Effect of Nanocrystallization on the Oxidation Behavior of a Ni–8Cr–3.5Al Alloy

Abstract

Magnetron-sputter deposition was used to produce a Ni–8Cr–3.5Al(wt.%) nanocrystalline coating on substrates of the same alloy. Theoxidation behavior of the cast Ni–8Cr–3.5Al alloy and itssputtered coating were investigated at 1000°C in air. Complex,layered-oxide scales composed of Cr₂O₃ outer layer,mixed spinel NiAl₂O₄ and NiCr₂O₄middle layer, and α-Al₂O₃ inner layer were formedon the Ni–8Cr–3.5Al nanocrystalline coating during 200-hroxidation, whereas Cr₂O₃, with some NiCr₂O₄external layer with internal Al₂O₃, formed on the castalloy. Because of the formation of this α-Al₂O₃inner layer on the coating, the sputtered Ni–8Cr–3.5Al coatingshowed better oxidation resistance than the cast alloy. The effect ofnanocrystallization on oxide formation is discussed. It was indicated thatthe formation of this α-Al2O3 inner layer was closely related to therapid diffusion of Al through grain boundaries in the nanocrystallinecoating and the relatively high Cr content in Ni–8Cr–3.5Al.