Effect of Co, Ni, Cu content on phase composition, microstructure and corrosion resistance of Co_1-xCrFeNi_1+xCu_y series high-entropy alloys

Co_1-xCrFeNi_1+xCu_y high-entropy alloys (HEAs) were fabricated by vacuum arc melting. With the change of Co, Ni and Cu content, the phases, microstructure and corrosion resistance of HEAs were investigated detailly. It is found that the microstructure of alloys is dendrite and all alloys have FCC phase. The phases of Cu-containing alloys include Cu-rich and Cu-poor phase, and there is a boundary zone with very high Cu content between the two phases. In 3.5 wt% NaCl solution, the higher the Cu content, the lower the corrosion resistance of alloys., which is attributed to the fact that the poor corrosion-resistance Cu₂O film formed on the surface of Cu-rich phase destroys the integrity of passive film, which makes the Cu-rich phase and the boundary zone exposed in the electrolyte. Due to the potential difference in the two areas, the Cu-rich phase is preferentially corroded. Then, other elements except Cu form spinel-type metal oxide with excellent corrosion resistance, so Cu in the copper-rich phase dissolves first. Increasing the Ni content and decrease the Co content is beneficial to increase the solid solubility of Cu in alloys which decrease the segregation of Cu at the grain boundary and promote the corrosion resistance of the HEAs.