Shunping Sun

The poor room-temperature toughness of MoSi₂ hinders its application as a structural material. A thorough analysis was conducted using arc cladding experiments and first-principles calculations to investigate the effect of La₂O₃ on the mechanical properties of the MoSi₂ coating. The ductility of La₂O₃ (Poisson’s ratio v > 0.26; B/G > 1.75) was demonstrated by the first-principles calculation, which enhanced the fracture toughness of MoSi₂. The bulk modulus, shear modulus, Young’s modulus, and Vickers hardness of La₂O₃ were calculated through Voigt-Ruess-Hill approximations. The findings of the experiment indicated that the pure MoSi₂ coating consisted of MoSi₂ and Mo₅Si₃ phases with cellular dendritic crystals. When the MoSi₂ coating was added with La₂O₃, the microstructure of the coating became denser and finer. La₂O₃ reacted with SiO₂, which formed La₂Si₂O₇ phases. When the La₂O₃ content was 3, 5, 7, and 9 wt%, the average hardness of the coatings with La₂O₃ was 9.73, 16.78, 29.98, and 52.67% more than that of the pure MoSi₂ coating, respectively. The MoSi₂ coating was added with 7 wt% La₂O₃ had the highest fracture toughness, approximately twice as high as the pure MoSi₂ coating. This was attributed to the fact that La₂O₃ could disperse the main stress of cracks and inhibit their propagation. Furthermore, the MoSi₂ coating added with 7 wt% La₂O₃ had the flattest wear surface and the lowest wear loss.

The design of new ultra-light Mg-Li alloys have great significance in aerospace and other fields. In this paper, Sn and Y elements were simultaneously added into Mg-8Li-3Al (wt%) alloy and the effect of Y content on microstructure evolution and tensile properties of Mg-8Li-3Al-2Sn-xY (wt%) alloys was studied. The as-cast Mg-8Li-3Al-2Sn alloy is composed of α-Mg + β-Li duplex-phase matrix, fine MgLiAl₂ phase and coarse fishbone-like Li₂MgSn phase. As Y element is added, granular Al₂Y phase is formed, and grain size of α-Mg phase decreases significantly with shape changing from well-developed coarse dendritic to equiaxed dendritic. After hot extrusion, α-Mg phase is elongated along the extrusion direction, and β-Li phase undergoes complete dynamic recrystallization and transforms into fine equiaxed grains. The addition of 0.5% Y improves the tensile properties of as-cast Mg-8Li-3Al-2Sn-0.5Y alloy. However, the tensile properties show a downward trend with further increase of Y content. Hot extrusion significantly enhances the tensile properties of Mg-8Li-3Al-2Sn-xY alloys. Similarly, strength reaches the highest when the Y content is 0.5% for extruded Mg-8Li-3Al-2Sn-xY alloys.

MoSi₂ is one of the most promising refractory metal silicide materials, but its further use as a structural material is limited by its drawbacks such as poor room-temperature toughness and low high-temperature strength. The work performed a comprehensive investigation combining first-principles calculations and arc cladding experiments to explore the effects of Nb and W doping on the mechanical properties and electronic structure of MoSi₂ coatings. The first-principles calculations revealed that Nb addition improved the B/G value and Poisson’s ratio of MoSi₂, indicating enhanced ductility. W addition yields the opposite effect and led to a higher elastic modulus and improved hardness. Experimental results demonstrated that the arc-cladding MoSi₂ coating mainly consisted of MoSi₂ and Mo₅Si₃ phases with a dendritic microstructure. Upon doping with Nb and W, additional t-(Mo,Nb)Si₂ and t-(Mo,W)Si₂ phases were formed, which resulted in a denser and finer microstructure. Nb addition contributed to the solid-solution toughening of the coating, while W addition enhanced hardness but reduced toughness. Remarkably, the synergistic alloying of Nb and W significantly increased the hardness and fracture toughness of the coating by 30.7 and 70.7%, respectively, compared to pure MoSi₂. The strengthening mechanism of the coating was attributed to solid-solution softening and fine-grain strengthening, while the crack extension mechanism involved the crack deflection and bridging. Furthermore, the coatings doped with 2% Nb and 4% W exhibited the lowest wear weight loss and superior wear resistance. The dominant wear mechanisms were oxidation wear and abrasive wear.

Different proportions of Al and Cr alloying MoSi₂ were prepared by arc cladding with different mass fractions of MoSi₂, Al, and Cr as raw materials. The work studied the effects about of Al and Cr on the phase, high-temperature oxidation morphology, products, and high-temperature oxidation property of MoSi₂. The results confirmed that the volume expansion coefficient of Al₂O₃ generated by material oxidation was low which could reduce the degree of mismatch and the cracks in the oxide film. Cr element had a poor effect on enhancing the continuity and compactness of the oxide film, however, it could improve the stability of t-MoSi₂ formation after arc cladding. MoO₃ was not formed in the MoSi₂ material added with Al and Cr show the surface oxidation. A dense and continuous Al₂O₃–SiO₂ oxide film was formed, and no cracks holes were found in the oxide film. After oxidation at 800 °C–1200 °C for 120 h, the sample still maintained good oxidation protection. According to the calculation of oxidation kinetics, the MoSi₂ material added with Al and Cr had good oxidation resistance. 3% Al + 9% Cr + 88% MoSi₂ had the optimal high-temperature oxidation resistance about four times than pure MoSi₂.

Molybdenum silicides are attractive high-temperature structural materials because of their excellent thermal stability and outstanding oxidation resistance at high temperatures. First-principles calculations were employed to investigate the effect of alloying elements (Cr, Nb, V, W, Al, Ga, and Ge) on the mechanical properties of Mo₃Si. The structural stabilities of doped Mo₃Si were calculated, showing that the Pm-3n structure was stable at the investigated low-doping concentration. The calculated elastic constants have also evaluated some essential mechanical properties of doped Mo₃Si. Cr- and V-doping decreased the elastic modulus, while Al- and Nb-doping slightly increased the shear and Young’s modulus of Mo₃Si. Furthermore, V-, Al- and Nb-doping decreased the B/G and Poisson ratio, suggesting that these elements could form strong covalent bonds, and decrease shear deformation and alloy ductility. Based on the three-dimensional contours and two-dimensional projection of the elastic modulus, Cr- and V-doping exhibited a significant influence on the anisotropy of the shear and Young’s modulus. According to charge density and density of states, the electronic structures of alloyed Mo₃Si were further analyzed to reveal the doping effects.