The 13 adiabatic elastic stiffness moduli of natural muscovite have been measured using Brillouin scattering. Following the standard Institute of Radio Engineers convention, these constants are (in gigapascals) C11 = 181.0 ± 1.2, C22 = 178.4 ± 1.3, C33 = 58.6 ± 0.6, C44 = 16.5 ± 0.6, C55 = 19.5 ± 0.5, C66 = 72.0 ± 0.7, C23 = 21.2 ± 1.8, C13 = 25.6 ± 1.5, C12 ± 48.8 ± 2.5, C15 = −14.2 ± 0.8, C25 = 1.1 ± 3.7, C35 = 1.0 ± 0.6, and C46 = −5.2 ± 0.9. The main observed features are the very great acoustic anisotropy due to the weak interlayer bonding and the small, but not insignificant, deviation from hexagonal symmetry (monoclinicity) in the velocity patterns.

Yield strength is measured at high pressure and temperature using a large volume, high pressure apparatus (SAM85) with synchrotron radiation. A macroscopic deviatoric stress is manifest as a uniform deviatoric strain that is oriented by the geometry of the pressurizing medium. Microscopic deviatoric stress is identified as the elastic broadening of diffraction lines. The deviatoric stress reaches the yield point as evidenced by the uniformity, the saturation, and the temperature dependence of the deviatoric stress. Yield strengths, which correspond to the stress saturation level at a few per cent strain, are determined for NaCl and MgO up to 8 GPa and 1200°C. The results are consistent at room temperature with previous diamond anvil studies and demonstrate the effect of pressure on yield strength. These data demonstrate the feasibility of determining high pressure, high temperature yield strengths for mantle phases.

The yield strength of diamond is measured under a pressure of 10 gigapascals at temperatures up to 1550 degrees C by the analysis of x-ray peak shapes on diamond diffraction lines in a powdered sample as a function of pressure and temperature. At room temperature, the diamond crystals exhibit elastic behavior with increasing pressure. Significant ductile deformation is observed only at temperatures above 1000 degrees C at this pressure. The differential yield strength of diamond decreases with temperature from 16 gigapascals at 1100 degrees C to 4 gigapascals at 1550 degrees C. Transmission electron microscopy observations on the recovered sample indicate that the dominant deformation mechanism under high pressure and temperature is crystal plasticity.

Articles you may be interested inHigh pressure equation of state and ideal compressive and tensile strength of MgO single crystal: Ab-initio calculationsThe 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 ͕110͖͗110͘, ͕111͖͗110͘, and ͕100͖ ϫ͗011͘ 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 ͕111͖ slip system is the most significant slip system in MgO at high pressure and high temperature.