Abstract
ACOM/TEM is an automated electron diffraction pattern indexing tool that enables the structure, phase and crystallographic orientation of materials to be routinely determined. The software package, which is part of ACOM/TEM, has substantially evolved over the last fifteen years and has pioneered numerous additional functions with the constant objective of improving its capabilities to make the tremendous amount of information contained in the diffraction patterns easily available to the user. Initially devoted to the analysis of local crystallographic texture, and as an alternative to both X-ray pole figure measurement and EBSD accessories for scanning electron microscopes, it has rapidly proven itself effective to distinguish multiple different phases contained within a given sample, including amorphous phases. Different strategies were developed to bypass the inherent limitations of transmission electron diffraction patterns, such as 180° ambiguities or the complexity of patterns produced from overlapping grains. Post processing algorithms have also been developed to improve the angular resolution and to increase the computing rate. The present paper aims to review some of these facilities. On-going works on 3D reconstruction are also introduced.
Highlights
Automated crystal orientation mapping tools (ACOM) are largely used in electron microscopy to quantitatively analyze grain size and crystallographic textures of materials at micron or sub-micron scales
Because the selection of the best template for a given diffraction pattern does not depend on the normalization of the pattern, this calculation is omitted to speed up the template matching process
Templates are computed in a straightforward manner: the positions of the reflections are determined by a simple geometrical construction that depends on the lattice parameters and the Euler angles, whilst the template intensities are proportional to the square of the structure factor: Fhkl = ∑ f i e2πi(hxi +kyi +lzi )
Summary
Automated crystal orientation mapping tools (ACOM) are largely used in electron microscopy to quantitatively analyze grain size and crystallographic textures of materials at micron or sub-micron scales. Its main features are the use of precession electron diffraction (PED) to improve the quality of the acquired patterns and the pioneering introduction of the template matching strategy for rapid orientation and phase identification. It is composed of four dedicated numerical software programs devoted, respectively, to pattern acquisition, template generation, pattern indexing, and results analysis. This package has significantly evolved over fifteen years by introducing novel functions and user-friendly facilities that increase the possibilities to analyze challenging materials and extract faint information from the diffracting signal. The final section introduces the ongoing works on 3D reconstructions that requires the diffracting signatures of all the overlapping grains to be distinguished
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