Abstract
The field of metallurgy has greatly benefited from the development of electron microscopy over the last two decades. Scanning electron microscopy (SEM) has become a powerful tool for the investigation of nano- and microstructures. This article reviews the complete set of tools for crystallographic analysis in the SEM, i.e., electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), and electron channeling contrast imaging (ECCI). We describe recent relevant developments in electron microscopy, and discuss the state-of-the-art of the techniques and their use for analyses in metallurgy. EBSD orientation measurements provide better angular resolution than spot diffraction in TEM but slightly lower than Kikuchi diffraction in TEM, however, its statistical significance is superior to TEM techniques. Although spatial resolution is slightly lower than in TEM/STEM techniques, EBSD is often a preferred tool for quantitative phase characterization in bulk metals. Moreover, EBSD enables the measurement of lattice strain/rotation at the sub-micron scale, and dislocation density. TKD enables the transmitted electron diffraction analysis of thin-foil specimens. The small interaction volume between the sample and the electron beam enhances considerably the spatial resolution as compared to EBSD, allowing the characterization of ultra-fine-grained metals in the SEM. ECCI is a useful technique to image near-surface lattice defects without the necessity to expose two free surfaces as in TEM. Its relevant contributions to metallography include deformation characterization of metals, including defect visualization, and dislocation density measurements. EBSD and ECCI are mature techniques, still undergoing a continuous expansion in research and industry. Upcoming technical developments in electron sources and optics, as well as detector instrumentation and software, will likely push the border of performance in terms of spatial resolution and acquisition speed. The potential of TKD, combined with EDS, to provide crystallographic, chemical, and morphologic characterizations of nano-structured metals will surely be a valuable asset in metallurgy.
Highlights
The understanding of the relationship between microstructure and macroscopic properties plays a key role in the development and manufacture of materials
This article reviews the complete set of tools for crystallographic analysis in the Scanning electron microscopy (SEM), i.e., electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), and electron channeling contrast imaging (ECCI)
In order to provide a quantitative example, it can be noted that Monte Carlo electron trajectory simulations performed with the software Casino, considering an electron beam in EBSD configuration with a primary electron energy of 20 keV interacting with a Fe substrate, indicate that the backscattering diffraction interaction depth is about 50 nm, whilst the interaction depth for X-rays is about 1 mm.[62,63]
Summary
The understanding of the relationship between microstructure and macroscopic properties plays a key role in the development and manufacture of materials. Quantitative metallography has made significant progress by the discovery of electron backscatter diffraction (EBSD) and the introduction of automated EBSD in the 1980s and early 1990s, and its concurrent commercial availability.[1–6] This technique has allowed fast and robust analysis of bulk polycrystalline materials and quantitative characterization of micro- and macro-texture, orientation correlations between phases, grains, and domains, as well as phase identification and size distributions of constituents. The main difference between standard EBSD and this new technique lies in the sample geometry While the former is performed over bulk materials, the latter is based on the use of electron transparent samples, so that the electrons are transmitted through the sample, reducing the volume of interaction between the electron beam and the specimen.
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