Microstructure and mechanical behaviors of CoFeNiMnTixAl1-x high entropy alloys

In this study, the microstructure and mechanical properties of CoFeNiMnTi_xAl_1-x (where the molar ratio (x) is 0, 0.1, 0.3, 0.5, 0.8, and 1) high entropy alloys (HEAs) were investigated. The body-centered cubic (BCC) phase, face-centered cubic (FCC) phase, and Laves phase were found in the alloy system. With an increase in the Ti content and a decrease in the Al content, the structure changes of the alloys from BCC to BCC + FCC to BCC + FCC + Laves, and then to Laves + FCC phases. The FCC phase is located at the BCC grain boundary. The CoFeNiMnTi_0.5Al_0.5 HEA with a BCC + FCC duplex phase structure shows the optimal performance of both strength and ductility. The yield strength, fracture strength, and fracture strain can reach 1052.8 MPa, 2402.4 MPa, and 20.5%, respectively. The hardness reaches HV518.7. Nanoindentation experiments show that the BCC phase is harder than the FCC phase in the CoFeNiMnTi_0.5Al_0.5 HEA. Furthermore, the deformation behaviors of the FCC phase and BCC phase are studied by measuring the first pop-in behavior.