Effect of Ti on microstructures and mechanical properties of high entropy alloys based on CoFeMnNi system

In this study, the microstructures and mechanical properties of CoFeMnNiTix (where the molar ratio (x) is 0, 0.25, 0.5, and 0.75) high entropy alloys were examined. Both the CoFeMnNi and CoFeMnNiTi0.25 alloys exhibit a simple face-centered cubic (FCC) solid-solution structure, whereas the CoFeMnNiTi0.5 and CoFeMnNiTi0.75 alloys exhibit a dual-phase structure consisting of a FCC solid-solution and a Laves phase. The Laves phase is identified as a Fe2Ti type and has a hexagonal C14 structure. The volume fraction of the Laves phase increases with increasing the Ti content. The Ti content has an obvious effect on the mechanical properties of the alloys. The yield strength and hardness of the alloys improve but ductility simultaneously decreases as the Ti content increases. The CoFeMnNiTi0.5 alloy has a good balance of both strength and ductility, and is superior to other similar CoCrFeNi-based and CoCrCuFeNi-based alloys. The yield strength, fracture strength and fracture strain of the CoFeMnNiTi0.5 alloy can reach 803 MPa, 2300 MPa and 34%, respectively. In addition, nanoindention tests show that the nanohardness value of the Laves phase is almost two times of the FCC phase, and the elastic modulus of the Laves phase is higher than that of the FCC phase in the current alloy system. By measuring the first pop-in event, the deformation behaviors of FCC and the Laves phases of the CoFeMnNiTi0.5 alloy were investigated.