High-Temperature Oxidation of High-Entropy Alcrfeconimnx Alloys

• Published in: Powder metallurgy and metal ceramics 2023-09, Vol.62 (5-6), p.360-371
• Main Authors: Rokytska, O. A., Karpets, M. V., Yakubiv, M. I., Krapivka, M. O., Samelyuk, A. V., Naumenko, M. P.
• Format: Article
• Language: English
• Subjects: Alloys; Aluminum oxide; Ceramics; Characterization and Evaluation of Materials; Chemistry and Materials Science; Composites; Entropy; Glass; High entropy alloys; High temperature; Intermetallic compounds; Internal oxidation; Manganese oxides; Materials Science; Metallic Materials; Microhardness; Mixed oxides; Natural Materials; Oxidation; Oxidation rate; Phase composition; Reaction kinetics; Scale (corrosion); Sigma phase; Solid solutions; Spinel; Spinodal decomposition
• ISSN: 1068-1302; 1573-9066
• DOI: 10.1007/s11106-023-00399-w

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 MeMn 2 O 4 , as well as Mn 3 O 4 and Al 2 O 3 , formed on the AlCrFeCoNiMn alloy, while only Mn 3 O 4 and Al 2 O 3 oxides emerged on the AlCrFeCoNiMn 0.5 alloy. Increase in the oxidation time for the equiatomic alloy up to 25 h led to spinel NiMn 2 O 4 and bixbyite FeMnO 3 in the oxide scale; Mn 3 O 4 and Al 2 O 3 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 NiMn 2 O 4 , FeMnO 3 , Mn 3 O 4 , and Al 2 O 3 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.