In this paper, the (CoCrFeNi)₉₅Nb₅ high-entropy alloy (HEA) coating with a thickness of 500 μm on Q235 steel substrate was fabricated by plasma spraying. The microscopic results showed that a new Laves phase is formed in the (CoCrFeNi)₉₅Nb₅ HEA coating compared to the HEA powder, and elemental segregation occurs between the dendrites and the interdendrites of the coating, while the interdendritic phase enriches with the Cr and Nb. The phase composition change and elemental segregation behavior were mainly due to the faster cooling rate of the plasma spraying technique. At the junction of the coating and the substrate, the HEA coating bonded well to the substrate; in addition, the width of transition zone was merely 2 μm. The microhardness of the (CoCrFeNi)₉₅Nb₅ HEA coating was 321 HV0.5, which is significantly higher than that of the substrate. In terms of corrosion resistance, the (CoCrFeNi)₉₅Nb₅ HEA coating has good corrosion resistance in NaCl solution. Although the corrosion form was pitting corrosion, the pitting potential of the (CoCrFeNi)₉₅Nb₅ HEA coating was significantly higher than that of other coatings, which was mainly because of the dense passivation film formed by Cr and Nb on the surface of the coating. Once the passivation film was destroyed by Cl⁻, the selective corrosion occurred on the surface of the (CoCrFeNi)₉₅Nb₅ HEA coating. In summary, the (CoCrFeNi)₉₅Nb₅ HEA coating prepared by plasma spraying technology can significantly improve the corrosion resistance and mechanical properties of the Q235 steel substrate.

The present work investigates the influence of micro-alloyed Mo on the corrosion behavior of (CoCrFeNi)_100−xMo_x high-entropy alloys. All of the (CoCrFeNi)_100−xMo_x alloys exhibit a single face-centered cubic (FCC) solid solution. However, the (CoCrFeNi)₉₇Mo₃ alloy exhibits an ordered sigma (σ) phase enriched in Cr and Mo. With the increase of x (the Mo content) from 1 to 3, the hardness of the (CoCrFeNi)_100−xMo_x alloys increases from 124.8 to 133.6 Vickers hardness (HV), and the compressive yield strength increases from 113.6 MPa to 141.1 MPa, without fracture under about a 60% compressive strain. The potentiodynamic polarization curve in a 3.5% NaCl solution indicates that the addition of Mo has a beneficial effect on the corrosion resistance to some certain extent, opposed to the σ phase. Furthermore, the alloys tend to form a passivation film in the 0.5 M H₂SO₄ solution in order to inhibit the progress of the corrosion reaction as the Mo content increases.

The recently developed dual-phase (DP) non-equiatomic Fe₅₀Mn₃₀Co₁₀Cr₁₀ (at.%) high-entropy alloy (HEA) showed much higher strength and ductility compared to the single-phase equiatomic Fe₂₀Mn₂₀Ni₂₀Co₂₀Cr₂₀ (at.%) HEA at room temperature. Herein we probe the cryogenic mechanical properties of the non-equiatomic DP-HEA with different grain sizes and compare with the equiatomic single-phase HEA. Our results show that the cryogenic ultimate tensile strengths of the coarse-grained (∼ 200 µm) and fine-grained (∼ 4 µm) DP-HEAs reach up to 1,133 and 1,342 MPa, respectively, which are significantly higher than that of the equiatomic single-phase HEAs with similar grain sizes. Furthermore, the fine-grained DP-HEA shows substantial improvement in both strength and ductility compared to the coarse-grained counterparts at cryogenic temperatures. Microstructural analysis reveals that the enhanced mechanical properties of the DP-HEA at cryogenic temperatures are attributed to a more extensive displacive transformation from the face-centered cubic (FCC) matrix into the hexagonal close-packed (HCP) phase compared to that at room temperature. Specifically, the HCP phase fraction in tensile tested fine-grained DP-HEAs increases from ∼ 39% to ∼ 79% with decreasing temperature from 298 to 77 K. The enhanced transformation behavior is enabled by the reduced stacking fault energy of the material with the decrease of deformation temperatures. The resulting outstanding combination of strength and ductility further suggests that the DP-HEAs are promising candidates as structural materials for cryogenic applications.

To solve the problem of poor corrosion and wear resistance of Mg-Li alloys, Fe-based amorphous coatings were prepared by high velocity oxygen-fuel spraying technology (HVOF) on the LA141 magnesium alloy substrate with a Ni60 intermediate layer. The microstructure and performance of Fe-based amorphous coatings with different oxygen flow and kerosene flow were characterized and analyzed. The results demonstrate that there is an optimal oxygen/kerosene ratio where the porosity of Fe-based amorphous coating is the lowest. Moreover, the amorphous content increases with the decrease in the oxygen/kerosene ratio. In particular, when the oxygen flow is 53.8 m³/h and the kerosene flow is 26.5 L/h, the Fe-based amorphous coating possesses the lowest porosity (0.87%), the highest hardness (801 HV₀.₁), the highest bonding strength (56.9 MPa), and an excellent corrosion and wear resistance. Additionally, it can be seen that the Fe-based amorphous coating is composed of amorphous splats and amorphous oxides, but the Ni60 intermediate layer exhibits an amorphous and crystalline multi-phase structure. The high bonding strength of the coating is attributed to the low porosity of Fe-based amorphous coating and the localized metallurgical bonding between different layers. Finally, the mechanisms on corrosion and wear of Fe-based amorphous coatings are also discussed.

We report that carbon honeycomb, a new three-dimension carbon allotrope, exhibits large negative Poisson’s ratio, as large as −0.32, in tensile revealed via molecular dynamics simulations. The Poisson’s ratio of carbon honeycomb is anisotropic, and sensitive to temperature. The carbon honeycomb has phase transformation from normal to auxetic by tensile, along both zigzag and armchair directions. The critical strain for the normal-auxetic transition along the cell-axis direction reduces with respect to an increase in temperature. Combined with high strength of 50 GPa, such a unique and adjustable negative Poisson ratio suggests broad engineering applications of carbon honeycomb.