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Fundamental and experimental aspects of diffraction for characterizing dislocations by electron channeling contrast imaging in scanning electron microscope

Abstract : Nowadays Field Emission Gun-Scanning Electron Microscopes provide detailed crystallographic information with high spatial and angular resolutions, and allow direct observation of crystalline defects, such as dislocations, through an attractive technique called Electron Channeling Contrast Imaging (ECCI). Dislocations play a crucial role in the properties of materials and ECCI has naturally emerged as an adapted tool for characterizing defects in bulk specimen. Nevertheless, fine control of the channeling conditions is absolutely required to get strong dislocation contrast for achieving comprehensive analysis. In this work, experiment-assisted fundamental aspects of the origin of dislocation contrast are studied. Experimentally, the potential of ECCI is explored in several dislocation configurations in Interstitial-Free steel (Fe − 1% Si) used as a model material. Full interpretations of dislocation contrast in (g, −g) and its evolution along the Kikuchi band are shown. Furthermore, a dislocation dipole is observed and fully characterized for the first time in an SEM. After the discovery of the channeling phenomenon by Coates in 1967 1 , where the term "channeling" describes the electrons propagation trough the crystal to a higher depth before scattering, Booker et al. suggested that it could be possible to use this phenomenon for imaging defects under the surface of a bulk sample 2. Therefore, an attractive technique called Electron Channeling Contrast Imaging (ECCI) was developed 3, 4. It provides microstructure analyses on bulk samples where crystalline defects can be imaged with a visibility depth of about a hundred nanometers below the surface (the same order of magnitude of the thickness of a Transmission Electron Microscope-TEM-thin foil) 5. Although the contribution of TEM in materials science is incontestable 6 , its requirements have slowed its large-scale exploitation. These constraints have encouraged the adoption of techniques utilizing an SEM, an easier to use electron microscope. For instance, ECCI allows observation of crystalline defects over larger areas and offers the ability to execute diffraction contrast imaging inside SEM with sufficient imaging resolution to analyze individual dislocations (Burgers vector b analysis…) in bulk specimens generally without special preparation. This ability is due to the fact that modern Field Emission Gun (FEG-SEM) microscopes offer high imaging performance 7 due to adapted characteristics such as high beam current mode, small beam convergence (quasi-parallel beam), and very small spot size that lead to high lateral resolution (a few nanometers) with a good signal-to-noise ratio (experimental details are presented in Methods section). Furthermore, accurate control of channeling conditions allows TEM style contrast analysis where the extinction criteria-g•b = 0 and (g•b) × u = 0 with g the diffraction vector, b the Burgers vector and u the dislocation line direction-are still applicable 8 .
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https://hal.univ-lorraine.fr/hal-02392256
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Submitted on : Friday, December 4, 2020 - 7:42:24 AM
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H. Kriaa, A. Guitton, N. Maloufi. Fundamental and experimental aspects of diffraction for characterizing dislocations by electron channeling contrast imaging in scanning electron microscope. Scientific Reports, Nature Publishing Group, 2017, 7 (1), ⟨10.1038/s41598-017-09756-3⟩. ⟨hal-02392256⟩

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