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ORM/OIM

ORM/OIM: Orientation (Imaging) Microscopy
Definition:A (scanning) electron microscopy (→SEM) technique where orientations are measured on a regular grid by electron diffraction and the so-determined data are used to produce orientation or phase maps of the scanned area on the sample.
Explanation:We refer here exclusively to the technique based on electron backscatter diffraction (→EBSD). When the electron beam is scanned over a crystalline sample, an EBSD pattern is obtained for every scan grid position. Using software, the orientation, crystallographic phase, and lattice imperfection can be obtained in a fully automated manner from each patterns. In this way, a large data set is produced that quantitatively represents the scanned area of the sample. These data can subsequently be used to reconstruct the microstructure of this area; for example, by assigning similar colours to points with similar crystal orientation or similar phases, orientation maps are obtained. By determining the orientation change from one position to another, misorientation maps can be produced as well. Besides a large variety of different maps, it is also possible to construct statistical data sets from the sample, such as orientation distributions (textures) displayed in the form of pole figures of different constituents in the mapped area (here, deformed and recrystallized grains in the figure below).

Some characteristics include a lateral resolution of 20 to 100 nm (depending on the sample material), a measurement rate of currently up to hundreds of patterns per second, investigated areas of few µm² to few cm², and angular resolution of 0.5° down to 0.01° with special techniques (→XR-EBSD). The technique can also be extended to a three dimensional technique.
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Diagram:

Some typical results of electron backscatter diffraction-based ORM.
SFB-Link:Orientation microscopy is one of the most comprehensive microstructure characterization methods for bulk materials. It allows, e.g., characterization of grain and phase morphology, many lattice defects, crystallographic texture.
References:Schwartz, A.J. et al: Electron Backscatter Diffraction in Materials Science, 2nd ed., Springer Verlag, Berlin, 2009