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

Previously, we reported our new invention of an ultralight (ρ = 1.61 g/cm) and super high modulus ( = 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 MgSn 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/MgSn, and β-Li/MgSn composite interfaces prefer to form α-Mg/MgSn/β-Li ternary composite structures due to the stable formation enthalpy (Δ: -1.95 eV/atom). Meanwhile, the interface cleavage energy and critical cleavage stress show that MgSn contribute to the interfacial bond strength better than the β-Li/α-Mg phase bond strength (σ(β-Li/MgSn): 0.82 GPa > σ(α-Mg/MgSn): 0.78 GPa > σ(β-Li/α-Mg): 0.62 GPa). Based on the interfacial electronic structure analysis, α-Mg/MgSn and β-Li/MgSn 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 MgSn 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.