CrossCourt4 includes a large selection of elastic constants to match against the material phase(s) of the data under analysis. The elastic constants are needed to evaluate the full strain tensor and to further calculate the stress tensor. The software will attempt to match these constants with the phase described in the source EBSD file but the appropriate set of constants can be selected from the library manually. It is also possible to enter and edit your own values and record them to the library.
Pre-Processing Data Filters
There are times when areas of the imported EBSD data are below an acceptable level of reliability (as indicated by results from maps such as the confidence level, image quality or KAM). In these cases it is possible to identify and remove/isolate these features from the data set before processing. Included for these purposes is a set of filter functions such as the Threshold filter, Grain filter, Pixel filter and Area filter.
The identification of the “zero strain” reference EBSD pattern is at the core of the preparation steps for cross correlation. This feature makes it easy to identify those pattern locations by providing automated tools based on either the KAM, Confidence Index or Pattern Quality data. It is also possible to manually identify the reference pattern and mix and match references between auto and manually selected.
This feature will automatically setup and align regions of interest (ROI’s) over selected EBSD patterns. CrossCourt4 divides the EBSD pattern into multiple regions, some overlapping to pass into the cross correlation process. This determines the average shift of the pattern within each ROI with respect to identically placed ROIs in a reference pattern. The use of multiple regions allows a better statistical sampling.
The Fast Fourier Transform (FFT) has been especially adapted for use with EBSD patterns. It is used to filter low and high frequency information in the image and enhance the signal being passed to the XCF function. The higher frequency components carry information with respect to the sharp changes in the pattern whilst the low frequency components carry information as regards long range contrast changes.
Cross Correlation Function
At the core of the HR EBSD method is cross correlation. The Cross Correlation Function or XCF is a well-established mathematical technique used in many fields from signal processing to astronomy. As used in HR EBSD it provides a comparative measure of the distortion between a test pattern and a reference pattern. Cross correlation is performed over numerous regions of interest (ROI) common to both patterns. These in turn are used to calculate the stress/strain state of the material.
The Remapping function allows for improved measurement of elastic strain in plastically deformed samples. It works by discriminating large lattice rotations from the elastic strains. In particular this is the situation in a plastically deformed metals. Remapping is a method of back rotating the EBSD patterns so that the rotations are ‘removed’ and ‘only’ the elastic stress and strains are left.