Samples for this study were extracted from a previously produced nominally equiatomic CrCoNi MEA whose microstructure and mechanical properties were reported in a recent paper8 (link) where details of its processing and mechanical characterization can be found. Atomic structures were investigated using the aberration-corrected TEAM0.5 transmission electron microscope (operating at 200 kV), housed at the National Center for Electron Microscopy at the Lawrence Berkeley National Laboratory (LBNL), and the in situ compression tests were performed using a Hysitron PI95 nanoindenter in a JEOL 3010 microscope at 300 KV. The nanopillars for the in situ compression tests were produced using focused-ion beam techniques; details of sample preparation and in situ compression have been described in previous studies65 (link)66 (link). The in situ TEM tensile tests were conducted at room temperature using a Gatan model 654 single-tilt straining holder in an FEI Tecnai G2 F20 TEM operating at 200 kV. Roughly 12 samples, thinned by jet polishing and well attached to the substrate, were selected for in situ tensile straining and detailed TEM investigation as described in a previous paper22 (link). Time-resolved TEM and HRTEM images of the regions of interest were recorded with a Gatan CCD camera at 10 frames per second.
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