Cu alloying enables superior strength-ductility combination and high corrosion resistance of FeMnCoCr high entropy alloy

The metastable dual phase FeMnCoCr high entropy alloy (HEA) has attracted great attention since it was first proposed, as it can overcome the strength-ductility trade-off at room temperature. For materials in engineering applications, it is also important to achieve high corrosion resistance while maintaining good mechanical properties. In this study, we simultaneously improved the mechanical properties and corrosion resistance of a FeMnCoCr-based HEA via Cu alloying and by applying a simple processing route that combined cold rolling and partial recrystallization annealing. The alloy doped with 0.3 at.% Cu and processed by annealing at 650 °C for 3 min exhibited the best combination of strength and ductility, i.e., yield strength of 768 MPa, tensile strength of 960 MPa and uniform elongation of 33.6%. The high yield strength majorly originated from high density dislocations and grain boundaries in the material, while the formation of deformation twins, martensite laths, stacking faults and Lomer Cottrell locks contributed to the continuous strain hardening and good ductility. The addition of Cu also enhanced corrosion resistance of the HEA in a NaCl solution by improving stability of the alloy matrix and elevating compactness of the passive film. The solid solution of Cu elevated equilibrium potential of the matrix and promotes transformation of microstructure from the face-centered cubic (fcc) plus hexagonal close-packed (hcp) dual phases to the fcc single phase, effectively inhibiting the accelerated corrosion of micro galvanic couple between different phases and enhancing stability of the matrix. At the same time, the enrichment of CuO in the inner layer of the passive film reduced the defect density within the Fe-rich oxide layer, improving the protection of the passive film against the matrix.