Development of a Stress Sensor for In-Situ High-Pressure Deformation Experiments in D-DIA

Radial X-ray diffraction has been used for decades to study mechanical properties of polycrystalline samples during in-situ high-pressure deformation experiments. When polycrystalline materials are deformed, deviatoric stresses develop in grains that leads to lattice distortion. Using X-ray diffraction we can estimate the lattice strain for each (hkl) diffraction planes and calculate the applied stress for each (hkl). In converting grain-scale lattice strain to macroscopic stress, a theory by Singh et al. (1998) has often been used. However, when this method is used for a sample deformed by dislocation creep, large anisotropy far exceeding elastic anisotropy is often observed leading to a large uncertainty in estimating the macroscopic strength of an aggregate.

In order to improve the estimate of the strength of an aggregate in such a study, we explore the use of stress sensor from which the macroscopic stress can be estimated without a large uncertainty. A guide to choose a material for a stress sensor is that its plastic anisotropy is small. We chose pyrope garnet (Mg₃Al₂Si₃O₁₂) as a stress sensor because its plastic properties are nearly isotropic.

To test pyrope stress sensor, we conducted in-situ uniaxial deformation experiments in D-DIA at 6-BM-B beamline at APS. Our polycrystalline samples (San Carlos olivine and olivine synthesized by a sol-gel method) were loaded with synthetic polycrystalline pyrope discs. Each sample were sandwich between platinum foils that are used as strain marker. The strain was measured in-situ using X-ray radiography image analyses, and the stress was calculated from energy dispersive X-ray diffraction collected on our pyrope stress sensor as well as olivine samples.

Here we present our results that show stresses estimated from pyrope present much smaller anisotropy and thus much smaller uncertainty (~10%) than stress estimated from X-ray diffraction in the olivine samples (San-Carlos olivine and olivine sol-gel).

References:

Karato, S.-I. (2009). "Theory of lattice strain in material undergoing plastic deformation: Basic formulation and applications to a cubic crytal." Physical Review. B 79(21): 214106-214106-214109.

Singh, A. K., et al. (1998). "Analysis of lattice strain measured under nonhydrostatic pressure." Journal of Applied Physic 83(12): 7567-7575.