A simultaneous deformation and diffusion experiment: Quantifying the role of deformation in enhancing metamorphic reactions

The influence of concurrent plastic deformation on a continuous Fe–Mg exchange reaction in ferropericlase/periclase was examined as a function of strain and changing microstructure in a torsion experiment performed at 300 MPa and 1523 K. The measured Fe–Mg interdiffusion coefficient increases non-linearly by a factor of 1.2 to 1.4 (depending on composition) with increasing shear strain (up to a γ of 5.2), strain rate (up to 2.9 × 10− 4 s− 1) and decreasing grain size (from 200 μm to 80 μm). The change in diffusion is attributed to the increase in density of large-angle (> 15°) grain boundaries, whereas both subgrain boundary density and dislocation density do not correlate well with the changing diffusion coefficients. The increased diffusivity due to grain boundary diffusion does not follow any of the proposed equations for combined volume and boundary diffusion, indicating that either the effect of grain boundary diffusion has been underestimated or that there is an additional effect on diffusivity due to intracrystalline deformation (movement of dislocations and/or point defects, grain boundary migration during dynamic recrystallization) even if there is no net stress or strain in the direction of diffusion.