Dislocation and disclination densities in experimentally deformed polycrystalline olivine

Abstract

Abstract. We report a comprehensive data set characterizing and quantifying the geometrically necessary dislocation (GND) density in the crystallographic frame (ραc) and disclination density (ρθ) in fine-grained polycrystalline olivine deformed in uniaxial compression or torsion, at 1000 and 1200 ∘C, under a confining pressure of 300 MPa. Finite strains range from 0.11 up to 8.6 %, and stresses reach up to 1073 MPa. The data set is a selection of 19 electron backscatter diffraction maps acquired with conventional angular resolution (0.5∘) but at high spatial resolution (step size ranging between 0.05 and 0.1 µm). Thanks to analytical improvement for data acquisition and treatment, notably with the use of ATEX (Analysis Tools for Electron and X-ray diffraction) software, we report the spatial distribution of both GND and disclination densities. Areas with the highest GND densities define sub-grain boundaries. The type of GND densities involved also indicates that most olivine sub-grain boundaries have a mixed character. Moreover, the strategy for visualization also permits identifying minor GND that is not well organized as sub-grain boundaries yet. A low-temperature and high-stress sample displays a higher but less organized GND density than in a sample deformed at high temperature for a similar finite strain, grain size, and identical strain rate, confirming the action of dislocation creep in these samples, even for micrometric grains (2 µm). Furthermore, disclination dipoles along grain boundaries are identified in every undeformed and deformed electron backscatter diffraction (EBSD) map, mostly at the junction of a grain boundary with a sub-grain but also along sub-grain boundaries and at sub-grain boundary tips. Nevertheless, for the range of experimental parameters investigated, there is no notable correlation of the disclination density with stress, strain, or temperature. However, a broad positive correlation between average disclination density and average GND density per grain is found, confirming their similar role as defects producing intragranular misorientation. Furthermore, a broad negative correlation between the disclination density and the grain size or perimeter is found, providing a first rule of thumb on the distribution of disclinations. Field dislocation and disclination mechanics (FDDM) of the elastic fields due to experimentally measured dislocations and disclinations (e.g., strains/rotations and stresses) provides further evidence of the interplay between both types of defects. At last, our results also support that disclinations act as a plastic deformation mechanism, by allowing rotation of a very small crystal volume.

Modeling of experimentally observed topological defects inside bulk polycrystals

“…Related work can be found in [23,24] focusing on defect kinematics where finite difference approximations are used for inferring the disclination density from observations, but no stress fields are calculated. A notable recent work based on EBSD measurements, and broadly complementary to our work is [25]; however, the rare topologically interesting features in the zirconium sample would be difficult to locate with a surface based measurement. The work in [26] utilizes grain averaged elastic strain measurements to compute intergranular elastic distortion and stress fields in a polycrystal using field dislocation mechanics theory (a subset of our equations), constraining the computed elastic strain field's grain-wise averaged field to equal the measured grain averaged elastic strain field.…”

Patterns of Grain Fragmentation During Plastic Deformation of Metals at Small to Medium Strains (Brief Review)

“…Such a variation can be described, in particular, as a distribution of disclination density on the boundary surface. A method for the recovery of components of the disclination density tensor from EBSD maps was suggested [88,89], and extensive presence of disclination densities has been revealed in various materials at low and high-angle boundaries as well as at triple junctions [88][89][90]. At the same time, the numerical approximation method applied in those studies to the mapped orientation data remains the subject of debate [91].…”