High temperature oxidation behavior of the dual-phase AlCoCr0.5Fe2.5Ni2.5 and single phase Al0.25CoCrFeNi high entropy alloys

This study examined both theoretically and experimentally the oxidation behavior of AlCoCr_0.5Fe_2.5Ni_2.5 and Al_0.25CoCrFeNi alloys in the time for 10 h and 180 h at 800 °C in air atmosphere. The results showed that the weight gain of the dual-phase AlCoCr_0.5Fe_2.5Ni_2.5 alloy followed the parabolic rate law, while this value persisted almost unchanged for the single-phase Al_0.25CoCrFeNi alloy at the same oxidizing condition. Surface analysis of the oxide scale confirmed the formation of Al₂O₃, Cr₂O₃ and Fe₃O₄ oxides. Comparing the experimental results with thermodynamic models and Density functional theory (DFT) calculations suggests that Al and Cr are first oxidized in the AlCoCr_0.5Fe_2.5Ni_2.5 and Al_0.25CoCrFeNi alloys, respectively, then outer Fe₃O₄ scale appeared in the two alloys. The weak resistance to high temperature oxidation of AlCoCr_0.5Fe_2.5Ni_2.5 alloy was mainly ascribed to its high fraction of grain and phase boundaries, as well as the fragmented oxide layer formed due to enhanced element diffusion at the FCC/BCC phase boundaries. The element migration and oxidation process of oxide layers in the two alloys were declared based on thermodynamic and DFT calculations.