Surface hardening of FCC phase high-entropy alloy system by powder-pack boriding

Face centred cubic (fcc) phase high-entropy alloys (HEA) exhibit a higher ductility compared to body centred cubic (bcc) phase or multiphase HEA systems. However, low hardness and low wear resistance prevent tribological applications. A precipitation hardening of the fcc phase CoCrFeMnNi and the manganese-free CoCrFeNi alloy was performed by powder-pack boriding. The microstructure, the phase formation and the wear resistance were investigated. The successful diffusion of elements is proven by the formation of a silicon-boron-rich double layer, causing precipitation which leads to segregation within the enriched surface layer. A multiphase microstructure can be observed for both alloys. The thickness of the silicon-rich surface layer extends for the manganese -free alloy with a higher nickel content. An increase in microhardness and wear resistance was measured for the borided samples. The correlation of structural and mechanical properties contributes to a better understanding of the material behaviour of thermochemically treated HEAs. In the context of current efforts to improve wear resistance, the presented results provide a new approach of surface hardening HEA systems.