Simultaneously increasing mechanical and corrosion properties in CoCrFeNiCu high entropy alloy via friction stir processing with an improved hemispherical convex tool

It generally shows the trade-off for the mechanical properties and corrosion resistance of high entropy alloys (HEAs). In this study, friction stir processing (FSP) with an improved hemispherical convex tool was applied to process the as-cast CoCrFeNiCu HEA, which simultaneously increased the mechanical properties and corrosion resistance of the HEA by a large degree. After FSP, the grains were significantly refined from 6 μm to ∼300 nm, with 91% of high angle grain boundaries and 18% of nanometer twins, leading to a remarkable increase in the yield strength from ∼194 MPa to ∼935 MPa. More importantly, the FSP HEA showed an excellent strength-ductility synergy with yield strength of ∼695 MPa and a uniform tensile elongation of ∼17.5% after annealing, which was mainly attributed to the comprehensive role of equiaxed ultrafine microstructure, second phase and the high fraction of annealing twins (27%). In addition, the FSP HEA showed higher corrosion resistance than the base material by electrochemical test, which was mainly attributed to the improvement of the self-corrosion potential and the stability of the passivation film caused by the mitigation of Cu segregation and grain refinement. This study provides a new way to simultaneously increase the mechanical properties and corrosion resistance in HEAs.