BS ISO 13067:2011 pdf free download – Microbeam analysis – – Electron backscatter diffraction – Measurement of average grain size.
5.5 Preliminary examination
A initial examination of the specimen shall be made to identify an initial set of operating parameters needed to map the orientation of the specimen with an acceptable level of accuracy and within an acceptable period of time over an area sufficient to give data on a statistically significant number of grains.
The reader is referred to ISO 241 73for information needed to measure the orientation.
5.6 Choice of step size
5.6.1 1 the grain size and shape are not known already ,an approximate grain size and shape estimation shall be performed by a quick imaging technique. Ai optical microscope might work on a region with only slight polishing relief or on an etched region adjacent to that to be examined by EBSD. Fbrescatter [10]or electron – channelling contrast imaging using diodes mounted on the EBSDdetector ,or imaging with the specimen current, can also produce images relatively quickly.
As an alternative to mapping ,some EBSDsoftware offers a line intercept method as a mapping mode .This can be used to quickly give an approximate grain size measurement.
5.6.2 The step size should be chosen in relation to the average grain size ,unless information on a particular minimum size is required. h either case ,it has to be recognized that a judgement is being made on the minimum number of pixels that are used to define a grain either by a lineal or areal method. e also 6.3and Figured), e )and f )for the effects of step size choice.
Asimple rule that can be applied to a preliminary scan is that the step size should be less than 10 %of the approximate mean grain size [2].To confirm the validity of the chosen step size ,repeat the mapping of a single area at several step sizes and determine the maximum size below which no significant difference in average grain size is determined .This choice has a direct influence on the accuracy of the grain size measurement.
5.6.3 h choosing the step size ,the spatial resolution of the system needs to be considered .The step size is preferably larger than the interaction volume ,which will be determined both by the material examined and the operating parameters of the SEM,such as the filament type ,accelerating voltage and aperture size.
5.7 Determination of the level of angular accuracy needed [7][8]
The speed with which EBSDpatterns are acquired (ncluding any averaging of patterns )affects the precision with which band edges can be detected and thus the angular accuracy of the calculated orientation. Oher factors 2such as the 1-bugh resolution and the number of bands chosen to match the calculated orientation, also affect the calculation time as well as the angular accuracy.
I too long a time is taken for acquisition and calculation ,problems of specimen drift can be increased significantly and fewer points will be acquired in a given time ,reducing the statistical significance of the data acquired To minimize drift ,it is recommended that the specimen have a good earth round )path and be securely fastened to the stage . A’oid carbon tabs. Athin carbon coating might also be necessary for insulating specimens.
I the time taken is too short ,then levels of indexing reliability will be reduced .The settings chosen as a compromise between the two opposing factors above shall be recorded.
To save time, EBSDpatterns may be saved without indexing during mapping and subsequently indexed off 4ine to investigate the effect of some of the above parameters on indexing accuracy.