Synergistic effect of Nb and Mo alloying on the microstructure and mechanical properties of CoCrFeNi high entropy alloy

Improving the strength of face-centered cubic high entropy alloy, CoCrFeNi, has been a research focus in recent years. In this paper, to further enhance its strength for engineering applications, a series of CoCrFeNi-(Nb, Mo) high entropy alloys were prepared using vacuum arc-melting. The synergistic effect of alloying by Nb and Mo on microstructure and mechanical properties of CoCrFeNi-based high entropy alloy were investigated. The criterion Md‾ was used to evaluate the phase evolution for current alloy system. Through synergistic alloying with Nb and Mo, a new kind of Laves phase with hexagonal close-packed structure was formed. The Laves phase is semi-coherent with the face-centered cubic matrix. Furthermore, through adjusting the ratio of Nb and Mo, fine lamellar structure Laves phase was obtained. The synergistic precipitation strengthening and solid-solution effect lead to high yield and fracture strength of 426 MPa and 714 MPa, maintaining high elongation of 17.4%. Fracture observation shows that the face-centered cubic matrix of the alloy system is ductile fracture, and the Laves phase is cleavage fracture. The results of this research would provide a basis for strengthening face-centered cubic high entropy alloy through synergistic alloying effect.