Electron backscatter diffraction

Indexing

The first step in the EBSD process after pattern collection, described above, is indexing. This allows for identification of the microstructure or orientation at the single point on the sample from where the pattern was collected. With EBSD software, pattern bands are detected via a mathematical routine called the Hough transform in which every pixel denotes a unique line in the EBSD pattern. Because angles between bands represent angles between lattice planes, when the position / angles between three bands are known an orientation solution can be determined.

A common standard for determining orientation from pattern bands is a triplet voting scheme. With this routine, the system will automatically calculate all orientations for all possible three band combinations (triplets) and count the number of votes for each candidate orientation. One orientation will have the highest number of votes and this is typically the correct solution. This method works very well for deformed materials and at overlapping patterns where triplets belonging to the stronger pattern determine the orientation solution. It can also be used as a basis for confidence index which is a measure of the difference in votes between the best and second best orientation solution (how well the solution matches).

An alternate routine is to start with all possibilities from three bands and converge to a single solution by adding bands as needed.

Orientation Mapping

A further function of EBSD is mapping of information collected from a prescribed grid of points on the sample. A sample area is designated and collection parameters selected. The system will then automatically raster the electron beam accordingly, and at each point the indexed orientation information is stored into a dataset. From this dataset numerous maps, charts and plots can be generated [1] . Some of these include grain orientation maps, grain boundary maps, image quality maps, grain size charts, misorientation charts and texture plots.

Integrated EBSD/EDS Mapping

When simultaneous EDS/EBSD collection can be achieved, the capabilities of both techniques can be enhanced. There are applications where sample chemistry or phase cannot be differentiated via EDS alone because of similar composition; and structure cannot be solved with EBSD alone because of ambiguous structure solutions. To accomplish integrated mapping, analysis area is scanned and at each point Hough peaks and EDS region-of-interest counts are stored. Positions of phases are determined in X-ray maps and measured EDS intensities are given in charts for each element. For each phase the chemical intensity ranges are set to select the grains. All patterns are then re-indexed off-line. The recorded chemistry determines which phase / crystal structure file is used for indexing of each point. Each pattern is indexed by only one phase and maps displaying clearly distinguished phases are generated.

EBSD when used together with other in-SEM techniques such as calcium carbonate (CaCO₃) therefore EDS/WDS cannot tell them apart, but they have different microcrystalline structures so EBSD can differentiate between them.

See also

• Crystallography
• Electron crystallography
• Diffraction