Achieving strength-corrosion synergy in a CoCrNi-based medium-entropy alloy via an electrochemically compatible hierarchical heterostructure

Achieving high strength without sacrificing corrosion resistance remains challenging, as strengthening-induced microstructural heterogeneity often triggers local electrochemical mismatch and destabilizes passivation. Here, we design a CoCrNiAl 3 Ti 3 Cu 6 Nb 0.8 medium-entropy alloy with an electrochemically compatible hierarchical heterostructure comprising multiscale coherent L1 2 precipitates, stacking faults (SFs), and long-period stacking ordered (LPSO) structures. The optimally processed alloy delivers a yield strength of 1108.4 MPa, ultimate tensile strength of 1446.6 MPa, and elongation of 23.8%, alongside the lowest passive current density, highest pitting potential, and largest low-frequency impedance among all aged states. Local surface potential mapping reveals that L1 2 precipitates and LPSO structures are only ∼ 10 mV nobler than the matrix, while SF-containing regions are only slightly less noble, indicating weak micro-galvanic coupling. X-ray photoelectron spectroscopy and cross-sectional TEM further reveal a compact ∼ 6 nm passive film with a Cr-rich inner barrier layer. The superior combination of properties arises from a hierarchical heterostructure that simultaneously delivers effective strengthening and strain hardening while preserving electrochemical compatibility and promoting stable passivation. This work establishes a practical microstructure design principle for overcoming the strength–corrosion trade-off in high-strength medium-entropy alloys.