"Prospects of high P-T elasticity and rheology measurements using
the D-DIA with synchrotron radiation"
Yanbin Wang
Senior Research Scientist, Center for Advanced Radiation Sources University of Chicago
26 August 2003 17:00-18:00
Meeting room, 6th floor, Advanced Research Building, Ehime University
Rheological properties of rocks and minerals under high pressure and
high temperature conditions control the dynamics of the Earth's interior.
A new solid-medium apparatus, the deformation-DIA (D-DIA), has been developed,
which is capable of investigating deformation behavior of materials under
pressures up to 15 GPa (Wang et al., RSI, 74, 3002, 2003), by employing
synchrotron X-ray imaging and diffraction techniques in an environment
where differential stresses can be controlled independently from the confining
pressure. In this presentation, we describe our first attempts towards
quantitative rheological studies in the D-DIA. Differential stresses in
cylindrical polycrystalline MgO and CsCl samples under controlled axial
strain are determined as a function of pressure and temperature in the
D-DIA using monochromatic diffraction, with a two-dimensional X-ray charge
coupled device (CCD) detector and X-ray transparent sintered cubic boron
nitride (cBN) anvils. Radiographic images and diffraction Debye rings are
repeatedly recorded at various pressures during constant strain-rate deformation.
>From the sample length change in the X-ray image, total sample axial
strain can be determined. From the distortion of the diffraction rings
recorded over the entire 360° azimuth angles, elastic lattice strains
can be accurately measured. Linear lattice elastic theory is applied to
convert these lattice strains to differential stress (Singh, J. Appl. Phys.,
73, 4278, 1993). The ability of the D-DIA to separate differential stress
from pressure (by deforming the sample axially while maintaining constant
pressure) allows us to establish criteria for detecting yielding and to
examine pressure and total strain dependence of yield strength. Our analyses
indicate that Singh's theory is adequate in describing the stress-strain
behavior within the elastic regime. However, as some crystallites begin
the yield, stress state becomes complex, resulting in significant change
in the apparent anisotropy factor. This change can be considered as an
indicator for the onset of yielding, and stress distribution in the sample
becomes heterogeneous. Such details contain important information on rheology
but previously could not be observed in conventional deformation and high
pressure (such as DAC or DIA) devices. Work is underway to model the stress
distribution in polycrystalline samples after yielding, in order to connect
stresses measured at the grain-to-grain level to the conventional the force-over-the-area
measurements. Our findings also raise questions on previous elastic constant
measurements using diffraction in the diamond-anvil cell, where differential
stress levels are expected to be high. These data are likely to be heavily
influenced by yielding, in which case the elastic data thus obtained would
be erroneous.
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主催：愛媛大学地球深部ダイナミクス研究センター