Strengthening in multi-principal element alloys with local-chemical-order roughened dislocation pathways

Abstract: 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.