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Electron backscatter diffraction (EBSD) is a microstructural-crystallographic characterisation technique to study any crystalline or polycrystalline material. The technique involves understanding the structure, crystal orientation and phase of materials in the Scanning Electron Microscope (SEM). Typically it is used to explore microstructures, revealing texture, defects, grain morphology and deformation. It can be combined with complementary techniques within the SEM for phase discrimination. Traditionally these types of studies have been carried out using X-ray diffraction (XRD), neutron diffraction and/or electron diffraction in a TEM. Experimentally EBSD is conducted using a SEM equipped with an EBSD detector containing at least a phosphor screen, compact lens and low light CCD camera. Commercially available EBSD systems typically come with one of two different CCD cameras: for fast measurements the CCD chip has a native resolution of 640×480 pixels; for slower, and more sensitive measurements, the CCD chip resolution can go up to 1600×1200 pixels. The biggest advantage of the high-resolution detectors is their higher sensitivity and therefore the information within each diffraction pattern can be analysed in more detail. For texture and orientation measurements, the diffraction patterns are binned in order to reduce their size and reduce computational times. Modern EBSD systems can index patterns at up to 1800 patterns / second. This enables very rapid and rich microstructural maps to be generated. For an EBSD measurement a flat/polished crystalline specimen is placed in the SEM chamber at a highly tilted angle (~70° from horizontal) towards the diffraction camera, to increase the contrast in the resultant electron backscatter diffraction pattern. The phosphor screen is located within the specimen chamber of the SEM at an angle off approximately 90° to the pole piece and is coupled to a compact lens which focuses the image from the phosphor screen onto the CCD camera. In this configuration, some of the electrons which enter the sample backscatter and may escape. As these electrons leave the sample, they may exit at the Bragg condition related to the spacing of the periodic atomic lattice planes of the crystalline structure and diffract. These diffracted electrons can escape the material and some will collide and excite the phosphor causing it to fluoresce. Inside the SEM, the electron beam is focussed onto the surface of a crystalline sample. The electrons enter the sample and some may backscatter. Escaping electrons may exit near to the Bragg angle and diffract to form Kikuchi bands which correspond to each of the lattice diffracting crystal planes. If the system geometry is well described, it is possible to relate the bands present in the diffraction pattern to the underlying crystal phase and orientation of the material within the electron interaction volume. Each band can be indexed individually by the Miller indices of the diffracting plane which formed it. In most materials, only three bands/planes which intercept are required to describe a unique solution to the crystal orientation (based upon their interplanar angles) and most commercial systems use look up tables with international crystal data bases to perform indexing. This crystal orientation relates the orientation of each sampled point to a reference crystal orientation. While this 'geometric' description related to the kinematic solution (using the Bragg condition) is very powerful and useful for orientation and texture analysis, it only describes the geometry of the crystalline lattice and ignores many physical processes involved within the diffracting material. To adequately describe finer features within the EBSP, one must use a many beam dynamical model (e.g. the variation in band intensities in an experimental pattern does not fit the kinematic solution related to the structure factor). ==Indexing== Often, the first step in the EBSD process after pattern collection is indexing. This allows for identification of the crystal orientation at the single volume of the sample from where the pattern was collected. With EBSD software, pattern bands are typically detected via a mathematical routine using a modified Hough transform, in which every pixel in Hough space denotes a unique line/band in the EBSP. The Hough transform is used to enable band detection, which are difficult to locate by computer in the original EBSP. Once the band locations have been detected it is possible to relate these locations to the underlying crystal orientation, as angles between bands represent angles between lattice planes. Thus when the position / angles between three bands are known an orientation solution can be determined. In highly symmetric materials, typically more than three bands are utilised to obtain and verify the orientation measurement. There are two leading methods of indexing performed by most commercial EBSD software: triplet voting; and minimising the 'fit' between the experimental pattern and a computationally determined orientation. Triplet voting involves identify multiple 'triplets' associated with different solutions to the crystal orientation; each crystal orientation determined from each triplet receives one vote. Should four bands identify the same crystal orientation then four (four choose three) votes will be cast for that particular solution. Thus the candidate orientation with the highest number of votes will be the most likely solution to the underlying crystal orientation present. The ratio of votes for the solution chosen as compared to the total number of votes describes the confidence in the underlying solution. Care must be taken in interpreting this 'confidence index' as some pseudo-symmetric orientations may result in low confidence for one candidate solution vs. another. Minimising the fit involves starting with all possible orientations for a triplet. More bands are included that reduces the number of candidate orientations. As the number of bands increases, the number of possible orientations converge ultimately to one solution. The 'fit' between the measured orientation and the captured pattern can be determined. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「electron backscatter diffraction」の詳細全文を読む スポンサード リンク
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