Microstructure, Mechanical Properties, and Damping Capacity of AlxCoCrFeNi High-Entropy Alloys Prepared by Spark Plasma Sintering

High-entropy alloys (HEAs) of AlxCoCrFeNi (x = 0.2, 0.5, and 1) were created using the spark plasma sintering (SPS) method in conjunction with an aerosolized powder. Their microstructure and phase constituents were characterized by X-ray diffractometry, scanning electron microscopy, and projection electron microscopy. The tensile properties, hardness, compactness, and damping properties were also tested. The results showed that the crystal structure of AlxCoCrFeNi HEAs changed significantly with the Al content, from the original single face-centered cubic FCC phase (Al0.2CoCrFeNi) to an FCC + BCC + B2 structure (Al0.5CoCrFeNi), and then to FCC + BCC + B2 + Sigma (σ) phase structures (AlCoCrFeNi). Twin crystals with FCC structure were also observed in the TEM of AlCoCrFeNi. A chemical composition analysis showed that the crystal structure transformation was related to the segregation caused by the increase of Al element content. The hardness of the AlxCoCrFeNi HEAs increased with the Al content, and the hardness of AlCoCrFeNi reached the highest value of 585.4 HV. The tensile properties of the alloy showed a trend of increasing and then decreasing values with the increase in Al content. The yield strength, ultimate tensile strength, and elongation of the Al0.5CoCrFeNi alloy reached the highest values of 557.7 MPa, 954.4 MPa, and 32.2%, respectively. Moreover, the fracture mechanism of the Al0.2CoCrFeNi and Al0.5CoCrFeNi alloys was that of a typical ductile fracture, while for the AlCoCrFeNi alloy, it was that of a cleavage fracture. The compactness of the alloy increased with the Al content. The combination of the FCC + BCC + B2 phase resulted in the damping capacity of Al0.5CoCrFeNi alloy reaching 0.018 at the corresponding strain amplitude of 6 × 10−4.