The impact of Al/Cr ratio on the oxidation kinetics of Y-doped AlCoCrFeNi high-entropy alloys at 1100 °C

Y/Hf-doped AlCoCrFeNi high-entropy alloys stand out for their potential application in high temperature coatings. Thereinto, both Cr and Al are crucial for improving oxidation properties. However, simultaneously increasing the content of Al and Cr is not advisable, since it can significantly reduce the ductility/toughness of the coating. In this research, we proposed an equivalent replacement method of Al and Cr, namely, tuning Al/Cr ratio (ACR), to enhance the elevated-temperature oxidation resistance of AlCoCrFeNi alloys. This strategy was verified by the 1000 h/1100 °C oxidation tests of three Y-doped AlCoCrFeNi alloys with different ACRs of 0.78, 0.58 and 0.41. The test results indicated an elusive transformation of oxidation rate occurred on these alloys, that the alloy with lowest ACR exhibited an initially higher oxidation rate but a lower oxidation rate over an extended period, in comparison to those higher ACR alloys. The underlying oxidation mechanisms were uncovered using microscopic techniques and thermodynamics calculations. The initial higher oxidation rate was ascribed to the rapid growth of spinel oxides, while the extended slower oxidation process was attributed to the resulting Al₂O₃ scale with larger grain sizes. Thermodynamic assessment revealed that larger Al₂O₃ grains corresponding to fewer grain boundaries decreased the diffusion coefficient of oxygen in Al₂O₃ scale. Our research is of both theoretical and industrial importance for clarifying the high temperature oxidation mechanism of Y-doped AlCoCrFeNi alloys and enhancing the oxidation resistance in multicomponent alloy systems.