The microstructure and oxidation behavior at high temperatures ranging from 900 °C to 1100 °C of equiatomic CrMoNbTaV high-entropy alloy produced by vacuum arc melting were investigated. The phase component, microstructure, and microhardness of the alloy were examined by using X-ray diffraction, scanning electron microscopy equipped with an energy-dispersive X-ray spectroscope, and Vickers hardness tests, respectively. The as-cast alloy consists of a single body-centered cubic (BCC) refractory metal solid solution due to the high mixing entropy effect and exhibits a dendritic microstructure. The alloy has a very high microhardness value of 923 HV due to the strong solid solution strengthening effect. The average microhardness in interdendrites (950 HV) was higher than that in dendrites (896 HV) because of composition segregation. The oxidation kinetic curves of the alloy after exposure to air at 900 and 1000 °C follow the pseudo-parabolic rate law, while the mass gain increases first and then decreases at 1100 °C. The thickness of the oxide layer increases with the increasing of oxidation time. The long rod-shaped oxidation products are composed of Nb2O5, NbO2, CrTaO4, CrNbO4, Ta9VO25, Nb9VO25, and TaO after oxidation at 900 and 1000 °C for 25 h. The oxides of CrTaO4 and CrNbO4 disappear as the oxidation temperature elevated to 1100 °C.

Al1.3CrFeNi eutectic high entropy alloy was designed and prepared by arc-melting to investigate the microstructure and oxidation behaviors at 1000 °C. The XRD pattern shows that this alloy had a double bcc/B2 structure. SEM images indicates that the microstructure of the alloy is composed of two precipitates of [Cr, Fe] solid solution and NiAl intermetallic, which form the typical eutectic structure. To explore the thermal application of Al1.3CrFeNi alloy, the oxidation behavior of Al1.3CrFeNi alloy at 1000 °C was investigated. From XRD and SEM results, it could be concluded that Al2O3 and Cr2O3 were the predominant oxides during the oxidation process. In addition, spinel like FeCr2O4 was also observed in the oxide scale. According to the analysis of oxide precipitates, the whole process of oxides’ formation was discussed and a simplified oxidation dynamic model of Al1.3CrFeNi alloy at 1000 °C was obtained. This could promote the development of thermal applications in multi-component alloys field.