Effect of Nb on microstructure and properties of AlCoCrFeNi2.1 high entropy alloy

To overcome the strength limitation of eutectic high entropy alloys (EHEAs) in industrial application, it is necessary to develop new EHEA materials with high strength. In this work, the AlCoCrFeNi2.1Nbx (0 ≤ x ≤ 0.5) high entropy alloys (HEAs) were synthesized by arc-melting introducing heterogeneous structures by adding to the Nb element. The microstructures and mechanical properties of the AlCoCrFeNi2.1Nbx alloys were investigated. The results show that all the AlCoCrFeNi2.1Nbx (x > 0) HEAs are composed of FCC solid solution, B2 solid solution and Fe2Nb-type Laves. The microstructures of the AlCoCrFeNi2.1Nbx alloys change from FCC + B2 dual-phase eutectic structure (x = 0) to FCC + B2/Laves bimodal eutectic structure (x = 0.1), then to the mixed microstructures of FCC + B2 dendrite and FCC + Laves eutectic (0.1 < x ≤ 0.4), and finally to the hypoeutectic structure with BCC primary phase (x = 0.5). With the addition of Nb, the Vickers hardness and compressive yield strength of the AlCoCrFeNi2.1Nbx alloys increase continuously from 294.8 HV to 618.9 HV and 626 MPa–1998 MPa, respectively. Wherein, the AlCoCrFeNi2.1Nb0.2 alloy exhibits the best comprehensive mechanical properties with the compressive yield strength of 1158 MPa and ductility of 29.7%. In addition, the precipitation strengthening of the hard Laves phase and solid solution strengthening is the main strengthening mechanism in the AlCoCrFeNi2.1Nb0.2 alloys.