Impact of in situ nanomechanics on physical metallurgy
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Introduction It is now well established that the mechanical properties of metals and alloys, such as strength, ductility, or toughness, result almost exclusively from the ability of dislocations to multiply and move through the crystalline lattice. Various approaches have been taken to tailor microstructures in order to improve these properties.1–5 Advanced chemistries have historically been the most popular approach to tune dislocation mobility; this is being explored currently for the design of high-entropy alloys.4 Multiplying grain boundaries (GBs), ultimately resulting in nanocrystalline (nc) materials,5 has also become a classical way to increase the yield strength of a material, but often at the expense of ductility. Combining both approaches can circumvent some of the drawbacks,6 which indicates that GB2 and twin7 engineering are probably key to fine-tuning the properties of future alloys. In situ mechanical testing has become an essential technique to investigate the impact of composition and microstructure on material properties, in particular because one can observe the mechanisms at play.8–10 Its versatility is further enhanced by recent quantitative capabilities, such as automated crystal orientation mapping (ACOM), or piezo-/microload
cells-equipped transmission electron microscopy (TEM) holders and scanning electron microscopy (SEM) stages.9–13 Visualization of the deformation of submicron volumes, a key feature of these recent developments, helps to close the gap between experiments and calculations. So does faster mechanical testing,14 which may also help unveil deformation mechanisms via the activation volume analysis15 of variable strain-rate experiments. Among all of the mechanisms studied in situ, those involving GBs are particularly difficult to decipher, despite their major implications for macroscopic properties. In particular, dislocation–GB interactions16–18 and GB-based plasticity where boundaries act as strain carriers,19–21 are critical processes that need both high-resolution imaging and statistical analysis.
Understanding strain accommodation at GBs Initially developed for crystal orientation mapping, quantitative diffraction methods may also be used to calculate geometrically necessary dislocation (GND) densities during deformation of metals by relating the curl of the orientation field to Nye’s dislocation density tensor.7,22–24 This approach was originally pioneered in the electron backscatter diffraction (EBSD)
J. Kacher, Georgia Institute of Technlogy, USA; [email protected] C. Kirchlechner, Max-Planck-Institut für Eisenforschung GmbH, Germany; [email protected] J. Michler, Laboratory for Mechanics of Materials and Nanostructures, Empa—Swiss Federal Laboratories for Materials Science and Technology, Switzerland; [email protected] E. Polatidis, Paul Scherrer Institute, Switzerland; [email protected] R. Schwaiger, Institute for Applied Materials, Karlsruhe Institute of Technology (KIT), Germany; [email protected] H. Van Swygenhoven, École Polytechn
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