Uncovering the Interfacial Strengthening Mechanisms of α-Mg/Mg₂Sn/β-Li Interfaces Using First-Principle Calculations and HAADF-STEM

Previously, we reported our new invention of an ultralight (ρ = 1.61 g/cm³) and super high modulus (E = 64.5 GPa) Mg-Li-Al-Zn-Mn-Gd-Y-Sn (LAZWMVT) alloy. Surprisingly, the minor additions of Sn contribute to significant strength and stiffness increases. In this study, we found that Mg₂Sn was not only the simple precipitate but also acted as the glue to bind the α-Mg/β-Li interface in a rather complicated way. To explore its mechanism, we have performed first-principle calculations and HAADF-STEM experiments on the interfacial structures. It was found that the interfacial structural models of α-Mg/β-Li, α-Mg/Mg₂Sn, and β-Li/Mg₂Sn composite interfaces prefer to form α-Mg/Mg₂Sn/β-Li ternary composite structures due to the stable formation enthalpy (ΔH: −1.95 eV/atom). Meanwhile, the interface cleavage energy and critical cleavage stress show that Mg₂Sn contribute to the interfacial bond strength better than the β-Li/α-Mg phase bond strength (σ_b(β-Li/Mg₂Sn): 0.82 GPa > σ_b(α-Mg/Mg₂Sn): 0.78 GPa > σ_b(β-Li/α-Mg): 0.62 GPa). Based on the interfacial electronic structure analysis, α-Mg/Mg₂Sn and β-Li/Mg₂Sn were found to have a denser charge distribution and larger charge transfer at the interface, forming stronger chemical bonds. Additionally, according to the crystal orbital Hamiltonian population analysis, the bonding strength of the Mg-Sn atom pair was 2.61 eV, which was higher than the Mg-Li bond strength (0.39 eV). The effect of the Mg₂Sn phase on the stability and interfacial bonding strength of the alloying system was dominated by the formation of stronger and more stable Mg-Sn metal covalent bonds, which mainly originated from the contribution of the Mg 3p-Sn 5p orbital bonding states.