In this study, we investigate the repeated local pinning along subsequent dislocations in the face-centered cubic (FCC) CoCrFeMnNi HEA by combining in-situ TEM deformation, atomic resolution analytical scanning transmission electron microscopy (STEM), and atomistic simulations. This is opposite to the strong-pinning model from Fleischer 23 where each solute corresponds to a site where the dislocation line is pinned. Here, the dislocation line interacts with the whole field of solutes within its elastic interaction range at the same time. A Labusch-type weak interaction 22 between solute atoms and dislocations is assumed in Varvenne’s model. The dominating mechanism of solid solution strengthening in HEAs is intricate as it becomes impossible to define distinct matrix and solute atoms. 21 predicts the flow stresses in an HEA solely based on misfit volumes and elastic properties. Lastly, the solid solution strengthening model developed by Varvenne et al. Such a mechanism would only exist in HEAs, but not in conventional dilute solid solutions as their local fluctuations in the SF energy remain comparatively small. Therefore, dislocation segments need to unzip from the local barriers while gliding. 7 argued that the wide range of SFE causes peaks in the energy landscape where lattice friction becomes enhanced locally. The resulting wavy dislocation lines have been observed for different HEAs in transmission electron microscopy (TEM) experiments 16, 17, 18 and atomistic simulations 7, 19, 20. The unexpected wide range of stacking fault (SF) energies observed for HEAs could provide an alternative explanation for the pinning experienced by dislocations 15, 16. 7 reported that even in a perfectly random CoCrNi alloy some preferential atomic arrangements, in the case of Co and Cr, exist, and breaking these randomly appearing favorable bonds requires additional energy. Indeed, dislocation pinning and the resulting strengthening are also observed in ideally random alloys 2. Yin and Curtin 8, on the other hand, showed that the extraordinary strength of CoCrNi 2 can also be explained based on solid solution strengthening without resorting to SRO. Since trains of dislocations traveling on the same plane were observed in this system, it is argued that the first dislocation gliding on a plane destroys SRO within that plane and facilitates the glide of subsequent dislocations 14. This strengthening mechanism has been reported for the CoCrNi, a subsystem of the Cantor (CoCrFeMnNi) alloy 7, 9, 13. Short-range ordering (SRO), i.e., a preferred ordering in the atomic-sized neighborhood, is well known to lead to a strength increase in conventional alloys 12. There are, however, four different origins explored in the literature to date: The origin of the high strength of HEAs is key for materials design but is still controversially discussed in the community 7, 8, 9, 10, 11. Several HEAs outrival the mechanical properties of conventional alloys, some possess exclusive property combinations such as high strength and high ductility-even down to cryogenic temperatures 2, 3, 4, 5, 6. Since entropy is not always the decisive design parameter, they are a subclass of concentrated random alloys. High- and medium-entropy alloys (HEAs and MEAs) are a new class of metallic materials that contain multiple elements at high concentrations and form solid solutions 1 in contrast to conventional alloys that typically consist of a single principal element with low concentrations of secondary elements.
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