Fundamental and experimental aspects of diffraction for characterizing dislocations by electron channeling contrast imaging in scanning electron microscope

Abstract

Nowadays Field Emission Gun-Scanning Electron Microscopes provide detailed crystallographic information with high spatial and angular resolutions, and allow direct observation of crystalline defects, such as dislocations, through an attractive technique called Electron Channeling Contrast Imaging (ECCI). Dislocations play a crucial role in the properties of materials and ECCI has naturally emerged as an adapted tool for characterizing defects in bulk specimen. Nevertheless, fine control of the channeling conditions is absolutely required to get strong dislocation contrast for achieving comprehensive analysis. In this work, experiment-assisted fundamental aspects of the origin of dislocation contrast are studied. Experimentally, the potential of ECCI is explored in several dislocation configurations in Interstitial-Free steel (Fe − 1% Si) used as a model material. Full interpretations of dislocation contrast in (g, −g) and its evolution along the Kikuchi band are shown. Furthermore, a dislocation dipole is observed and fully characterized for the first time in an SEM.

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

“…Therefore, the variation of I BSE given by Equation (8) is in good agreement with the experimental observations already reported in literature [3,7]. …”

“…Note also that the maximum intensity does not coincide with the exact position of the dislocation core ($\mathrm{x}=0\mathrm{nm}$) but it is at $\mathrm{x}\approx \pm 4\mathrm{nm}$: it is displaced from one side of the dislocation position to the other side when changing from $+\mathbf{g}$ to $-\mathbf{g}$. This result is analogous to that obtained in TEM and can be used to characterize a dislocation configuration consisting of two parallel dislocations, such as dipole [3,16]. …”

“…$\mathrm{s}$ is the deviation from the exact Bragg position in the perfect crystal, which can be experimentally measured [3]. The scalar product $\mathrm{boldg}·\frac{sans-serif\partial \mathrm{boldR}}{sans-serif\partial \mathrm{normalz}}{)}_{{\mathrm{z}=\mathrm{z}}_{\mathrm{D}}}$ represents the supplementary deviation ${\mathrm{s}}_{\mathrm{D}}$ due to the variation of the incidence angle between the primary beam and the distorted crystalline planes near the dislocation core.…”

“…Their corresponding experimental ECC micrographs are also shown (Figure 2a’,b’). The g and s vectors are, respectively, determined experimentally through the pseudo-band indexation of the High Resolution Selected Area Channeling Pattern (HR-SACP) assisted by Electron BackScatter Diffraction (EBSD) experiment [2,3]. …”

“…Therefore, any slight local distortion of the crystal lattice, produced for instance by a dislocation, leads to a BSE intensity (I BSE ) modulation, thus generating several contrasts such as a bright line on a dark background [2] or a black line on a bright background [3]. …”