Enhanced Strength-Ductility Synergy Achieved Through Al-Addition in CoCrFe2Ni-Based High Entropy Alloys

The AlxCoCrFe2Ni (x = 0, 0.4, 0.6, 0.8, and 1.0) high entropy alloys (HEAs) were designed to achieve exceptional comprehensive mechanical properties. The results show that the phase structure of the AlxCoCrFe2Ni alloys underwent a transformation from a single-phase FCC (x <= 0.4) to a combination of FCC + BCC/B2 phases, and eventually to mixed BCC/B2 phases (x >= 0.8). This transformation resulted in a morphological change from equiaxial grains to dendrites, and finally back to equiaxial grains. In the alloys with Al contents of x <= 0.6, the supercooling caused by Al segregation significantly refined the average grain sizes, reducing them from approximately 125 mu m in the base alloy to around 63 and 58 mu m. Nanoscale B2 strips or particles formed within the BCC matrix in the alloys with x >= 0.8 through spinodal decomposition. The incremental presence of BCC/B2 phases gradually improved the hardness and ultimate tensile strength (UTS) of the AlxCoCrFe2Ni alloys. This transition occurred from 154 HV and 472 MPa at x = 0 to 520 HV and 1.2 GPa at x = 0.8, albeit at expense of the ductility. In combination, the Al0.6CoCrFe2Ni alloy achieved enhanced strength-ductility synergy, boasting a high UTS of approximately 1.1 GPa primarily attributed to solid solution strengthening. Additionally, it exhibited a high elongation of 41.7%, owing to the dominant FCC phase, exceeding those reported for most (FeCoCr, CoCrFeNi)-based HEAs.