Electron backscatter diffraction

Electron backscatter diffraction (EBSD) is a Scanning Electron Microscope based microstructural-crystallographic characterization technique commonly used in the study of crystalline or polycrystalline materials. The technique can provide information about the structure, crystal orientation, phase or strain in the material. Traditionally these types of studies have been carried out using X-ray diffraction (XRD), neutron diffraction and/or electron diffraction in a Transmission electron microscope. Electron backscatter diffraction (EBSD) is a Scanning Electron Microscope based microstructural-crystallographic characterization technique commonly used in the study of crystalline or polycrystalline materials. The technique can provide information about the structure, crystal orientation, phase or strain in the material. Traditionally these types of studies have been carried out using X-ray diffraction (XRD), neutron diffraction and/or electron diffraction in a Transmission electron microscope. 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 electron beam scattering pattern (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).