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Développement et application d’une méthode à haute résolution angulaire pour la mesure des gradients d’orientation et des déformations élastiques par microscopie électronique à balayage

Abstract : Understanding the deformation mechanisms in crystalline materials requires a fine characterization of microstructures. The precise measurement of lattice rotations and elastic strains in the scanning electron microscope is the aim of the so-called high-angular resolution methods. For this purpose, digital image correlation techniques are used in order to register electron diffraction patterns. In this thesis, an original registration approach is proposed. The displacement field across the whole scintillator is modelled by a linear homography. Such a shape function is often met is the field of computer vision to describe projective transformations. The homography between two patterns is measured from a single and large region of interest using a numerically efficient inverse-compositional Gauss-Newton algorithm. It integrates a correction of optical distortions caused by camera lenses and its convergence is ensured by a pre-alignment step of the patterns. The latter relies on global cross-correlation algorithms based on Fourier-Mellin and Fourier transforms. It fairly accounts for rotations up to approximately ten degrees with an accuracy typically between 0.1 and 0.5°. The homography is measured independently from the projection geometry, which is only considered afterwards to analytically deduce the rotations and elastic strains. The proposed method is validated numerically from simulated and optically distorted patterns showing disorientations up to 14° in the presence of elastic strains up to 5×10⁻². The accurate measurement of elastic strains between 1×10⁻⁴ and 2×10⁻³ requires a correction of radial distortion effects, even when the disorientation angle is small. Finally, the method is applied to patterns acquired by means of electron backscatter diffraction (EBSD) and in transmission using the new on-axis transmission Kikuchi diffraction (TKD) configuration. Plastically deformed polycrystalline metals as well as semiconductors are characterized. The method highlights fine details of the microstructure of a quenched and tempered martensitic steel and of an interstitial free steel deformed by 15% in tension, although plastic deformation deteriorates the diffraction contrast. The deformation structures in a nanostructured aluminium obtained by severe plastic deformation are also analysed by coupling the image registration method to the on-axis TKD configuration. This coupling allows a high spatial resolution (3 to 10 nm) and a high angular resolution (0.01 to 0.05°) to be reached simultaneously. Elastic strain maps are obtained at the nanoscale in a SiGe thin foil. The geometrically necessary dislocation densities in a GaN single crystal are mapped with a resolution of about 2.5×10⁻³ µm⁻¹ (i.e. 8×10¹² m⁻²).
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  • HAL Id : tel-03254732, version 1

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Clément Ernould. Développement et application d’une méthode à haute résolution angulaire pour la mesure des gradients d’orientation et des déformations élastiques par microscopie électronique à balayage. Science des matériaux [cond-mat.mtrl-sci]. Université de Lorraine, 2020. Français. ⟨NNT : 2020LORR0225⟩. ⟨tel-03254732⟩

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