HR: 15:25h
AN: V32E-08    [PDF]
TI: Effect of Plasticity on Stress Heterogeneity at High Pressure
AU: * Weidner, D J
EM: dweidner@sunysb.edu
AF: Stony Brook University, Department of Geosciences State University of New York, Stony Brook, NY 11790-2100 United States
AU: Li, L
EM: lilli@notes.cc.sunysb.edu
AF: Stony Brook University, Department of Geosciences State University of New York, Stony Brook, NY 11790-2100 United States
AB: It has become routine to measure the differential strain field in both diamond anvil and multi-anvil cells using synchrotron x-rays. In these experiments, the x-rays pass through the sample along a path perpendicular to the compressive axis of a cylindrical stress field. Then the diffracted x-rays sample the lattice spacings both parallel and perpendicular to the maximum stress. Detection can be by means of a 2-D system such as an imaging plate or a CCD detector with monochromatic x-rays, where the Debye rings are recorded as a function of the azimuthal angle, �O. Then the lattice spacings parallel to the maximum stress axis can be compared with those parallel to minimum stress axis. White x-rays provide the same results with multiple solid state detectors, used in conjunction with a conical slit. These measurements have been possible with the use of x-ray transparent gaskets, usually Be for the diamond cell, and x-ray transparent anvils, such as cubic boron nitride, for the multi-anvil system. Each diffraction line will give rise to a measure of the stress field in the sample, giving stress measures for distinct populations of grains, namely those whose orientations meet the necessary diffraction conditions. If the sample has been compressed and strained elastically, then the Reuss �V Voight bounds become the defining relationship among the stress �V strain fields of the different populations of grains that are sampled by x-ray diffraction lines. Singh indicates the relationship between the strain field and the average stress field using the parameter, alpha = 1 to indicate a ��Reuss�� solid (uniform stress) and alpha = 0 indicating a ��Voight�� solid (uniform strain). A value between 0 and 1 indicates a mixed boundary condition solid. However, when the polycrystal is plastically deformed, several important changes occur. First, the x-ray diffraction is only sampling a portion of the total strain field. It does not reflect the plastic portion. Second, the stress in an individual grain is no longer controlled by the elastic properties, but rather by the plastic properties. The Schmidt factor, which represents the ratio of a stress on a dislocation to the stress on the grain, becomes the relevant measure of anisotropy and the stress fields in the different grain populations now evolve to maintain the necessary stress on the dislocations as to enable plastic deformation. The Taylor �V Sachs bounds replace the Reuss �V Voight bounds. In general, the stress field of the various populations of grains will change radically. Calculations, using a self-consistent polycrystalline model indicate that once plastic deformation has occurred in the sample, it will generally be no longer possible to use the Singh formalism to define the single-crystal elastic properties.
DE: 3902 Creep and deformation
DE: 3904 Defects
DE: 3924 High-pressure behavior
DE: 3994 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting