Effects of Co/Ni ratio on microstructure and mechanical properties of as-cast Cr-Fe-Ni-Co-Al-Si-Ti-Cu high entropy alloys

In this paper, partial replacement of Ni for Co was adopted in FCC + BCC heterostructure as-cast CrFeNi1-xCoxAl0.28Si0.09Ti0.02Cu0.01 high entropy alloys (HEAs), and the modification in microstructure and mechanical properties of these alloys with the increase in the Co/Ni ratio has been systematically investigated. It was observed that all these designed HEAs consist of FCC sideplates with a surrounding BCC phase, as well as well-dispersed B2 phase in as-cast state. Additionally, higher Co/Ni ratio leads to a larger proportion of BCC phase, which has following impacts on the mechanical properties of this series of HEAs: when such ratio begins to increase, it has obvious enhancement in strength and gradual decrease in plasticity; when it reaches 0.15/0.85, it has high strength of similar to 1.4 GPa and good plasticity of >10 %. While further enhancing Co/Ni ratio leads to significant connection of BCC phase, resulting in drastic increase in brittleness. The detailed mechanism for such phenomenon has been discussed in detail. This study provides a novel route on improving the comprehensive mechanical properties of this series of as-cast HEAs.