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Based on density functional theory (DFT), a new silicon allotrope C2-Si is proposed in this work. The mechanical stability and dynamic stability of C2-Si are examined based on the elastic constants and phonon spectrum. According to the ratio of bulk modulus and shear modulus, C2-Si has ductility under ambient pressure; compared with Si64, Si96, I4 /mmm and h-Si6, C2-Si is less brittle. Within the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional, C2-Si is an indirect narrow band gap semiconductor, and the band gap of C2-Si is only 0.716 eV, which is approximately two-thirds of c-Si. The ratios of the maximum and minimum values of the Young’s modulus, shear modulus and Poisson’s ratio in their 3D spatial distributions for C2-Si are determined to characterize the anisotropy. In addition, the anisotropy in different crystal planes is also investigated via 2D representations of the Young’s modulus, shear modulus, and Poisson’s ratio. Among more than ten silicon allotropes, C2-Si has the strongest absorption ability for visible light.

We predict two novel group 14 element alloys Si2Ge and SiGe2 in P6222 phase in this work through first-principles calculations. The structures, stability, elastic anisotropy, electronic and thermodynamic properties of these two proposed alloys are investigated systematically. The proposed P6222-Si2Ge and P6222-SiGe2 have a hexagonal symmetry structure, and the phonon dispersion spectra and elastic constants indicate that these two alloys are dynamically and mechanically stable at ambient pressure. The elastic anisotropy properties of P6222-Si2Ge and P6222-SiGe2 are examined elaborately by illustrating the surface constructions of Young’s modulus, the contour surfaces of shear modulus, and the directional dependence of Poisson’s ratio; the differences with their corresponding group 14 element allotropes P6222-Si3 and P6222-Ge3 are also discussed and compared. Moreover, the Debye temperature and sound velocities are analyzed to study the thermodynamic properties of the proposed P6222-Si2Ge and P6222-SiGe2.

In this work, a new superhard material named Pm BN is proposed. The structural properties, stability, mechanical properties, mechanical anisotropy properties, and electronic properties of Pm BN are studied in this work. Pm BN is dynamically and mechanically stable, the relative enthalpy of Pm BN is greater than that of c-BN, and in this respect, and it is more favorable than that of T-B3N3, T-B7N7, tP24 BN, Imm2 BN, NiAs BN, and rocksalt BN. The Young’s modulus, bulk modulus, and shear modulus of Pm BN are 327 GPa, 331 GPa, and 738 GPa, respectively, and according to Chen’s model, Pm BN is a novel superhard material. Compared with its original structure, the mechanical anisotropy of Young’s modulus of Pm BN is larger than that of C14 carbon. Finally, the calculations of the electronic energy band structure show that Pm BN is a semiconductor material with not only a wide band gap but also an indirect band gap.

Explosive welding is widely applied to joining Ti with steel in recent years. However, joining the explosion-bonded CP-Ti/Q345 bimetallic sheets has not been thoroughly investigated. In this paper, explosion-bonded CP-Ti/Q345 bimetallic sheets were applied as base metal and intermediate materials were employed to join the bimetallic sheets. Based on the element metallurgical compatibility, Cu/V based filler metals (solid wire and flux-cored wire) were designed as intermediate layers for TIG welding method. To determine the feasibility of these filler metals, microstructures were analyzed by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Microhardness tests were used to compare the brittleness of the intermetallics in the weld joints. Tensile strength test was performed to evaluate the strength of the joint. The results showed that the feasibility of the Cu/V based solid wires was poor due to metallurgical incompatibilities. The joint filled with Cu/V solid wires suffered severe cracks. Such defects reduced if a Cu–V flux-cored wire was used. It was attributed to continuously distributed microstructures in weld joint and low heat input with the Cu–V flux-cored wire.

Two new Group IV element allotropes Si3 and Ge3 in P6222 phase are predicted in this work and their physical properties are investigated using the density functional theory. Each of the newly predicted allotropes has a superdense structure, which is mechanically, dynamically, and thermodynamically stable, as verified by elastic constants, phonon dispersion spectra and relative enthalpies, respectively. The mechanical anisotropy properties are studied in detail by illustrating the directional dependence of Young’s modulus, discussing the universal anisotropic index, and calculating shear anisotropy factors together with bulk moduli. It shows that P6222–Si3 exhibits the greater anisotropy than P6222–Ge3, and interestingly both of the newly predicted crystals appear to be isotropic in the (001) plane. Additionally, the Debye temperature, sound velocities, and the minimum thermal conductivity are examined to evaluate the thermodynamic properties of C3, Si3, and Ge3 in P6222 phase, and the electronic band structures are achieved by HSE06 hybrid functional, which indicate that P6222–C3 and –Si3 are indirect band gap semiconductors and P6222–Ge3 exhibits the metallic feature.