Ordered magnesium-lithium alloys: First-principles predictions

Magnesium-lithium (Mg-Li) alloys are among the lightest structural materials. Although considerable work has been done on the Mg-Li system, little is known regarding potential ordered phases. A first and rapid analysis of the system with the high-throughput method reveals an unexpected wealth of potentially stable low-temperature phases. Subsequent cluster expansions constructed for bcc and hcp superstructures extend the analysis and verify our high-throughput results. Of particular interest are those structures with greater than $13\text{at}\text{.}\mathrm{\%}$ lithium, as they exhibit either partial or complete formation as a cubic structure. Order-disorder transition temperatures are predicted by Monte Carlo simulations to be in the range 200–500 K.

Figure 1

High-throughput formation enthalpy calculations for Mg-Li. Structures resting on the convex hull are labeled according to their Strukturbericht designation and are displayed prominently. Note that many computed structures have negative formation enthalpy, suggesting an ordering system. ${\text{MoTi}}^{\ast }$ prototype from Ref. 15.

Figure 2

Schematic LiMg phase diagram constructed after Refs. 8, 17, 18, 19, 20, 21.

Figure 3

Formation enthalpy versus concentration for many derivative superstructures. The random-alloy energy is included as well as the convex hull. Points on the convex hull indicate thermodynamically stable ground-state structures.

Figure 4

CE formation enthalpies as a function of concentration. Energy predictions from the hcp CE are shifted according to the difference in free energy of Mg and Li on bcc and hcp lattices. Note the two-phase region of bcc and hcp phases on the Mg-rich side consistent with known phase data (Fig. 2).

Figure 5

Final ground-state predictions for bcc and hcp Mg-Li using energies computed with VASP. The tie lines are shown. The hcp ground states do not correspond to commonly known structure types.

Figure 6

Final ground-state predictions for hcp and bcc Mg-Li. The hcp energies are shifted relative to bcc energies. Note that there is no hcp phase predicted other than pure Mg and the two-phase region extending to 2/3 Mg. The structures B2 and C11b are also shown from left to right.

Figure 7

Specific heat $C_p$ of Mg-Li ordered phases with temperature increased in increments of 10 K. Formation enthalpy is overlaid including the scale indicated on the left.