High-Temperature Oxidation of High-Entropy Alcrfeconimnx Alloys

The evolution of phase composition and thermal oxidation behavior of high-entropy AlCrFeCoNiMn_x alloys (x = 0.5 and 1) during long-term oxidation at 900°C were studied. A single- phase ordered (B2) bcc alloy formed in the starting as-cast state regardless of manganese content. The scale phase composition varied with exposure time and manganese content. After 10 h of oxidation, high-entropy spinel-type MeMn₂O₄, as well as Mn₃O₄ and Al₂O₃, formed on the AlCrFeCoNiMn alloy, while only Mn₃O₄ and Al₂O₃ oxides emerged on the AlCrFeCoNiMn_0.5 alloy. Increase in the oxidation time for the equiatomic alloy up to 25 h led to spinel NiMn₂O₄ and bixbyite FeMnO₃ in the oxide scale; Mn₃O₄ and Al₂O₃ were also present. The phase composition of the oxidized layer on the AlCrFeCoNiMn_0.5 alloy did not change. After 50 h, the structure of the oxide scale was similar for both alloys and consisted of NiMn₂O₄, FeMnO₃, Mn₃O₄, and Al₂O₃ in different ratios. The oxidation kinetics of the alloys naturally depended on the manganese content: the higher the manganese content, the higher the oxidation rate. A continuous layer of the fcc solid solution rich in chromium, iron, and cobalt was observed under the scale in both alloys. An internal oxidation area was also found in the subscale layer of the AlCrFeCoNiMn alloy. Long-term (more than 50 h) oxidation at 900°C substantially changed the phase composition of the alloy matrices: the bcc (B2) solid solution underwent spinodal decomposition to form bcc and fcc phases and tetragonal σ phase. Analyses of the alloy matrices showed a sharp increase in their microhardness after annealing. This can be attributed to the formation of a significant amount of the σ phase.