Improving mechanical properties of (Co1.5FeNi)88.5Ti6Al4R1.5 (R = Hf, W, Nb, Ta, Mo, V) multi-component high-entropy alloys via multi-stage strain hardening strengthening

In this study, (Co1.5FeNi)88.5Ti6Al4R1.5 (R = Hf, W, Nb, Ta, Mo, V) high entropy alloys were fabricated by vacuum arc melting, followed by cold rolling and aging treatment. The effects of refractory elements on the phase structure, microstructure and mechanical properties of high entropy alloys (HEAs) were systematically studied by means of XRD, SEM and TEM. The results show that the phase composition of all alloys is the face-centered-cubic (FCC) + L12 dual-phase structure, and the addition of refractory elements has not changed the phase structure of the alloys. The tensile results show that the yield strength and elongation of the matrix HEA are 750 MPa and 18 %, respectively. The strength of HEAs is improved variously by adding refractory elements. The HEAs with Ta element have the best comprehensive mechanical properties, exhibit excellent strength-ductility combination, with the yield strength and elongation of 950 MPa and 20 %, respectively, and the fracture mechanism changes from complete ductile fracture to mixed brittle-ductile fracture mode. In addition, the HEAs have multi-stage strain-hardening behavior, which has higher strength when the strain-hardening curve has larger fluctuations, which indicates that the HEAs can improve the strength-plasticity matching relationship by a multi-stage strain hardening behavior.