First-principles simulations of plasticity in body-centered-cubic magnesium-lithium alloys

First-principles quantum mechanics is an increasingly important tool for predicting material properties when designing novel alloys with optimized mechanical properties. In this study, we employ first-principles orbital-free density functional theory (OFDFT) to study plastic properties of body-centered-cubic (bcc) Mg-Li alloys as potential lightweight metals for use in, e.g., vehicle applications. The accuracy of the method as a predictive tool is benchmarked against the more accurate Kohn-Sham DFT (KSDFT). With a new analytic local electron-ion pseudopotential, OFDFT is shown to be comparable in accuracy to KSDFT with the conventional non-local pseudopotential for many properties of Mg-Li alloys, including lattice parameters and energy differences between phases. After this validation, we calculate generalized stacking fault energies (SFEs) of a perfect lattice and Peierls stresses (σp's) for dislocation motion in various bcc Mg-Li alloys. Such predictions have not been made previously with any level of theory. Based on analysis of SFE barriers, we propose that alloys with 31-50 at.% Li will exhibit the greatest strength. Their σp's are predicted to be 0.18-0.31 GPa. The Li concentration in this range (31-50 at.%) has little impact on plastic properties of bcc Mg-Li alloys, while atomic-level disorder may decrease the σp. This range of σp is similar to the industrial goal for potential lightweight Mg alloys.