Oxidation behavior of arc melted AlCoCrFeNi multi-component high-entropy alloys

Multi-component, high-entropy alloys (HEAs) are being investigated as potential alternatives for high temperature structural materials due to their reported high symmetry crystal structures, favorable mechanical properties, high temperature phase stabilities, and resistances to degradation in oxidizing/corrosive environments. However, their high temperature oxidation behaviors are poorly understood. In this work, the as-cast microstructures and 1050 °C oxidation behaviors of a series of arc-melted Alx(NiCoCrFe)100-x HEAs where x = 8, 10, 12, 15, 20, and 30 (at.%) were investigated. The dominant structure of the low Al concentration HEAs was determined to be FCC, while the high Al concentration HEAs were BCC dominant. A structural transition point at ∼15 at. % Al exists where a large fraction of both FCC and BCC are present. Each HEA exhibited initial transient oxidation followed by various degrees of parabolic oxide growth. All of the HEAs formed a combination of Al2O3 and AlN beneath an external Cr2O3 scale. Increased Al content improved the continuity and internal position of the Al2O3 scale, resulting in enhanced oxidation resistances. These results are discussed relative to chemically similar conventional alloys and existing Ni-Cr-Al oxide formation models. The resulting phase equilibria has been compared to thermodynamic predictions made using the CALPHAD method.