Electron Backscatter Diffraction - Indexing

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.