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Recent OMG Papers & Pre-prints

  • Forescattered electron imaging of nanoparticles in scanning electron microscopy

    Liu, J, Lozano-Perez, S, Karamched, P, Holter, J, Wilkinson, AJ, Grovenor, CRM
    2019
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    Journal article
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    Materials Characterization
    © 2019 Elsevier Inc. In this study, we have used a Zr-Nb alloy containing well-defined nano-precipitates as a model material in which to study imaging contrast inversions (atomic number or diffraction contrast) observed with the forescattered electron imaging system, ARGUS™, in a scanning electron microscope (SEM) when imaging a thin foil in a transmission geometry. The study is based on Monte Carlo simulations and analysis of micrographs experimentally acquired under different imaging conditions. Based on the results, imaging conditions that enhance atomic number or diffraction contrast have been proposed. Data acquired from the ARGUS™ imaging system in an SEM has also been compared with results from standard transmission electron microscopy and scanning transmission electron microscopy imaging of the same material. These results demonstrate the capability of the ARGUS™ system to investigate microstructures in nano-scale materials.

  • Pattern matching analysis of electron backscatter diffraction patterns for pattern centre, crystal orientation and absolute elastic strain determination – accuracy and precision assessment

    Tanaka, T, Wilkinson, AJ
    2019
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    Journal article
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    Ultramicroscopy

  • On the depth resolution of transmission Kikuchi diffraction (TKD) analysis.

    Liu, J, Lozano-Perez, S, Wilkinson, AJ, Grovenor, CRM
    2019
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    Journal article
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    Ultramicroscopy
    In this paper, we have analyzed the depth resolution that can be achieved by on-axis transmission Kikuchi diffraction (TKD) using a Zr-Nb alloy. The results indicate that the signals contributing to detectable Kikuchi bands originate from a depth of approximately the mean free path of thermal diffuse scattering (λTDS) from the bottom surface of a thin foil sample. This existing surface sensitivity can thus lead to the observation of different grain structures when opposite sides of a nano-crystalline foil are facing the incident electron beam. These results also provide a guideline for the ideal sample thickness for TKD analysis of ≤ 6λTDS, or 21 times the elastic scattering mean free path (λMFP) for samples of high crystal symmetry. For samples of lower symmetry, a smaller thickness ≤ 3λTDS, or ≤ 10λMFP is suggested.
    Depth resolution, Nanocrystalline material, Thermal diffuse scattering, Transmission Kikuchi diffraction (TKD), Zr alloys

  • Microstructural Evolution of Mechanically Deformed Polycrystalline Silicon for Kerfless Photovoltaics

    Wu, M, Murphy, JD, Jiang, J, Wilshaw, PR, Wilkinson, AJ
    2019
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    Journal article
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    Physica Status Solidi (A) Applications and Materials Science
    © 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.

  • Mapping the full lattice strain tensor of a single dislocation by high angular resolution transmission Kikuchi diffraction (HR-TKD)

    Yu, H, Liu, J, Karamched, P, Wilkinson, AJ, Hofmann, F
    2019
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    Journal article
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    Scripta Materialia
    © 2019 Elsevier Ltd The full lattice strain tensor and lattice rotations induced by a dislocation in pure tungsten were mapped using high-resolution transmission Kikuchi diffraction (HR-TKD) in a SEM. The HR-TKD measurement agrees very well with a forward calculation using an elastically isotropic model of the dislocation and its Burgers vector. Our results demonstrate that the spatial and angular resolution of HR-TKD in SEM is sufficiently high to resolve the details of lattice distortions near individual dislocations. This capability opens a number of new interesting opportunities, for example determining the Burgers vector of an unknown dislocation in a fast and straightforward way.

  • The impact of water on slip system activity in olivine and the formation of bimodal crystallographic preferred orientations

    Wallis, D, Hansen, LN, Tasaka, M, Kumamoto, KM, Parsons, AJ, Lloyd, GE, Kohlstedt, DL, Wilkinson, AJ
    2019
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    Journal article
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    Earth and Planetary Science Letters
    © 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.
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