2-11. Microstructure characterization in 2D and 3D using advanced SEM-based electron diffraction techniques

Stefan Zaefferer
Max-Planck-Institute for Iron Research, Germany

Abstract: The scanning electron microscope (SEM) offers two powerful electron diffraction techniques, namely the well-known and much exploited electron backscatter diffraction (EBSD) and the still less used electron channelling contrast imaging (ECCI). While the EBSD technique yields diffraction patterns, i.e. Fourier-space information about the crystal lattice, the ECCI technique allows direct observation of extended defect structures like dislocations, stacking faults or even elastic strain fields. These two techniques, therefore, complement each other as well as diffraction and imaging mode in classical TEM. For example, EBSD is used to determine the diffraction conditions that are used for well-controlled ECC observations; another example is the use of cross correlation (CC) EBSD to quantify stress fields and geometrically necessary dislocation (GND) densities while ECCI yields the local distribution of dislocations and the total dislocation density including GNDs and non-GNDs in these stress fields.
      Both, EBSD-based orientation microscopy and ECCI are originally 2-dimensional (2D) techniques, which reveal details of the microstructure from a rather thin layer at the surface of the sample. Nevertheless, both techniques can be extended to 3D techniques by combining them with some sort of serial sectioning. Since a long while, this has been done by removing thin layers from the surface using gracing-incidence focussed ion beam (FIB) sputtering. This technique, however, leads to rather small observation volumes and is prone to significant beam damage in some cases. Therefore, mechanical-chemical polishing appears to be a very powerful alternative as it allows removal of very large surface areas under perfect conservation of the crystal lattice.

      In our lecture the combination of EBSD and ECCI in 2D and 3D will be illustrated by different examples: (i) in-situ observation of dislocation structure evolution during fatigue loading of TWIP steels, (ii) dislocation observations in steels loaded with hydrogen, (iii) residual stress measurements using CC EBSD in an additive manufactured sample of stainless steel, and (iv) on 3D observations of grain boundaries and grain boundary corrosion using fully automated large-area mechanical-chemical serial sectioning of a stainless steel.