literature, which account for the thermodynamics and kinetics of oxidation. The present thermodynamic remodeling of the Ni–O system results in a better agreement with experimental data of oxygen solubility in Ni at high temperatures. The oxygen solubility is combined with kinetic parameters to determine oxygen permeability in Ni, and the critical Al concentration needed to establish an α-Al₂O₃ scale at a given exposure temperature. Good agreement is found with available experimental data for both oxygen permeability and critical Al concentration, indicating the capacity of the CALPHAD approach to tailor oxidation resistance for materials of interest using thermodynamic and kinetic knowledge.