Effects of Cr and Al additions on the oxidation of FeCoNi-based medium- and high-entropy alloys in CO2/CO mixed gases.

Abstract

The effects of Cr and Al additions on the oxidation of FeCoNi-based medium- and high-entropy alloys containing FeCoNi, FeCoNiCr, and FeCoNiCrAl_0.3 were studied at 600-950 ℃ in a 9:1 CO₂/CO gas mixture. The oxidation kinetics of all three alloys obeyed the parabolic-rate law, and their oxidation rates steadily increased with temperature. The scales formed on FeCoNi consisted of an intermixed layer of CoO and Fe₃O₄ and internal FeO precipitates (observed at 950 ℃). Those formed on FeCoNiCr consisted of an outer layer of CoNiO₂ intermixed with FeCoCrO₄ and an inner layer of Cr₂O₃. The scale formed on FeCoNiCrAl_0.3 was strongly temperature-dependent, consisting of discontinuous Cr₂O₃ granules at 600℃, internal Al₂O₃ precipitates formed beneath a Cr₂O₃ layer at 760 ℃, and an exclusive α-Al₂O₃ layer at 950 ℃.