Effect of Al/Cu Ratio on Microstructure and High-Temperature Oxidation Resistance of AlxCoCrCuyFeNi High-Entropy Alloy Coatings

To improve high-temperature oxidation resistance for Ti6Al4V alloy, AlxCoCrCuyFeNi (x = 0, 0.3, 0.5, 0.7, 1.0; y = 1.0, 0.7, 0.5, 0.3, 0, x + y = 1.0) high-entropy alloy (HEA) coatings were prepared on the Ti6Al4V alloy substrate by a laser cladding technique. The results show that the coatings were mainly composed of FCC, BCC, and Ti-rich phases. Severe segregation of the Cu element occurred in the CoCrCuFeNi HEA coatings as a Cu-rich phase (FCC2). The Cu-rich phases decreased with a decreasing Cu content and completely disappeared until the Al content reached 1.0. The microhardnesses of the Cu1.0, Cu0.7Al0.3, Cu0.5Al0.5, Cu0.3Al0.7, and Al1.0 HEA coatings were 2.01, 2.06, 2.08, 2.09, and 2.11 times that of the substrate, and compared with those of a Ti6Al4V alloy substrate, the oxidation rates of the HEA coatings decreased by 55%, 51%, 47%, 42%, and 35%, respectively. The surface oxides of the five coatings were mainly composed of CuO, TiO2, Fe3O4, Cr2O3, and Al2O3. The increase in the Al content promoted the generation of Al2O3 film and Cr2O3 on the surfaces of the coatings, which significantly improved the high-temperature antioxidant performance of the high-entropy alloy coatings for 50 h at 800 °C. When x = 1.0, the coating showed the best high-temperature antioxidant performance.