Behaviour of Nickel-Rich Non-Equimolar High Entropy Alloys in High-Temperature Oxidizing Conditions

Abstract

This study aims to compare the behaviour of two Ni-rich non-equimolar AlCoCrFeNi high entropy alloys, i.e. Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ (at%) in isothermal high-temperature oxidizing conditions. In both cases, mass gain after 100-hr oxidation at 1173 K in synthetic air atmosphere does not exceed 1 mg/cm², indicating good resistance against the corrosive environment. Investigations on the morphology, chemical and phase composition of the alloys after the oxidation process indicate that a mixture of Al₂O₃ and Cr₂O₃ is responsible for the protective properties of the scale formed on Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅, whereas an additional chromium–iron–nickel–cobalt spinel structure was determined on the Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ sample after prolonged exposure to the above-mentioned conditions. The oxidation kinetics are slightly better in the case of Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and lower amounts of the remaining constituent elements were detected in the protective scale compared to Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅. Furthermore, the Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ substrate was able to maintain its initial morphology throughout the entire alloy after the corrosion process. From all of the above, it can be concluded that Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ seems to demonstrate better oxidation properties at a high temperature than Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅.

Fig. 9 XRD patterns obtained from (a) Al₂₀Co₅Cr₂₀Fe₂₀Ni₃₅ and (b) Al₁₀Co₁₅Cr₂₀Fe₂₀Ni₃₅ after oxidation at 1173 K in synthetic air for 100 h.