Effects of Nb addition on structural evolution and properties of the CoFeNi2V0.5 high-entropy alloy

A series of CoFeNi2V0.5Nbx (x = 0–1) high-entropy alloys (HEAs) were synthesized to investigate the alloying effect of the Nb element on the microstructures and properties. It was found that the microstructures change from initial single-phase FCC solid solutions (x = 0), to a hypoeutectic microstructure (0 < x < 0.75), then to a fully eutectic microstructure (x = 0.75), and finally to a hypereutectic microstructure (0.75 < x < 1). The eutectic microstructure consists of the FCC solid solution phase and the Fe2Nb-type Laves phase. Nb0 (x = 0) and Nb02 (x = 0.2) alloys exhibit excellent ductility, the compressive plastic elongation of which can reach up to 80 % without fracture. With increasing Nb content, increased brittle Laves phase leads to the decrease of the plastic strain and the increase of the yield strength, and the Vickers hardness shows an approximately linear increase from HV 133.4 to HV 687.7. Besides, the CoFeNi2V0.5Nbx HEAs exhibit excellent soft magnetic behavior, in which the Nb0 alloy shows the maximum saturation magnetization (M s) of 70.33 emu/g, and minimum coercivity (H c) of 1.62 Oe.