X-ray strain analysis at high pressure: Effect of plastic deformation in MgO

Abstract

The factors that control the stress–strain state of a polycrystal under differential stress depend on whether or not plastic deformation has occurred in the solid. If not, then the elastic properties with the constraints of the Reuss–Voigt bounds limit this relationship. If plastic deformation becomes important then the Taylor and Sachs models are relevant. These models assume that the plastic process is enabled by dislocation flow on specific lattice planes and specific Burger’s vectors. Then, the relationship between stress and strain is controlled by the orientation of an individual grain with respect to the stress field, von Mises criterion, and the critical resolved stress on the dislocation that is necessary for flow. We use a self-consistent model to predict the flow stress during the plastic deformation of polycrystalline MgO with a slip system of {110}〈11̄0〉, {111}〈11̄0〉, and {100}〈011〉 at different critical resolved shear stress ratios for the different slip systems. The prediction of the models is correlated with the results of x-ray diffraction measurements. Uniaxial deformation experiments on polycrystalline and single-crystal MgO samples were conducted in situ using white x-ray diffraction with a multielement detector and multianvil high-pressure apparatus at a pressure up to 6 GPa and a temperature of 500 °C. A deformation DIA was used to generate pressure and control at a constant deformation rate. Elastic strains and plastic strains were monitored using x-ray diffraction spectra and x-ray imaging techniques, respectively. The correlation of the data and models suggests that the plastic models need to be used to describe the stress–strain observations with the presence of plasticity, while the Reuss and Voigt models are appropriate for the elastic region of deformation, before the onset of plastic deformation. The similarity of elastic strains among different lattice planes suggests that the {111} slip system is the most significant slip system in MgO at high pressure and high temperature.