Cost-effective duplex Fe-Cr-Ni-Si-Al-Cu medium-entropy alloys with outstanding corrosion resistant and strength-ductility synergy

Corrosion resistance, strength, ductility, and cost are important indicators for the large-scale use of alloys, which are often mutually exclusive, such as strength-ductility/cost. In this study, we designed novel cost-effective, duplex Fe_67-xCr₁₆Ni₁₀Si₅Al₂Cu_x (x = 0, 2 and 4 at%) medium entropy alloys (MEAs) containing the B2 nanoprecipitate-reinforced transformable FCC and Cr-rich BCC phases to mitigate above conflicts. This hierarchical structure shows superior mechanical properties and corrosion resistance, comparable to those of 304 stainless steels and some reported MEAs. In particular, the Fe₆₅Cr₁₆Ni₁₀Si₅Al₂Cu₂ MEA exhibits yield strength of 720 MPa at 298 K and 1175 MPa at 77 K, with elongations of 43 % and 49 %, respectively. The excellent mechanical properties at low temperature origin from the synergistic mechanisms, such as strengthening from dislocations, grain boundaries and nanoprecipitation, and ductilizing from Lüders banding and slip band-mediated transformation-induced plasticity. First principles calculations show that Cu enhance the work function, promotes the oxygen adsorption by Cr, and then facilitates the formation of a dense passive film. The Cu and Cu oxide within the surface passive film also enhances the corrosion resistance. These findings provide new insights to design advanced alloys with superior comprehensive performance.