Corrosion resistant and tough multi-principal element Cr-Co-Ni alloys

Cr-Co-Ni Multi-Principal Element Alloys (MPEAs) with good combinations of strength and ductility were studied to determine their attendant corrosion performance. Three alloys – Cr₄₅Co_27.5Ni_27.5, Cr_33.3Co_33.3Ni_33.3, and Cr₂₅Co_37.5Ni_37.5 – were produced in recrystallized and cold-worked states, and their electrochemical response was tested in a simulated seawater electrolyte. Increasing the Cr content improved the yield strength (σ_y) and ultimate tensile strength (UTS), while maintaining high (>40%) uniform elongation. Potentiodynamic polarization and electrochemical impedance spectroscopy revealed high corrosion resistance of the alloys in simulated seawater, in particular the Cr-rich alloy (Cr₄₅Co_27.5Ni_27.5). Furthermore, following 40% cold work, the Cr₄₅Co_27.5Ni_27.5 alloy displayed a further improvement in corrosion resistance. The Cr₄₅Co_27.5Ni_27.5 alloy displays mechanical and corrosion properties that exceed those of conventional structural alloys such as Ni-superalloys, stainless steels and most 3d-transition metal MPEAs, including those without appreciable ductility. Therefore, in the present work it is shown that increasing the Cr content in Cr-Co-Ni alloys leads to a better combination of mechanical properties and corrosion resistance in saline environment, as observed especially for the Cr₄₅Co_27.5Ni_27.5 alloy.