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

The Al_xCoCrFe₂Ni (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 Al_xCoCrFe₂Ni 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 μm in the base alloy to around 63 and 58 μm. 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 Al_xCoCrFe₂Ni 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 Al_0.6CoCrFe₂Ni 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.