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Balancing mechanical and corrosion performance is a decisive factor in the use of high entropy and medium entropy alloys (H & MEAs) as advanced engineering structural materials. This study investigates the microstructure of Fe77.3-xMnxSi9.1Cr9.8C3.8 (x = 14, 23.2) HEAs by combining experiments and first-principles calculations to elucidate the effects of adjusting Mn content on the mechanical and corrosion properties of FCC and FCC + HCP phases. The results demonstrate that HEAs with low Mn content exhibit a single FCC phase, whereas increasing Mn content not only precipitates the HCP phase but also refines the grain size. The yield strength, compressive fracture strength, work hardening index, and plasticity of Fe77.3-xMnxSi9.1Cr9.8C3.8 (x = 14, 23.2 at.%) HEAs are 1300 and 1526 MPa, 2380 and 2332 MPa, 0.42 and 0.36, 16.7% and 16%, respectively. Both HEAs exhibit superior electrochemical and salt spray corrosion performance compared to 304 stainless steels, with the Fe63.3Mn14Si9.1Cr9.8C3.8 alloy performing the best, exhibiting a self-corrosion potential of −0.523V and a self-corrosion current density of 2.605 × 10−6 A cm−2. The excellent corrosion resistance of Fe63.3Mn14Si9.1Cr9.8C3.8 can be attributed to the moderate grain size, resulting in excellent passivation film characteristics, and the lower Mn content, leading to a higher Cr2O3 content in the passivation film. First-principles calculations reveal that Fe63.3Mn14Si9.1Cr9.8C3.8 has a lower density of states number at the Fermi level (i.e., Fe63.3Mn14Si9.1Cr9.8C3.8 is 214.145eV, Fe54.1Mn23.2Si9.1Cr9.8C3.8 is 337.077eV), a wider pseudogap width, higher work function (5.700ev, 5.189ev), and better stability. Additionally, there exists a strong trade-off between corrosion and mechanical properties due to the influence of the HCP phase and grain size.

Vacuum brazing of GH3535 alloy was conducted using Au–18Ni (wt%) filler metal. The effect of brazing temperature on the microstructure and mechanical properties was investigated. Furthermore, the microstructural evolution and fracture mechanisms of brazed joints were discussed. The cross section of brazed joint could be divided into two distinctive reaction zones. The typical microstructure of the joint brazed at 1030 °C/10min was GH3535 substrate/Ni (s.s) + Mo-rich Ni (s.s) + Au-rich phase/Au (s.s) + FeNi3/Au-rich phase + Mo-rich Ni (s.s) + Ni (s.s) + GH3535 substrate. Raising the brazing temperature promoted the diffusion of elements, Ni (s.s) and Au (s.s) gradually increased, and the second phase particles consisting of FeNi3 gradually decreased. With the increase in brazing temperature, the shear strength of brazed joints first increased and then decreased. The maximum room shear strength of 420 MPa was obtained at 960 °C/25min and the high temperature shear strength of 153 MPa was obtained at 600 °C. Fracture analysis indicated that cracks initiated at the interface of Au (s.s) and then propagated along the zone of FeNi3 with dimple and tearing ridges patterns.

Micro/nano-structure is one of the important types of multilayer construction, which can make nanocrystalline metals and alloys with excellent comprehensive mechanical properties. By means of Aluminothermic Reaction (AR) and subsequent rolling with deformation of 50% at room temperature (RT) and followed by annealed at 973 K for 1 h, a heterogeneous composite structure of micro/nano-structured 304 stainless steel (SS) is obtained. Such a stainless steel specimen exhibits a tensile strength as high as 1023 MPa and an elongation-to-failure of about 27.3%. In the process, the much elevated strength originates from martensite strengthening, austenite grain refinement and dislocation density increasing, while ductility promotes from the recrystallization of deformed austenite and recovery of strain induced martensite. Superior strength–ductility combination achieves in micro/nano-structured 304 SS, which demonstrates a novel approach for optimizing the mechanical properties in engineering materials.