Microstructure, mechanical and corrosion properties of Al–Co–Cr–Ni near-eutectic medium entropy alloys

In this work, a series of Fe-free AlCoaCrbNic (AlCo1.31Cr0.5Ni2.4, AlCo1.71Cr0.5Ni2.0, AlCo1.8Cr0.6Ni1.8, and AlCo1.16CrNi2.0 denoted as E1, E2, E3, and E4, respectively) medium-entropy alloys (MEAs) were designed though the mathematical model. Among the four MEAs, E2 and E3 exhibit uniform and typical eutectic structural characteristics, consisting of face-centered cubic (FCC) and ordered body-centered cubic (B2) phases, with ordered FCC (L12) nanoprecipitates uniformly distributed in the FCC matrix. The E4 alloy is composed of primary B2 phase and eutectic FCC + B2 phases, and the high-density Co- and Cr-rich BCC-structured spherical precipitates are distributed in the B2 phase. Importantly, the E1 alloy is characterized by a phase structure comprising both FCC and body-centered tetragonal (BCT) phases, a combination that is notably infrequent among high-entropy alloys. There are high-density twinning and stacking faults (SFs) in the BCT phase. All four alloys exhibit excellent compressive properties, with fracture strength surpassing 2000 MPa and compression ratio exceeding 30 %. The potentiodynamic polarization curves implied that the E4 MEA presented excellent corrosion resistance with the lowest corrosion current density of 4.05 × 10−7 A/cm2 and the lowest passive current density of 7.01 × 10−6 A/cm2 in 3.5 wt% NaCl solution. Moreover, the electrochemical impedance spectroscopy (EIS) test results also indicating that the E4 MEA has the best corrosion resistance among the four alloys because it has the highest electrode resistance (Rp = 171.76 × 103 Ω cm2). The ordered FCC/BCT phase structure in E1 alloy provides effective exploration for the development of new structured MEAs.