The influence of anisotropic diffusion on the high-temperature creep of a polycrystalline aggregate

Abstract

The influence of anisotropic diffusion coefficients on diffusion-controlled high-temperature creep is examined. Anisotropic diffusion affects anisotropy of diffusion-controlled deformation of a single crystal. The shape change of a single crystal by diffusional mass flux is controlled directly by the anisotropy in diffusion coefficients. The rate of shape change of a single crystal by diffusion-controlled dislocation glide is controlled by the anisotropy of diffusion coefficients on the plane normal to the dislocation line. Consequently, if a polycrystalline aggregate is deformed uniformly by diffusion creep, then the rate of deformation is controlled by that of diffusion (of the slowest diffusion species) along the slowest direction. In contrast, when a polycrystalline aggregate is deformed by climb-controlled dislocation creep, the rate of deformation is controlled by the diffusion (of the slowest diffusion species) along the direction where the diffusion coefficient has the intermediate value. The results are applied to olivine and post-perovskite. For olivine, the observed large plastic anisotropy and small anisotropy in diffusion suggests that high-temperature power-law creep is controlled not only by diffusion but also by some other factors such as jog density. For post-perovskite, the results of numerical calculations on the mobility of vacancies would suggest that the viscosity of post-perovskite aggregates is higher than or comparable to that of the perovskite aggregates if the defect concentrations were the same among these minerals. However, currently nothing is known about the defect concentrations in post-perovskite and other coexisting phases, and therefore it is impossible to compare the creep strength among coexisting phases from these results.