Effects Of Elements Doping On Structural Stability And Mechanical Properties Of Mg₂Sn Phase

Magnesium alloys have been paid extensive attention as the lightest structural material. In recent years, first-principles simulations about magnesium alloys have emerged in endlessly. The properties of magnesium alloys under extreme conditions?high temperature, high pressure and corrosion? can be predicted by first-principles simulation. Solid solution strengthening is a main way to improve the properties of magnesium. In first-principles simulation field, one can provide useful guiding information and save cost and time by adding alloying elements to investigate solid solution process. In this paper, we investigated the effects of Al, Zn, Ag or Zr doping on structural stability, mechanical properties and electronic structures of Mg₂Sn phase.We determined structural stability and mechanical stability of the doping phases and compared the influences of different doping atoms on mechanical properties of Mg₂Sn phase. Meanwhile, we calculated the band structures, density of states and ionic configuration of doping phases to study their metallicity and ionic property. In the end, we investigated the stability and electronic structures of Mg₂Sn?100? surface.The main conclusions of this paper are as follows:?1? The lattice parameters of Mg₂Sn phase varied after adding 4 doping atoms.Zr atom doping increased the lattice parameters while Al, Zn and Ag atom doping decreased the lattice parameters of Mg₂Sn phase. Among different doping phases,Mg₂Sn0.75Zr0.25 doping phase has the largest lattice parameter while Mg₂Sn0.75Zn0.25 possesses the smallest lattice parameter. The variation of lattice parameters of Mg₂Sn phase after atom doping is consistent with the atomic radius of doping atoms. The larger the atomic radius of the doping atom is, the larger the lattice parameter of the doping phase is. The formation enthalpy of Mg₂Sn phase increased after atom doping,which indicated the structural stability of Mg₂Sn phase became lower. Mg₂Sn0.75Zr0.25 phase is not stable.?2? The elastic constants C₁₁ and C₄₄ of Mg₂Sn phase decreased after doping while C12 of Mg₂Sn phase increased after adding doping atoms. The elastic constants of Mg₂Sn and its 7 doping phases all satisfy the Born mechanical stability criterion.From the perspective of Cauchy pressure, the bonding in Mg₂Sn and its 7 doping phases is mainly metallic bonding and these phases are all ductile materials. The values of all the anisotropy factors of Mg₂Sn and doping phases are not 1, which demonstrates that these phases are all anisotropic materials. Among these phases, the anisotropies of Mg1.75Zr0.25 Sn and Mg₂Sn0.75Al0.25 are the largest. After doping, the shear modulus of Mg₂Sn phase presents a declining tendency, Young's modulus decreases a little and the bulk modulus nearly keeps unchanged. According to Poisson's ratio and G/B, Mg₂Sn and its doping phases are ductile materials and the ductility of Mg1.75Al0.25 Sn is the best.?3? There are not band gaps in the band structures of Mg₂Sn and its 7 doping phases, which indicates that the phases are metallic and conductive. Atom doping doesn't change the metallicity of Mg₂Sn phase. From the figures of density of states,we found that the density of states in low energy area of Mg₂Sn phase increased after atom doping. There are orbital hybridization in Mg₂Sn and its doping phases, which implies that there exists covalent bonding in these phases. By calculating the ionic configuration of doping phases, we found that the doping atoms participated the charge transfer process in these phases.?4? The effects of relaxation are mainly localized within the three outermost atomic layers for Mg-termination surfaces as well as Sn-termination surfaces. The changes between atomic layer distances of Sn-termination surfaces are slightly larger than those of Mg-termination surfaces. Mg-termination surfaces are more stable than Sn-termination surfaces since the surface energy of Mg-termination surfaces is lower than that of Sn-termination surfaces over nearly entire area of Mg chemical potentials.There are no band gaps in Mg-termination and Sn-termination surfaces. Both Mg-termination and Sn-termination surfaces are metallic characterization. It is found that covalent bonding exists in both Mg-termination and Sn-termination surfaces by analyzing the PDOS, and the covalent bonding in Sn-termination surfaces is stronger than that in Mg-termination surfaces.