Angus Wilkinson

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Silicon wafers for photovoltaics could be produced without kerf loss by rolling, provided sufficient control of defects such as dislocations can be achieved. A study using mainly high resolution electron backscatter diffraction (HR-EBSD) of the microstructural evolution of Siemens polycrystalline silicon feedstock during a series of processes designed to mimic high temperature rolling is reported here. The starting material is heavily textured and annealing at 1400 °C results in 90% recrystallization and a reduction in average geometrically necessary dislocation (GND) density from >1014 to 1013 m−2. Subsequent compression at 1150 °C – analogous to rolling – produce sub-grain boundaries seen as continuous curved high GND content linear features spanning grain interiors. Post-deformation annealing at 1400 °C facilitates a secondary recrystallization process, resulting in large grains typically of 100 μm diameter. HR-EBSD gives the final average GND density in as 3.2 × 1012 m−2. This value is considerably higher than the dislocation density of 5 × 1010 m−2 from etch pit counting, so the discrepancy is investigated by direct comparison of GND maps and etch pit patterns. The GND map from HR-EBSD gives erroneously high values at the method's noise floor (≈1012 m−2) in regions with low dislocation densities.

© 2018 The Authors Crystallographic preferred orientations (CPOs) in olivine are widely used to infer the mechanisms, conditions, and kinematics of deformation of mantle rocks. Recent experiments on water-saturated olivine were the first to produce a complex CPO characterised by bimodal orientation distributions of both [100] and [001] axes and inferred to form by combined activity of (001)[100], (100)[001], and (010)[100] slip. This result potentially provides a new microstructural indicator of deformation in the presence of elevated concentrations of intracrystalline hydrous point defects and has implications for the interpretation of seismic anisotropy. Here, we document a previously unexplained natural example of this CPO type in a xenolith from Lesotho and demonstrate that it too may be explained by elevated concentrations of hydrous point defects. We test and confirm the hypothesis that combined (001)[100], (100)[001], and (010)[100] slip were responsible for formation of this CPO by (1) using high-angular resolution electron backscatter diffraction to precisely characterise the dislocation types present in both the experimental and natural samples and (2) employing visco-plastic self-consistent simulations of CPO evolution to assess the ability of these slip systems to generate the observed CPO. Finally, we utilise calculations based on effective-medium theory to predict the anisotropy of seismic wave velocities arising from the CPO of the xenolith. Maxima in S-wave velocities and anisotropy are parallel to both the shear direction and shear plane normal, whereas maxima in P-wave velocities are oblique to both, adding complexity to interpretation of deformation kinematics from seismic anisotropy.

We have used cross-correlation-based analysis of electron backscatter diffraction patterns to map lattice rotations in polycrystalline samples of Ti-6Al-4V before and after moderate (6%) tensile deformation. The sensitivity is improved compared to conventional Hough-based indexing and allows the density of geometrically necessary dislocations (GNDs) to be determined to ∼3 × 1012 m-2 at a step size of 250 nm. In the undeformed sample there were a few grains with GND density significantly higher than the background level. These tended to be of small area and associated with neighbouring regions of β-phase exhibiting the Burgers relation. After deformation the overall GND density increased, with 〈a〉 dislocations some 20 times more frequent than 〈c + a〉 ones. Evidence is given of elevated GND densities close to some, but not all, grain boundaries after deformation. This was also the most likely reason for the trend for the grain-averaged GND density to be higher for grains with small area on the section plane. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.