Modeling Dislocation Contrasts Obtained by Accurate-Electron Channeling Contrast Imaging for Characterizing Deformation Mechanisms in Bulk Materials

Abstract

Electron Channeling Contrast Imaging (ECCI) is becoming a powerful tool in Materials Science for characterizing deformation defects. Dislocations observed by ECCI in Scanning Electron Microscope, exhibit several features depending on the crystal orientation relative to the incident beam (white/black line on a dark/bright background). In order to bring new insights concerning these contrasts, we report an original theoretical approach based on the dynamical diffraction theory. Our calculations led, for the first time, to an explicit formulation of the backscattered intensity as a function of various physical and practical parameters governing the experiment. Intensity profiles are modeled for dislocations parallel to the sample surface for different channeling conditions. All theoretical predictions are consistent with experimental results.

Inelastic Scattering in Electron Backscatter Diffraction and Electron Channeling Contrast Imaging

“…Ten phonon configurations were averaged. The ECCI profile shows a left-right asymmetric intensity, consistent with previous experimental and simulated results (Wilkinson & Hirsch, 1997; Zaefferer & Elhami, 2014; Kriaa et al, 2019). The asymmetry is due to the dislocation strain field causing a reversal in lattice plane bending, and therefore electron beam channeling, either side of the dislocation.…”

“…This can be understood by comparing plasmon de-channeling with the change in channeling conditions induced by the dislocation strain field. The change in deviation parameter s d for a reciprocal vector g due to a displacement vector field u is given as follows (Hirsch et al, 1965; Kriaa et al, 2019):…”

“…Instead, the focus is on simulating the dislocation core region, which in real materials is of potential interest due to dissociation into partial dislocations (Balk & Hemker, 2001; Hull & Bacon, 2001; Mendis et al, 2006). The core region is also not accessible in conventional Bloch wave ECCI simulations (Picard et al, 2014; Kriaa et al, 2019), due to the large strain fields and reliance on the column approximation (Hirsch et al, 1965).…”

“…The supercell slice thickness was a o /4 or 1.36 Å. It is computationally expensive to simulate the full, long-range strain field of the dislocation, which can extend to several tens of nanometers (Picard et al, 2014; Kriaa et al, 2019). Instead, the focus is on simulating the dislocation core region, which in real materials is of potential interest due to dissociation into partial dislocations (Balk & Hemker, 2001; Hull & Bacon, 2001; Mendis et al, 2006).…”

“…EBSD provides information on crystal phases, their orientation, grain boundaries, and strain (Zaefferer, 2007; Schwartz et al, 2009), while ECCI is used for imaging crystal defects such as dislocations, stacking faults, and subgrain boundaries (Wilkinson & Hirsch, 1997; Zaefferer & Elhami, 2014). Simulations are often essential for interpreting subtle features in the data, such as ferroelectric domains (Burch et al, 2017), chirality (Winkelmann & Nolze, 2015), and strain (Britton et al, 2010) in EBSD, as well as the contrast due to elastic strain fields in ECCI (Picard et al, 2014; Kriaa et al, 2019). While there have been several attempts at modeling electron backscattering under channeling conditions (Spencer et al, 1972; Dudarev et al, 1995), current Bloch wave-based simulation methods (Winkelmann et al, 2007; Winkelmann, 2008, 2009; Callahan & De Graef, 2013; Picard et al, 2014; Pascal et al, 2018) rely on the observation that electron backscattering is largely incoherent, so that the backscatter signal from any given atom is proportional to the local electron beam intensity.…”