Measuring elastic strains and orientation gradients by scanning electron microscopy: Conventional and emerging methods

Understanding the deformation mechanisms in crystalline materials requires a fine characterization of microstructures. The precise measurement of lattice rotations and elastic strains in the scanning electron microscope (SEM) is the aim of the so-called high-angular resolution technique. It is known as HR-EBSD or HR-TKD, depending on whether it is applied to diffraction patterns acquired using the electron backscatter diffraction (EBSD) technique or the transmission Kikuchi diffraction (TKD) technique. Nevertheless, principle remains the same. The displacement field between a pair of electron diffraction patterns is measured with a subpixel resolution by means of digital image correlation algorithms. Knowledge of the SEM projection geometry allows the displacement field to be interpreted in terms of lattice rotations and elastic strains. This series of five chapters proposes a novel HR-EBSD/TKD approach based on a linear homography, namely a geometric transformation often met in computer vision to describe projections. The displacement field is assessed using global digital image correlation. A unique and large region of interest is considered, in opposition to the original HR-EBSD/TKD approach, which involves local translation measurement from small square subsets. The latter method is described in this first chapter, which is a literature review of crystallographic orientation mapping and high-angular resolution techniques in the SEM.