In situ TEM nanomechanics
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Introduction The mechanical properties of any material are highly correlated with its defect structure. Starting in the late 1950s,1 in situ straining stages for use in transmission electron microscopes that could provide dynamic observations of dislocation motion in metals were developed. In the following years, research teams fabricated various transmission electron microscopy (TEM) straining holders, including some that could operate at low or high temperature. Since then, in situ TEM straining tests have been performed to study the behavior of dislocations and their interactions with other defects such as twins, grain boundaries (GBs), and interfaces. After early load sensor integration developments,2,3 in situ TEM mechanical testing with quantitative load measurement expanded toward commercialization in the beginning of this century, including mechanical probing modes such as nanoindentation.4 These developments are particularly useful for studying size effects in materials, whereby mechanical properties and microstructure can be tuned by the physical dimensions (internal such as grain size or external such as film thickness) of materials.5–7 At the micrometer or nanometer scale, the sample dimensions can be of the same order as the defects that carry the deformation, such as the length of a dislocation line or the thickness or spacing of twins. When this occurs, strong size effects can change the deformation
mechanisms and the corresponding mechanical properties. Quantitative in situ TEM mechanical testing is therefore an ideal tool for studying the origin and principles of these size effects. This review provides an overview of available in situ TEM techniques and recent studies performed on dislocation behavior, dislocation/boundary interactions, deformation twinning, and GB-mediated plasticity. We also provide a critical discussion of the effects of size on different deformation mechanisms and the correlated mechanical properties of materials. Some of these mechanisms are summarized in Figure 1.
In situ TEM straining techniques Various tools for mechanical testing by a transmission electron microscope that exist today8 are briefly detailed in this section. They span from simple motorized grips to elaborate mechanical testing units capable of returning values for load and displacement. In classical tensile holders for TEM, samples are attached to a pair of jaws, and one side is slowly pulled by an electric motor located in a housing unit outside the TEM instrument. A rigid shaft ensures strain transmission at speeds ranging from 10 nm/s to 10 µm/s. This simple technique9 has various advantages because of its high versatility. Virtually any thin or thinned sample can be strained as soon as it is attached to
Q. Yu, Center of Electron Microscopy and State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, China; [email protected] M. Legros, Centre d’Elaboration des Matériaux et d’Etudes Structurales, Centre National de la Recherche Scientifique, France; ma
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