  High-entropy alloys (HEAs) were presumed to have a configurational entropy as
high as that of an ideally mixed solid solution (SS) of multiple elements in
near-equal proportions. However, enthalpic interactions inevitably render such
chemically disordered SSs rare and metastable, except at very high
temperatures. Here we highlight a structural feature that sets these
concentrated SSs apart from traditional solvent-solute ones: the HEAs possess a
wide variety of (local) chemical ordering (LCO). Our atomistic simulations
employing an empirical interatomic potential for NiCoCr reveal that the LCO of
the multi-principal-element SS changes conspicuously with alloy processing
conditions, producing a wide range of generalized planar fault energy in terms
of both its sample-average and spatial variation. We further demonstrate that
the LCO heightens the ruggedness of the energy landscape and raises activation
barriers governing dislocation activities. This not only influences the
selection of dislocation pathways in slip, faulting, twinning, and martensitic
transformation, but also increases the lattice friction to dislocation motion
via a new mechanism of nanoscale segment detrapping that elevates the
mechanical strength. All these open a vast playground not accessible to
ground-state SSs or intermetallics, offering rich opportunities to tune
properties.
