High-Pressure Deformation of Single-Crystal Garnet in the D-DIA using Quasi Laue Diffraction

White-beam Laue diffraction on single crystals is a standard technique to determine crystallographic orientation and deviatoric lattice strain tensor without rotating the crystal. Theoretically, it is possible to determine the orientation and deviatoric lattice strain components of a crystal with four known Laue reflections. In order to obtain the full lattice strain tensor, the dilatation strain (unit-cell volume) must be measured separately. By determining the photon energy of a few known Laue reflections, the full strain tensor can be readily obtained. If the elastic constants are known, it is straightforward to convert the lattice strain tensor to the full stress tensor for the crystal. We have adopted this technique for single-crystal deformation in the D-DIA, where stress field of the sample can be carefully controlled to minimize gradients. Here we report preliminary data on deformation of YAG single crystals. The large unit-cell dimensions (~12 Angstrom) of YAG ensured sufficient number of Laue spots to be detected by an MAR165 CCD. Each crystal, about 0.3 mm in linear dimensions with random orientation, was loaded into a cell assembly made of a mixture of amorphous boron and epoxy and compressed isotropically while monitoring the sample length using radiographic imaging. Care was taken to avoid fracturing and significant plastic deformation during loading. At high pressure, the sample was deformed by driving the two differential pistons in the D-DIA at controlled speeds (hence strain rate). The sample length was monitored constantly to obtain information on plastic strain and strain rate. The Si (111) monochromator was scanned from 25 to 50 keV at a step of 0.25 keV during data collection to generate quasi-Laue patterns. Scanning was automated with a controlling software developed in-house. At a typical distance of 300 mm from the sample, the detector area (165 mm in linear dimensions) allowed reflections up to ±10 degrees in two-theta to be collected. Preliminary results show that (1) single crystals of several hundred microns can maintain their integrity during cold compression in carefully designed solid medium assemblies, (2) at a constant hydrostatic pressure, deformation was successfully imposed on single crystals with linear strains up to ~5% without introducing significant streaking in the Laue spots, making full lattice strain tensor determination possible, and (3) full lattice strain tensors and full stress tensors have been determined at various plastic strains, making it possible to measure mechanical properties of single crystals. Further developments and potential future applications will be discussed.