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In this paper, based on the density functional theory, through thermodynamic and mechanical stability criteria, the crystal cell model of intermetallic compounds with vacancy and anti-site point defects is constructed and the lattice constant, formation heat, binding energy, elastic constant, and elastic modulus of Mg2X (X = Si, Ge) intermetallics with or without point defects are calculated. The results show that the difference in the atomic radius leads to the instability and distortion of crystal cells with point defects; Mg2X are easier to form vacancy defects than anti-site defects on the X (X = Si, Ge) lattice site, and form anti-site defects on the Mg lattice site. Generally, the point defect is more likely to appear at the Mg position than at the Si or Ge position. Among the four kinds of point defects, the anti-site defect x M g is the easiest to form. The structure of intermetallics without defects is more stable than that with defects, and the structure of the intermetallics with point defects at the Mg position is more stable than that at the Si/Ge position. The anti-site and vacancy defects will reduce the material’s resistance to volume deformation shear strain, and positive elastic deformation, and increase the mechanical instability of the elastic deformation of the material. Compared with the anti-site point defect, the void point defect can lead to the mechanical instability of the transverse deformation of the material and improve the plasticity of the material. The research in this paper is helpful for the analysis of the mechanical stability of the elastic deformation of Mg2X (X = Si, Ge) intermetallics under the service condition that it is easy to produce vacancy and anti-site defects.

Being a positive candidate reinforcement material for laminar composites, the Mg2X (X = Si, Ge, Sn) based intermetallics have attracted much attention. The elastic properties, anisotropy, and electronic properties of intermetallic compounds with Bi-doped Mg2X (X = Si, Ge, Sn) are calculated by the first principles method. Results show that the lattice parameters of Mg2X are smaller than those of Bi-doped Mg2X. The element Bi preferentially occupies the position of the X (X = Si, Ge, Sn) atom than other positions. Mg2X (X = Si, Ge, Sn), Mg63X32Bi, Mg64X31Bi, Mg64Ge32Bi, and Mg64Sn32Bi are mechanically stable, while Mg64Si32Bi indicates that it cannot exist stably. The doping of alloying element Bi reduces the shear deformation resistance of the Mg2X (X = Si, Ge, Sn) alloy. The pure and Bi-doped Mg2X (X = Si, Ge, Sn) exhibits elastic and anisotropic characteristics. The contribution of the Bi orbitals of Mg63X32Bi, Mg64X31Bi, and Mg63X32Bi are different, resulting in different hybridization effects in three types of Bi-doped Mg2X.

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