The Effects of Al Addition on the Oxidation Behavior of Ni–Fe–Cr–Nb Alloys at 800 °C in Air

High temperature oxidation of Ni–19Fe–19Cr–5Nb alloys (based on the chemical composition of Alloy718) with different Al contents were investigated to elucidate the effect of Al on oxidation kinetics and scale formation during exposure at 800 °C in air. The experimental results indicated that the oxidation kinetics decreased with increasing Al content. The beneficial effect of Al could be attributed to be a result of the formation of γ′-Ni₃Al, which stabilizes γ′′-Ni₃(Nb,Al) and kinetically prohibits the coarsening of γ′′-Ni₃(Nb,Al) phases. Faster dissolution of finer γ′′-Ni₃(Nb,Al) precipitates due to greater γ-matrix / γ′′-Ni₃(Nb,Al) interface region in as-received Al-containing alloys could supply greater Nb flux for the formation of CrNbO₄, which promotes the formation of an exclusive Cr₂O₃ scale at the initial stage of oxidation.