. Fig. 29.27  TKD phase and orientation maps acquired from the thin sample shown in .  Fig. 29.18. The data was acquired at a beam voltage of 30 kV with a step size of 4 nm. This data was collected using an on-axis TKD detector, as shown in .  Fig. 29.13a. a Map with austenite

colored red and ferrite green. b Orientation map of both the austenite and ferrite phases. Note the fine-scale twinning that occurs in the austenite precipitates

texture of the samples and an understanding of the fine details present in the microstructure were extremely useful in determining the cooling history and the microstructural development in this meteorite. It should also not be lost on the reader that EBSD and the related TKD span a huge range of length scales from collecting large mm sized regions to using extremely small step sizes to elucidate the nm scaledetails of the microstructure.

29.2.11\ Summary

EBSD and the related technique of TKD are an important part of crystalline materials characterization in the SEM. With EBSD the SEM can now be considered a complete materials characterization tool that can not only take excellent quality images of samples but can determine the elemental constituents of the sample as well as the detailed crystallography.

29.2.12\ Electron Backscatter Diffraction

Checklist

Specimen Considerations

EBSD requires a properly prepared sample that is securely attached to an appropriate support as the sample will be tilted to high angles to facilitate EBSD pattern acquisition. The sample surface should be free of artifacts due to sample preparation. Due to the high sample tilt and the generally long acquisition times required for EBSD, carbon tape is not a good choice as it tends to creep allowing the sample to move causing drift related image issues. Samples mounted in epoxy metallographic mounting materials are subject to drift caused

by charging of the polymer material. It is satisfactory to lightly conductively coat the specimen for EBSD analysis provided the coating is kept as thin as possible while remaining adequate for charge removal.

Non-conductive samples may also be lightly coated for analysis as discussed above. It is also possible to utilize the variable pressure mode of operation to reduce charging related artifacts. One must carefully choose the correct pressure to be used as too high of a pressure will result in blurred patterns due to scattering in the gas and too low of a pressure may not control the charging.

Proper positioning of the specimen within the SEM is critical. One must remember that the sample will be tilted to a high angle for analysis. The high tilt required for EBSD will limit how tall the sample is and how it must be mounted on the SEM sample stage. A sample that is too short may also present difficulties in positioning the sample in the proper location.

EBSD Detector

Due to the limited space in most SEM sample chambers, it is best to position the sample so that the sample is tilted appropriately and the area to be analyzed is in the field of view. Once the sample position has been established the EBSD detector should be introduced. The exact location of the detector is not extremely important, but the sample to detector distance should be sufficiently short so that a large solid angle can be obtained and the brightest part of the pattern located in the upper half of the detector image. It is very important to keep in mind the position of the sample and the detector as there are two kinds of people who use EBSD: Those who have hit the EBSD detector and those who will. So be careful when moving the sample and the detector. A chamberscope is absolutely required for safe EBSD operation.