Geometrically necessary dislocation densities in olivine obtained using high-angular resolution electron backscatter diffraction

Abstract

Dislocations in geological minerals are fundamental to the creep processes that control large-scale geodynamic phenomena. However, techniques to quantify their densities, distributions, and types over critical subgrain to polycrystal length scales are limited. The recent advent of high-angular resolution electron backscatter diffraction (HR-EBSD), based on diffraction pattern cross-correlation, offers a powerful new approach that has been utilised to analyse dislocation densities in the materials sciences. In particular, HR-EBSD yields significantly better angular resolution (<0.01°) than conventional EBSD (~0.5°), allowing very low dislocation densities to be analysed. We develop the application of HR-EBSD to olivine, the dominant mineral in Earth's upper mantle by testing (1) different inversion methods for estimating geometrically necessary dislocation (GND) densities, (2) the sensitivity of the method under a range of data acquisition settings, and (3) the ability of the technique to resolve a variety of olivine dislocation structures. The relatively low crystal symmetry (orthorhombic) and few slip systems in olivine result in well constrained GND density estimates. The GND density noise floor is inversely proportional to map step size, such that datasets can be optimised for analysing either short wavelength, high density structures (e.g. subgrain boundaries) or long wavelength, low amplitude orientation gradients. Comparison to conventional images of decorated dislocations demonstrates that HR-EBSD can characterise the dislocation distribution and reveal additional structure not captured by the decoration technique. HR-EBSD therefore provides a highly effective method for analysing dislocations in olivine and determining their role in accommodating macroscopic deformation.

The recrystallized grain size piezometer for quartz: An EBSD‐based calibration

“…This effect may compromise grain separation in the lowest-stress sample presented here (W1126), where recrystallized grain sizes are very similar to those of the starting material used in these experiments. Nevertheless, we suggest that high angular resolution operating procedures [Prior et al, 1999;Wilkinson et al, 2010;Wallis et al, 2016] should give improved performance for lower stress samples. These microstructures imply that the grain separation in W1126 is robust, and not an artifact of local variations in EBSD indexing accuracy.…”

Crystallographic Preferred Orientation of Olivine in Sheared Partially Molten Rocks: The Source of the “a‐c Switch”

“…Diffraction patterns were collected with 2 3 2 binning of CCD pixels and saved for HR-EBSD processing. This improved angular resolution lowers the noise floor of associated GND density Geochemistry, Geophysics, Geosystems 10.1002/2017GC007309 estimates, permitting more details of the substructure to be resolved than would be possible using misorientation measurements from conventional EBSD (Jiang et al, 2013;Wallis et al, 2016). HR-EBSD utilizes image cross correlation of diffraction patterns to achieve precisions on the order of 0.018 in measurements of lattice orientation gradients (Wilkinson et al, 2006).…”

“…The transverse section was chosen so that most [100] and [001] directions are at low angles to the plane of the map, which is overall the optimal configuration for measuring curvature arising from dislocations with these Burgers vectors (Wallis et al, 2016;Wheeler et al, 2009). Lattice orientation gradients were converted to lower-bound estimates of GND densities following the approach of Wallis et al (2016Wallis et al ( , 2017, who considered six dislocation types. This improved angular resolution lowers the noise floor of associated GND density Geochemistry, Geophysics, Geosystems 10.1002/2017GC007309 estimates, permitting more details of the substructure to be resolved than would be possible using misorientation measurements from conventional EBSD (Jiang et al, 2013;Wallis et al, 2016).…”

Texture, Microstructure and Mechanical Properties of 6111 Aluminum Alloy Subject to Rolling Deformation

“…To coordinate the deformation, geometrically necessary dislocations (GND) are generated and play an important role as a kind of coordination dislocations. Many early works has explored the GND during deformation by EBSD 22,23 . The density of GND with different deformation samples from EBSD is shown in Figure 6.…”