Three-Dimensional Reconstruction of Planar Deformation Features from Single Electron Micrographs
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INTRODUCTION
MULTIPLE electron microscopy techniques allow the visualization of plastic deformation features such as dislocations, stacking faults, and twins. However, the resulting micrographs are two-dimensional (2D) projections of the underlying structures. Information regarding the depth of these features can be inferred within a transmission electron microscope (TEM), but the true three-dimensional (3D) geometry remains hidden due to the nature of the technique. Elaborate methodologies have been built to recreate the real shape of the deformation features, motivated by the need to understand the complex deformation mechanisms taking place in crystalline materials. However, these tools are typically time-consuming and labor-intensive. The current study aims to minimize the time and work required to obtain a realistic model of the region imaged for the case of fcc crystals where the deformation features are planar in nature. Some of the 3D reconstruction techniques from the literature are discussed first to offer a wider perspective of the alternatives. The scanning electron microscope (SEM) serial sectioning method uses electron channeling
F.D. LEO´N-CA´ZARES, C. KIENL, and C.M.F. RAE are with the Department of Materials Science & Metallurgy, University of Cambridge, 27 Charles Babbage Rd, Cambridge, CB3 0FS, UK. Contact e-mail: [email protected] Manuscript submitted August 29, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
contrast imaging (ECCI) at a surface of the sample to observe dislocations with a backscattered detector. Alternating focused ion beam to remove a layer of material with imaging of the sample results in sequential planar datasets that can be then combined to form a 3D model.[1] This technique appears to be scalable for analyses at the micrometer length scale, although being a labor-intensive destructive technique limits its applicability. The same disadvantage goes to atom probe tomography (APT), a chemistry-sensitive technique with atomic resolution in which the locations of up to 60 pct of the atoms in a thin needle of material are traced as it is evaporated.[2] Electron tomography is a well established technique in which tenths or hundreds of TEM images manually focused and with a similar reciprocal-lattice g-vector are taken at short tilt intervals.[3–5] The variation in contrast for the different orientations is corrected via image processing before the data acquired are merged via a sequentially iterated reconstruction technique. The resolution obtained depends on the sample characteristics and magnification used, and it may go up to atomic resolution for small volumes (~ 2 nm).[6] Nonetheless, heavy computational requirements, long imaging times, and good sample conditions are drawbacks of the technique. Ferromagnetism in steel samples is an additional limitation, although improved sample preparation and imaging techniques have circumvented this problematic.[7] The use of scanning TEM (STEM) and automated image acquisition and processing have significantly improved electron tomography,
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