Fig. 1. Comparison of intergranular and intragranular atomic structures between elemental metals and high-entropy alloys.

Fig. 2. X-ray diffraction patterns of the CrMnFeCoNi HEA.

Fig. 3. EBSD and cross-sectional TEM images of CoCrFeMnNi alloys with different grain sizes show the microstructural evolution of (a) ～30 μm (cg), (b) ～15 μm (fg), (c) ～1 μm (ufg) and (d-f) ～7 nm (nc) samples.

Fig. 4. Composition maps by SEM-EDX for the five elements that comprise the (a) cg, (b) fg, (c) ufg and (d) nc CoCrFeMnNi HEAs.

Fig. 5. (a) Hardness plotted as a function of loading strain rate for the HEA alloys with different grain sizes and (b) hardness with different grain sizes of FCC HEAs from the present results and literature [13,26,73].

Fig. 6. Comparing the trend lines of (a) strain rate sensitivity m and (b) activation volume v* changed with grain size between the present FCC HEA alloys and conventional FCC Cu gathered by Chen et al. [10].

Fig. 7. (a) Strain rate sensitivity m and (b) activation volume values v* with different grain sizes of FCC HEAs from the present results and literature [13,26,41,47,73,74]. The dotted black line represents the trend derived from the present HEA samples, while the dotted red line describes the trend considering all the data involving both the HEA herein and those reported in the literature. Then, the gap magnified in the inset of (a) demonstrate that whatever the dominant mechanism is, that should not be GB-self deformation mechanisms, as sufficiently relaxed GB in the present nc HEA sample should possess lower GB dislocation density and effectively released local residual stress that is favorable for GB sliding, GB rotation corresponding to higher SRS index m.

Fig. 8. Schematic illustrations showing distinct grain size dependent strain rate sensitivities that correlated with various deformation-carries for both conventional FCC metals and FCC HEAs. For conventional FCC metals, a mechanism transition occurs from dislocation-mediated to GB-mediated processes as grain size reduces, resulting in increased SRS index at reduced d. For HEA FCC metals, the wary morphology of dislocations leads to lower dislocation velocity and therefore more dislocations (corresponding to higher dislocation density) for a given strain rate, results in higher hardness and smaller SRS index in coarse grained regime, and the identical dislocation processes extended to even nanoscale grained regime, where the probability of a dislocation absorbed by GB, Pdis, play a crucial role that leads to unusual small SRS index. As grain size reduced, the wary dislocation might be smoothed because of the increased intrinsic stress, especially in nanoscale grained HEA metals. If it is true, the smoothing process may also affect the SRS index and need to be further explored.