  The light elemental solutes B, C, N, and O can penetrate the surface of Mg
alloys and diffuse during heat treatment or high temperature application,
forming undesirable compounds. We investigate the diffusion of these solutes by
determining their stable interstitial sites and the inter-penetrating network
formed by these sites. We use density functional theory (DFT) to calculate the
site energies, migration barriers, and attempt frequencies for these networks
to inform our analytical model for bulk diffusion. Due to the nature of the
networks, O diffuses isotropically, while B, C, and N diffuse anisotropically.
We compute the elastodiffusion tensor which quantifies changes in diffusivity
due to small strains that perturb the diffusion network geometry and the
migration barriers. The DFT-computed elastic dipole tensor which quantifies the
change in site energies and migration barriers due to small strains is used as
an input to determine the elastodiffusion tensor. We employ the elastodiffusion
tensor to determine the effect of thermal strains on interstitial diffusion and
find that B, C, and N diffusivity increases on crystal expansion, while O
diffusivity decreases. From the elastodiffusion and compliance tensors we
calculate the activation volume of diffusion and find that it is positive and
anisotropic for B, C and N diffusion, whereas it is negative and isotropic for
O diffusion.
