Microstructural stability and high temperature isothermal oxidation behavior studies of high entropy alloy Al0.5CoCrFeNi

• The activation energy for the oxidation of Al 0.5 CoCrFeNi HEA is 195.7 kJ/mol. • The oxidation kinetics of the alloy follow a parabolic law. • The oxidation mechanism of the alloy is similar to the MCrAl alloy ( M =Fe/NiAl). • Shrinkage and spheroidization of the B2 phase reduce the hardness of the alloy. High entropy alloys of Al-Co-Cr-Fe-Ni have been developed for high-temperature applications due to their high melting points, superior microstructural stability, and the ability to form a protective oxide scale on the surface. This paper presents the results of research on the isothermal oxidation of the Al0.5CoCrFeNi alloy at temperatures of 900, 1000, and 1100 °C for 2, 16, and 40 h. A series of experiments were conducted to investigate the microstructural stability and oxidation behavior of the alloy. During oxidation, the alloy forms various oxides, including Fe 2 O 3 , Co 3 O 4 , spinel of Cr 2 NiO 4 , Cr 2 O 3 , and Al 2 O 3 , as well as AlN. It was found that the oxidation behavior of the alloy is closely related to that of the general medium entropy M -Cr-Al system ( M =Fe, Ni, Al). The oxidation kinetics of the alloy follow a parabolic rate law with an activation energy of 195.7 kJ/mol. The instability of the microstructure of the alloy is indicated by the spheroidization of the B2 phase due to thermal stress occurring at the A1/B2 phase interface, in addition to a decrease in its volume fraction. These microstructural changes result in a decrease in the alloy's hardness.