Adding appropriate amounts of Al and Cu atoms to high-entropy alloys （HEAs） can significantly improve mechanical properties of the alloy
Adding appropriate amounts of Al and Cu atoms to high-entropy alloys （HEAs） can significantly improve mechanical properties of the alloys， but there are few research reports on the corrosion resistance of Al and Cu atoms in HEAs. To reveal the influence of Al and Cu atoms on the corrosion behavior of HEAs， this study focuses on FeCoNi based medium entropy alloys with excellent mechanical properties. FCC single-phase Fe25Co25Ni25Al10Cu15（Al10Cu15） alloy and BCC+FCC dual-phase Fe25Co25Ni25Al15Cu10（Al15Cu10） and Fe25Co25Ni25Al20Cu5（Al20Cu5） alloys are designed using empirical formulas for high-entropy alloy composition design. XRD analysis shows that the amount of FCC phase decreases and the amount of BCC increases with the increase of Al content， which is consistent with the theoretical calculation. SEM microstructure and EDS analysis show that increasing the amount of Al added and decreasing the amount of Cu added result in a transformation of the grain morphology from dendritic （Al10Cu15， Al15Cu10） to equiaxed （Al20Cu5）， and the composition of the interdendritic also changes significantly. The Al10Cu15 interdendritic microstructure is a Cu-rich FCC phase， the Al15Cu10 interdendritic microstructure is an Al-， Ni- and Cu-rich BCC phase， and the Al20Cu5 grain boundaries microstructure is a Fe- and Co-rich FCC phase. The potentiodynamic polarization（PDP） experiments show that alloys with high Al content have a dual-phase structure and are prone to galvanic corrosion during long-term immersion. The integrity of the passivation film is easily damaged， resulting in poor corrosion resistance of the alloy. The electrochemical impedance spectroscopy （EIS） tests show that the reaction resistance of alloys with higher Al additions decreases significantly with the prolongation of immersion time， which is consistent with the results of PDP analysis. Static immersion experiments at room temperature show that compared with Al10Cu15 alloy， Al15Cu10 and Al20Cu5 alloys are more susceptible to galvanic corrosion under prolonged immersion. It can be concluded that the addition of an excessive amount of Al atoms induced by the second phase significantly deteriorates the corrosion resistance of the material. Ensuring the homogeneity of alloy structure composition is an effective means to improve the corrosion resistance of materials.
