Dislocation Bands and Subboundaries in Experimentally Deformed Olivine

Abstract

Transmission electron microscopy imaging of dislocations in olivine indicates heterogeneous structures and diversity of dislocation types. However, the volumes imaged at high resolution are smaller than individual grains even of fine‐grained samples. Electron backscatter diffraction (EBSD) mapping of large areas provides a statistical view of the distribution of dislocations also in coarse‐grained samples. Maps of a deformed single crystal confirm that conventional EBSD can image mobile dislocations. Dislocation densities in polycrystalline samples deformed under high temperature or wet conditions are heterogeneous, both within grains and from grain to grain. Slip systems with Burgers vector b  = [100] and [001] dominate, but dislocations with b = [010] are also present. The most prominent dislocation structures are bands of high dislocation density evolving into subboundaries. Dislocation densities along bands and subboundaries and the resulting misorientation are variable, reflecting non‐uniform grain‐internal strain. Bands of high dislocation density and subboundaries commonly terminate at concave parts of grain boundaries of grains containing the bands. Areas of adjacent grains with matching convex grain boundaries generally have low dislocation density, consistent with dislocation density‐driven grain boundary migration. No characteristic structure distinguishing wet from dry deformed samples was found, but the densities of the dislocation types differ. The observed dislocation structures indicate that formation and migration of bands and subboundaries (requiring both glide and climb of dislocations) are an integral part of deformation in the dislocation creep regime at temperatures >1200°C.