Effects of Al and Co contents on the microstructure and properties of AlxCoCryFeNi high-entropy alloys

AlxCoCryFeNi high-entropy alloys (HEAs) were prepared by vacuum arc melting. The effects of Al and Cr contents on the microstructure, corrosion resistance and mechanical properties of the alloys were evaluated. Results showed that with an increase in Cr content, the phase of the alloys changed from FCC + B2 + BCC to FCC + B2 + BCC + FeCr when the Al content was 0.75 (X = 0.75). Coarse FCC dendrites decreased. The alloys exhibited two mixed BCC/B2 structures with different morphologies: (i) the granular B2 phase distributed in the BCC matrix and (ii) the maze-like spinodal decomposition structure. At X = 1.5, the FCC and FeCr disappeared, and the microstructure of the alloy transformed into a mixed structure with a nanoparticle-like BCC phase distributed in the B2 matrix. During compression testing, the fracture type was brittle fracture for all alloys, except for X0.75Y1, which exhibited excellent mechanical properties with a fracture strain of 40% due to partial plastic tearing. Both B2 and FeCr phases, which were rich in Al-Ni and Cr, respectively, showed reduced corrosion resistance. For the former, the loose structure of Al2O3 reduced the compactness of the passivated film; for the latter, the precipitation of the Cr-rich phase led to a Cr-poor region in the periphery. The Al0.75CoCrFeNi alloy exhibited the highest corrosion resistance, which was attributed to its reduced FeCr and B2 phases.