  Multi-principal element alloys (MPEAs), commonly termed as medium- or
high-entropy alloys containing three or more components in high concentrations,
render a tunable chemical short-range order (SRO). Leveraging large-scale
atomistic simulations, we probe the limit of Hall-Petch strengthening and
deformation mechanisms in a model CrCoNi alloy and unravel chemical short-range
ordering effects. It is found that, in the presence of SRO, the maximum
strength is appreciably increased, and the strongest grain size drifts to a
small value. Additionally, the propensity for faulting and deformation
transformation is reduced and accompanied by the intensification of planar slip
and strain localization. We reveal strikingly different deformation
microstructures and dislocation patterns that prominently depend on
crystallographic grain orientation and the number of slip planes activated
during deformation. Grain of single planar slip attains the highest volume
fraction of deformation-induced structure transformation, and grain with double
active slip planes develops the densest dislocation network. These results
advancing the fundamental understanding of deformation mechanisms and
dislocation patterning in MPEAs suggest a mechanistic strategy for tuning
mechanical behavior through simultaneously tailoring grain texture and local
chemical order.
