Solute transport database in Mg using ab initio and exact diffusion theory

Magnesium alloys have seen renewed interest for lightweight applications in aerospace and automotive; design of new alloys focuses on solute additions to produce microstructure for improved mechanical properties. Predictive control of alloy processing requires accurate transport coefficients: both solute diffusivity and "off-diagonal" Onsager coefficients impact non-equilibrium transport. We systematically study the diffusion of 69 solutes including rare-earths in hexagonal close packed magnesium using density functional theory calculations and our newly developed green function approach for dilute diffusion coefficients. The fundamental symmetry analysis of Mg crystal identifies distinct solute-vacancy complexes and transitions between them, while DFT calculates the site and transition state energies. We find 17 symmetrically inequivalent transition states corresponding to 27 frequencies to build a solute transport database comprising solute diffusivities and drag ratios. Our methodology avoids the uncontrolled approximations in the 8- and 13-frequency models by accounting for the symmetrically inequivalent transitions which were assumed to be equal in these previous models. We find improved predictions of activation energies of diffusion for the rare-earths and Ca. We show significant changes in drag ratios for the common alloying additives Al, Zn and rare-earths compare to previous models and explain their physics through the "ring network" of jumps of a vacancy around a solute. The solute transport database generated from exact theory of diffusion through green function approach coupled with density functional theory calculations can serve as the basis for the development of new materials and materials processes.