Fig. 1. Crystal slips in a high entropy alloy nanopillar. The nanopillar of ~600 nm in diameter was observed by SEM after being compressed to ~30% strain, showing single slip steps with size from a few to hundred nm.

Fig. 2. Nanopillar geometry and observation directions. (a) Schematic illustration of pillar orientation and observation direction. (b) Selected area electron diffraction pattern recorded from one pillar. The pillar axis is approximately along [647] as determined by EBSD. The primary slip plane is $(1\bar{1}1)$. The observation direction is close to $[1\bar{2}0]$.

Fig. 3. Dislocation activities leading to the formation of a slip band and crystal slip in a HEA nanopillar of ~500 nm in diameter. (a) The measured shear stress is plotted versus time for the nanopillar recorded in (b). (b) Frames 1 to 6 are the difference of two consecutive images (0.1 s apart) at the specified starting time. The white arrows indicate the initial dislocation source position and dislocation pile-up. The yellow arrow in Frame 5 indicates the jump of indenter at the time of nanopillar slip. (Reproduced with permission from Ref. [14]).

Fig. 4. The dislocation microstructure in a HEA nanopillar before deformation. A BF-TEM image of an as-prepared HEA nanopillar, recorded near $[1\bar{2}0]$ zone axis using a small aperture placed around the transmitted beam.

Fig. 5. Dislocation nucleation and pinning during the early stage of deformation. (a) and (b) Two sets of consecutively captured frames and their different images taken at 0.1 s time apart at the early stage of deformation. The bright lines in the two different images mark the same location.

Fig. 6. Dislocation structure in a stabilized slip band. (a)?(c) BF-TEM images of the cross-sectional sample of a nanopillar after compression to a strain of ~30%. The images are recorded at three different two-beam conditions near $[1\bar{1}0]$ zone axis as shown in the inset diffraction patterns. Yellow lines mark the slip band boundary. The images here are rotated with the nanopillar top at the top. The top white regions in (b) and (c) mark the cut off by the camera.

Fig. 7. Weak-beam dark-field TEM imaging of dislocation band. (a) and (b) are obtained using the (002) and $(00\bar{2})$ beams, respectively, under the g-3g and sg > 0 diffraction condition. Yellow lines mark the distances between the observed dislocations.

Fig. 8. Kikuchi patterns near $[1\bar{1}0]$ direction taken from the region (a) in the back and (b) in front of the slip band at the same stage tilt. The red line marks the change of orientation between (a) and (b). The line direction of the slip band is marked by white arrow in (a). The solid and dotted yellow lines mark the $(11\bar{1})$ Kikuchi lines in (a) and (b), respectively.

Fig. 9. Crystal slip via the formation of slip band. Some of the previously proposed dislocation mechanisms are highlighted here, including (a) Seeger's model with groups of dislocations emitted by a source piled-up at obstacles [31], (b) elongated ellipsoidal slip band model of Brown [9], (c) multipole structure. (d) Geometry and (e) model of the slip band in compressed Al0.1CoCrFeNi nanopillar. (a), (b), and (c) are after Ref. [32].

Fig. 10. Ex-situ BF-TEM imaging of the cross-section of a nanopillar deformed to ε = 1%. (a) Image taken using g = (111) where both primary and secondary dislocations are visible, (b) image recorded with g = $(11\bar{1})$ with only secondary dislocations visible. The pillar diameter is 630 nm.