High-Pressures Rheological Study with Synchrotron X-Ray Diffraction. H. Mao, J. Shu, R. J. Hemley (Center for High Pressure Research and Geophysical Laboratory, Carnegie Institution of Washington), A. K. Singh (Material Science Division, National Aeronautical Laboratory, INDIA)

Rheological properties of hcp-Fe, including its preferred crystallographic orientation, shear strength, and single-crystal elasticity tensor, have been studied as a function of pressure to 220 GPa with a novel x-ray diffraction method. We apply uniaxial stress on samples in a diamond cell and measure strains with energy dispersive x-ray diffraction (EDXD) at superconducting wiggler beamline of the National Synchrotron Light Source. X-ray transparent beryllium gasket provide access for the EDXD probe of the polycrystalline Fe sample in a full 180deg. of [phi], the angle between the unique stress axis (diamond-cell axis) and the vector of the diffraction planes. At each [phi], EDXD pattern for all crystallographic orientations (hkl) are obtained. Relative intensity of EDXD peaks as a function of [phi] reveals the preferred orientation of the polycrystalline sample. As the sample flows in the radial direction perpendicular to the uniaxial stress, the intensity of the 002 reflection of the hcp-Fe reaches a maximum at [phi] = 0deg. and a minimum at [phi] = 90deg., indicating the c-axis of hcp-Fe preferentially aligns with the maximum stress direction and the a-axis aligns with the maximum shear (flow) direction. The dependence of d-spacings as a function of [phi] and hkl provide information of strength and elasticity. By fitting the equation d_hkl to d_hkl = d_0hk1 [1 - F_hkl (1 - 3cos²f)], the parameter F _hkl is determined as a function of hkl . With additional constraints provided by hydrostatic x-ray diffraction data, the full elastic tensors can be determined. The elasticity tensor and acoustic anisotropy of the hcp-Fe show notable differences with recent theoretical predictions, the elastic wave velocities calculated from the experimental tensor show a much stronger anisotropy. In addition, the compressional wave velocity reaches a maximum at 45deg. rather than along the a or c axis. The surprising finding of a large body-diagonal elastic anisotropy points out the need to re-examine models for the seismic velocity anisotropy of the Earth inner core and to further improve the theoretical treatment of iron.