Data collection methodology
The way EBSD data is collected will vary depending upon the type of analysis desired. There are three methods for data collection (1) collecting individual points (2) collecting a line of points (3) collecting an XY controlled systematic grid of points usually known as a map (single or multiple maps can be collected). EBSD data collection can be integrated with chemical analysis by simultaneously collecting EDS at each analysis point. Performing integrated analysis increases the time required to collect the data at each point but has the advantages of having co-located structural and geochemical data.
Point analysis is most commonly used to determine the orientation of a single crystal, to confirm that the crystal is a single orientation (take multiple points in one crystal and check the orientation does not change), to verify or identify phases or to compare the orientation of multiple crystals (collect a single point in each crystal and compare the orientations). When point analyses are being performed the researcher controls the location of the beam, choosing the sites to be analysed manually. Once the EBSP is generated the software will provide indexing solution(s) which the researcher goes through manually to choose the correct solution, which will then be saved. With this manual data collection method no non-indexed or mis-indexed points will be saved. This data can be presented using either pole figures or orientation distribution functions (ODF).
Line scans collect a line of systematically spaced data points and are usually performed to analyse grain sizes, it is the same principle as the linear intercept method (Thorvaldsen, 1997). The advantages of performing line scans using EBSD, rather than on an optical microscope are that smaller grains can be identified, it is more accurate and you also get orientation information on the grains analysed which may reveal information about internal substructure. Again this data can be presented using either pole figures or ODFs.
Maps are required to answer more complicated questions about issues such as rheology or controlling deformation mechanisms or active slip systems. Maps provide the most information but also take the longest time to collect. Map data can be collected in two ways, either by using low magnification and collecting one large map, or by using higher magnification and collecting multiple smaller maps. The multiple maps can be combined into one map using software called Map Stitcher. The downside of multi-mapping is that occasionally the stitched maps do not fit together exactly and low angle boundaries are artificially produced at the overlapping locations. Multi-mapping tends to be used when a large area needs to be covered at a magnification that does not allow the entire area of interest to fit on one screen. This is common when you have bi-modal grain sizes associated with, for example, recrystallised or porphyritic textures.
As maps are collected automatically the data will inevitably contain non-indexed points (points with no orientation solution generated at grain boundaries, cracks and surface particles) and mis-indexed points (points with an incorrect solution, either the wrong orientation or the wrong phase). Therefore maps will require data processing to decrease/remove any non-indexed points and mis-indexed points. This has to be done carefully so that no artifacts (incorrect data) are produced (this is discussed later in Data Processing). The data acquisition settings for EBSD should be optimized to produce as few non-indexed and mis-indexed points as possible. The percentage of indexed analysis points required for the raw (uncorrected) data to be useable depends on the type of sample being analysed and what the aims of the research are. If the sample is made up of phenocrysts contained within a non-crystalline groundmass and maps are performed to capture multiple phenocrysts, the indexing could be as low as 5-10% but the data is useable as the phenocrysts are completely indexed and all the non-indexed points are located in the groundmass. In a sample which contains several phases or just multiple grains, 100% indexing cannot be achieved due to the grain boundaries. In these situations maps with raw indexed rates of less than 50% (more incorrect solutions than correct solutions) should be used with caution. XY grid collected data can be presented as maps, pole figures or ODFs.
Both line scan and map data collection can be automated by defining the start and end location (in the Flamenco software) but there are two data collection methods used in automated EBSD, stage scan or beam scan. In stage-scan mode the stage mechanically steps between each data point and the incident beam remains stationary (Adams et al., 1993), whereas in beam-scan mode the incident beam is deflected across the sample surface while the stage remains stationary. Currently it is much faster to deflect the beam rather than move the stage when performing line scans or maps. If multiple maps are being collected then the beam will be scanned across the defined area of a single map then the stage will move to the XY starting coordinates of the next map and the mapping process will resume until all maps have been collected. It is also possible to save all of the generated EBSPs so the patterns can be reanalysed offline by hand to check for and correct mis-indexed and non-indexed points. This can be extremely important when analysing challenging materials.