The passivation phenomenon of metallic iron was discovered as early as the 18th century, and the related passive-film theory was accepted by the early 20th century, after which corrosion-resistant alloys (represented by austenitic stainless steels (SS) and duplex SS) were successfully designed and widely used in our daily life and industries However, traditional corrosion-resistant alloys that are based on one principal element often exhibit low to moderate strength and insufficient work-hardening ability, which cannot guarantee long-term use in harsh environments requiring high structural load-bearing capacity. How to simultaneously improve the mechanical properties and corrosion resistance has become a thorny issue since the development of traditional structural materials hit the wall. On the other hand, the advent of HEAs has broken through the design concept of one single principal element of traditional alloys, greatly expanding the alloy design freedom and the realm of possibilities for mechanical or functional performance, making them up-and-coming metallic alloys.

In the first part of this thesis, a prototype (Ni₂Co₂FeCr)₉₂Al₄Nb₄ HEAs was selected as our starting material. We systematically studied the microstructural evolutions and mechanical behaviors of (Ni₂Co₂FeCr)₉₂Al₄Nb₄ HEAs at different aging temperatures (700-900 ℃). We found that only spherical L1₂ phase existed in the specimen peak-aged at 700 °C without other precipitates, while in the specimen peak-aged at 800 °C, irregular precipitation of ε phase appeared on the grain boundaries in addition to the intragranular L1₂ phase. In strong contrast, upon peak aging at 900 ℃. the fine interleaving Widmanstätten-type ε phase with an ordered hexagonal structure (D0₁₉) became the dominated precipitate. We further revealed that the coherent L1₂ phase provided a more effective strengthening effect than that of the ε phase, leading to the highest yield strength of ~950 MPa in the alloy peak-aged at 700 °C. The stacking faults shearing of L1₂ phase and the planar dislocations shearing of ε phase were identified as the main deformation mechanisms of the specimens peak-aged at 700 and 900 °C, respectively.

Second, the electrochemical corrosion behavior of (Ni₂Co₂FeCr)₉₂Al₄Nb₄ HEAs with different phase structures (i.e., FCC, FCC+L1₂, FCC+D0₁₉) was systematically investigated in 3.5 wt.% NaCl solution, and it was found that the temperature-dependent microstructural evolutions also play an essential role in the aqueous corrosion performance and passive film features of the precipitation-strengthened HEAs. Compared with single-phase FCC alloys, the appearance of nano-sized L1₂ particles increases the metastable pitting activities and slightly reduces the pitting potential, while the lamellar D0₁₉ phase further deteriorates the localized corrosion resistance. The chemical composition and oxide state of passive films are closely related to the different precipitation scenarios. The corrosion morphologies of the single-phase and L1₂-strengthened alloys are dominated by pitting, whereas the intergranular corrosion (IGC) and intragranular dealloying-like corrosion occur in the D0₁₉-strengthened alloy.

Third, we first report a HEA with good corrosion resistance and comprehensive mechanical properties, namely Co₄₀Cr₂₀Ni₃₀Al₅Ti₅ (at.%) HEA. The effect of the L1₂ phase on the corrosion and passivation of the Co₄₀Cr₂₀Ni₃₀Al₅Ti₅ HEA was then elucidated by comparing the electrochemical corrosion properties and passive film characteristics of the alloy in the aged (dual-phase FCC+L1₂ structure) and recrystallized (single-phase FCC structure) states. We found that the chemical inhomogeneities induced by the L1₂ phase resulted in a noticeable increase in Cr(OH)₃ content in the passive films. This, combined with the high galvanic corrosion tendency between the two phases, was identified as the key reason behind the alloys' increased susceptibility to localized corrosion attacks.

Lastly, based on the above results, we designed a novel L1₂-strengthened HEA of composition (NiCo)₇₇Cr₁₃Al₅Nb₅ (at.%) that combines excellent mechanical properties and good corrosion resistance. The formation of a thin passive film of non-stoichiometric compounds with an amorphous structure, characterized by an enrichment of Cr₂O₃ and Nb₂O₅, contributes to the superior localized corrosion resistance. The precipitation strengthening brought about by coherent L1₂ particles and the multi-stage plastic deformation dominated by stacking faults (SFs) provides ultrahigh strength and appreciable ductility, respectively.

We believe that our present findings are of both technical and scientific importance for accelerating the design of damage-tolerant and high-performance L1₂-strengthened HEAs, enabling them as promising corrosion-resistant materials applied in aggressive environments.