Case studies on the application of high-resolution electron channelling contrast imaging – investigation of defects and defect arrangements in metallic materials

Abstract

In 1967, Coates discovered the electron channelling contrast of backscattered electrons (BSEs) in scanning electron microscopy, and by this the possibility to investigate arrangements of lattice defects in deformed microstructures of materials. Since that time, a straightforward development of the scanning electron microscopes as well as of the electron channelling contrast technique took place. Nowadays, the performance of scanning electron microscopes is high enough that the resolution of electron channelling contrast imaging (ECCI) micrographs is comparable with conventional bright field transmission electron microscopy (TEM) micrographs. In the first part of the present paper, a historical review on the development of the ECCI technique starting from its discovery more than 45 years ago up to the combination with other advanced methods of scanning electron microscopy like electron backscatter diffraction or high-resolution selected area channelling patterning in the last few years is given. Major important investigations using this technique for the visualization of individual lattice defects like stacking faults (SFs) and dislocations or dislocation arrangements are chronologically summarized. The second part demonstrates that nowadays, ECCI micrographs taken in high-resolution scanning electron microscopes can be called high-resolution ECCI (HR-ECCI). It is shown that the resolution of individual SFs and dislocations in the HR-ECCI micrographs is comparable to that of conventional TEM (about 15 nm defect image width). Furthermore, the paper is demonstrating that HR-ECCI micrographs can be obtained for various types of materials after different mechanical loadings and different grain sizes ranging from large grain size of 500 μm (cast steel) down to less than 2 μm (γ-TiAl).

Recent Developments of Crystallographic Analysis Methods in the Scanning Electron Microscope for Applications in Metallurgy

“…Since the overall BSE signal is lower in the low-tilt configuration, in order to ensure adequate performance, it is necessary to operate a beam with small convergence angle and sufficient probe current, and to use a BSE detector with high collection efficiency. 25 This new configuration provided dislocation contrast characteristics comparable to those obtained by the high-tilt configuration. It is known that operational parameters can be adjusted to optimize BSE contrast.…”

“…At the beginning of the 1990s, for instance, ECCI was conducted with specimen tilt of »45 with respect to the incident electron beam, with a side-mounted BSE detector, as shown in Figure 2c under high-tilt configuration. 25 On the other hand, a low-tilt configuration, schematically described in Figure 2c, was used from the beginning of the 1980s, 133 e.g., for applications in geosciences, 135 and later by Simkin et al (1999). Sample tilting angles below 10 enabled the use of general-purpose pole piece-mounted backscatter detectors instead of specialized forward-scattered electron detectors, allowing access to more advantageous ECC imaging conditions.…”

“…ECCI enables the analysis of large areas with good statistics, and facilitates easier implementation of in-situ studies compared to TEM. 25 An additional benefit of ECCI over TEM with regards to sample geometry is that in ECCI only one free surface needs to be exposed, as opposed to the two free surfaces required by TEM, which may cause significant relaxation of stresses.…”

“…ECCI has been available for 50 years, but is currently being applied in larger extent in metallurgy due to recent technical developments that have simplified the analysis. [23][24][25] Moreover, the improved SEM performance together with the operation of ECCI in controlled diffraction condition, i.e., in combination with EBSD, has enabled high-resolution studies of nano-scale structures. 26,27 The application of EBSD, TKD, and ECCI for the characterization of metallic microstructures is likely to continue expanding.…”

Accurate electron channeling contrast analysis of a low angle sub-grain boundary

“…Electron channeling contrast imaging (ECCI) is a powerful technique for observing and characterizing crystallographic defect such as dislocations, stacking faults, and grain boundaries in scanning electron microscopy (SEM) [17][18][19][20][21][22][23]. In order to detect defects, ECCI uses the fact that the backscattered electron yield is very sensitive to the angle between the incident beam and the crystal lattice.…”