New techniques for imaging and identifying defects in electron microscopy
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Introduction The paradigm underpinning materials science and engineering, that structure controls properties and performance, is determined by the governing role of defects in mediating properties ranging from mechanical strength and damage tolerance1–3 to optoelectronic response4 to phase transformation phenomena.5 Direct experimental characterization of crystalline defects dates back to the beginnings of electron microscopy.6 The past decade especially has witnessed tremendous advances in new electron imaging and diffraction-based modalities for quantifying defects and their corresponding ensembles, interactions with other microstructural features, and dynamics. In the case of extended defects, such as dislocations and planar faults, pioneering developments have taken an orthogonal tack from the race for spatial resolution in electron microscopy, and instead have targeted correlative characterization techniques, temporal resolution to capture dynamics in situ, and statistical quantification of defect evolution and organization. These techniques are required to deploy modern materials for emerging technologies such as additive manufacturing and optoelectronics, and materials used in extreme environments, where a toolbox of materials characterization probes is necessary to advance our understanding of the links between defects and materials properties. This article focuses on methods for identifying and quantifying defects that are amenable to scanning electron microscopy
(SEM) platforms, which when compared with transmission electron microscopy, offers versatility for multimodal and in situ characterization, as well as differences in scattering physics owing to different primary electron-beam energies. We highlight recent advances and trends in defect imaging and characterization using electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI), and diffraction-contrast scanning transmission electron microscopy (STEM) approaches. We conclude by assessing the emerging role of the interplay between multimodal microscopy and data science on our understanding of defect–property relationships in advanced materials.
Defect characterization using EBSD Progressive development of EBSD has increased in sophistication—it presently provides fast automated indexing of electron diffraction patterns in the SEM.7–10 For defect analysis (Figure 1), this has been augmented by recent developments of the high (angular) resolution-EBSD (HR-EBSD) method initially advanced by Wilkinson and colleagues,11,12 which uses direct cross correlation of EBSD patterns, enabling resolution better than 10–4 in (relative, deviatoric) elastic strain and 10–4 rads in (relative) lattice rotation. For metallic structures, recent advances have included pattern remapping of diffraction pattern intensities to reduce complications due to the gnomonic projection,13,14 which has improved the robustness of elastic strain measurements in metals.
Daniel S. Gianola, Materials Department, University of California, Santa Barbara, USA; gi
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