The electron backscatter diffraction technique – A powerful tool to study microstructures by SEM

Abstract

In the last 10 years the electron backscatter diffraction (EBSD) technique has developed into a powerful tool for the crystallographic analysis of materials in the scanning electron microscope. In particular the emergence of com- puter algorithms for the fully automated analysis of diffraction patterns has pushed the technique to develop into a new kind of scanning microscopy technique, known as “orientation imaging microscopy, OIM1” or “automatic crystal orientation mapping, ACOM”. The ACOM technique is based on the consecutive acquisition of electron dif- fraction patterns obtained from every point of a scan grid on a flat surface of a steeply inclined sample in the SEM. The automatic analysis of these EBSD patterns yields for every scan point the crystallographic orientation and phase and a value indicating the quality of the diffraction pattern. From these data the microstructure of the scanned area can be reconstructed. The resulting crystal orientation maps give a vast amount of information on the sample, including kind and distribution of different phases, size, form and defect condition of grains, kind and posi- tion of grain boundaries, local crystal orientation and misorientation distribution (texture) and others more. Furthermore, EBSD can be used to investigate the structure of crystals, i.e. lattice symmetry and lattice parameters and may therefore take over a large amount of crystallographic work that was performed before by TEM. The EBSD technique usually requires less complicated sample preparation and allows the observation of much larger areas, yet with a quite high spatial resolution. Although the EBSD technique can be applied with good success on a standard tungsten filament SEM it profits enormously - in terms of spatial resolution and orientation accuracy - from using a thermal field emission gun instrument with a high beam current. The present text briefly describes some of the physical fundamentals and the practical set-up of the technique before it presents a couple of application exam- ples that illustrate some of its possibilities. All examples stem from problems of physical metallurgy – the author's field of expertise – although the technique is also intensively applied to the study of non-metallic, crystalline materi- als like semiconductors, ceramics and minerals