HIGH-ANGULAR RESOLUTION ELECTRON BACKSCATTER DIFFRACTION AS A NEW TOOL FOR MAPPING LATTICE DISTORTION IN GEOLOGICAL MATERIALS

摘要

Electron backscatter diffraction (EBSD) in the scanning electron microscope has become centralto microstructural analysis in a wide range of geoscience subdisciplines. In particular, EBSD isemployed intensively to investigate the microstructures of deformed rocks. However, conventionalEBSD, based on indexing the Hough transform of each diffraction pattern, reveals only a fractionof the information on intragranular deformation that is stored in each pattern. Recent work in thematerials sciences has generated a new approach to analysing diffraction patterns based on crosscorrelation of multiple regions of interest between patterns. This method, termed high-angularresolution electron backscatter diffraction (HR-EBSD), offers several advantages overconventional EBSD for analysis of intragranular lattice distortions. The cross-correlation basedapproach improves precision in misorientation angles by approximately an order of magnitude toaround 0.01°. This precision allows lower densities of geometrically necessary dislocations to beresolved. Moreover, the associated misorientation axes are much better constrained, even for smallmisorientation angles. These improved constraints aid determination of the types of dislocationsthat generate lattice curvature. A further major advantage of HR-EBSD is that it maps elastic strainof the lattice, which is not analysed at all by conventional EBSD, with precision on the order of10-4. These measurements are relative to the strain state of a reference point within each grain andtherefore quantify intragranular strain heterogeneity. With knowledge of the elastic constants, thestrains are used to calculate intragranular residual stress heterogeneity, with precision on the orderof tens of megapascals. Simple examples of HR-EBSD datasets from experimentally deformedolivine and naturally deformed quartz demonstrate each of these advantages of HR-EBSD analysis.These capabilities open a wealth of opportunities to gain new insights into the details ofdeformation microstructures and the geological processes that cause and are influenced by them.