Effect of Mo on microstructure, mechanical and corrosion properties of FeCrNiMnMox high-entropy alloys

Four FeCrNiMnMox (x=0, 0.1, 0.3, 0.5, in molar ratio) high-entropy alloys (HEAs) were synthesized by vacuum arc melting to explore the potential impact of Mo on the microstructure, mechanical properties, and passivation and electrochemical behaviors in 0.5 M H2SO4 solution. The results display that the FeCrNiMn alloy exhibits a single face-centered cubic (FCC) structure while the microstructures of the FeCrNiMnMo0.1, FeCrNiMnMo0.3, and FeCrNiMnMo0.5 alloys consist of the FCC and σ phase. The appear of the σ phase ascribed to the addition of Mo enhances the hardness and yield strength with the sacrifice of plasticity. The FeCrNiMnMox HEAs achieve the maximum hardness of 414 HV0.2 and the highest compressive yield strength of 830 MPa when x=0.5, but compressive fracture strain is lowered to 10.8%. X-ray photoelectron spectroscopy (XPS) and electrochemical analysis show that the passivation film in FeCrNiMnMox alloy mainly consists of chromium oxides and molybdenum oxides. Mo has a beneficial effect on the corrosion resistance of the FeCrNiMnMox HEAs in a 0.5 M H2SO4 solution by increasing the corrosion potential (Ecorr) and decreasing the corrosion current density (Icorr) and passivation current density (Ipass). The FeCrNiMnMo0.1 alloy shows the best corrosion resistance, mainly due to its passivation film consisting of a large proportion of chromium oxide (Cr2O3). More Mo additions promote the formation of the precipitate of a phase and the matrix regions depleted Cr and Mo elements adverse to the resistance to preferential localized corrosion.