Effect of Doping with Different Nb Contents on the Properties of CoCrFeNi High-Entropy Alloys

A series of five-element CoCrFeNi-Nbx (x = 0, 1, 3, 5, 7, and 9 wt%) high-entropy alloys were prepared using high-energy ball milling and discharge plasma sintering methods. Then, the effects of doping with Nb elements on the organization and properties of the CoCrFeNi HEAs were systematically investigated by tensile testing, hardness testing, and examining their micro-morphologies. The results show that with the addition of the Nb element, the lattice distortion of the alloy due to the large size of the Nb atoms causes the microstructure of CoCrFeNi HEAs to change from a single-phase FCC structure to a dual-phase structure of FCC and Laves. With the increase in the Nb content, the increase in the volume fraction of the hard and brittle Laves phase leads to the enhancement of the HEA's tensile strength, yield strength, and hardness, and a decrease in plasticity. The Nb5 alloy showed the most excellent comprehensive performance, with a tensile strength, yield strength, and plasticity of 879.1 MPa, 491.8 MPa, and 39.8%, respectively, and all the properties were improved compared with those of the HEAs obtained by the arc melting method. The increase in the hardness of the HEAs was nearly proportional to the increase in the volume fraction of the Laves phase, which was the direct cause of the increase in the hardness of the HEA. Therefore, since the Laves phase is the direct cause of the increase in HEA hardness, the doping of CoCrFeNi HEAs with Nb can significantly improve the properties of HEAs.