Effect of Co Content on the Microstructure and Corrosion Behavior of Al_0.3Co_xCrFeNi High Entropy Alloys

The influence of Co content on the microstructure and corrosion behavior of Al0.3CoxCrFeNi (x = 0.25, 0.5 and 1.0) high entropy alloys were investigated. The results indicated that the increased content of Co improve the corrosion resistance of alloy, which can be attributed to the distinction of phase composition in alloys caused by different Co content. The Al0.3CoxCrFeNi alloy shows a double-phase (FCC + BCC) structure for x = 0.25 and 0.5, and BCC phase consists of (Al, Ni)-rich B2 phase and (Cr, Fe, Co)-rich A2 phase. The increase of Co content induces the transformations of crystal-structures from BCC to FCC, resulting in the decrease of BCC phase proportion. The Al0.3CoCrFeNi alloy exhibits the single FCC structure, and all elements tend to be homogeneously distributed. The difference in the potential of FCC and BCC phases indicates the BCC phase acts as anode resulting to localized corrosion, while B2 phase preferentially dissolves compared to the A2 phase due to the lower potential. Therefore, the existence of selective corrosion is found in the alloys with double-phase, by contrast, the alloy with single FCC phase shows random pitting corrosion.