Full local elastic strain tensor from Laue microdiffraction: simultaneous Laue pattern and spot energy measurement

In sample-scanning Laue microdiffraction, the local crystal orientation and local deviatoric strain tensor are obtained by illuminating the polycrystalline sample with a broadband `white' (5–30 keV) X-ray microbeam and analyzing the spot positions in the resulting local Laue pattern. Mapping local hydrostatic strain is usually slower, owing to the need to alternate between white and tunable-energy monochromatic microbeams. A technique has been developed to measure hydrostatic strain while keeping the white beam. The energy of one of the Laue spots of the grain of interest is measured using an energy-dispersive point detector, while simultaneously recording the Laue pattern on the two-dimensional detector. The experimental spot energy, E_exp, is therefore measured at the same time as E_theor, the theoretical spot energy for zero hydrostatic strain, which is derived from the analysis of the Laue pattern. The performance of the technique was compared with that of the monochromatic beam technique in two test cases: a Ge single crystal and a micrometre-sized UO₂ grain in a polycrystal. Accuracies on the hydrostatic strain Δa/a of ±0.4 × 10⁻⁴ and ±1.3 × 10⁻⁴ were obtained for Ge and UO₂, respectively. Measurement strategies to limit the remaining uncertainties on E_theor are discussed.