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.