Effect of Al doping on the early-stage oxidation of Ni-Al alloys: A ReaxFF molecular dynamics study

The way how oxides form is critical to surface mechanical properties and also crack initiation and propagation in Ni-based superalloys, but the mechanism is still unclear in atomistic scale. In this work, we employ molecular dynamics simulation in combination with first-principles calculation to investigate the effect of Al doping on the early-stage oxidation of Ni-Al alloys. The activation energies for the oxidation are one order of magnitude lower compared to that for long-time oxidation reported in experiments. The oxidation of Ni is found to continuously increase the surface roughness due to the formation of island-like oxides which is accelerated by raising temperature. For alloys with Al doping, the surface roughness is generally low and its temperature dependence is weak. The distributions of bond angles reveal that the oxides are in intermediate states. The migration energy is high for oxygen to diffuse into Ni matrix, while it can be remarkably reduced by Al doping. The reason is that the covalent component of Ni-O bond is largely suppressed by Al doping, which is responsible for the effect of Al on the evolution of surface morphology during oxidation