Grain-size effects on the deformation in nanocrystalline multi-principal element alloy

Multi-principal element alloys (MPEAs) continue to garner great interest due to their potentially remarkable mechanical properties, especially at elevated temperatures for key structural and energy applications. Despite extensive literature examining material properties of MPEAs at various compositions, much less is reported about the role of grain size on the mechanical properties. Here, we examine a representative nanocrystalline BCC refractory MPEA and identify a crossover from a Hall-Petch to inverse-Hall-Petch relation. While the considered MPEA predominantly assumes a single-phase BCC lattice, the presence of grain boundaries imparts amorphous distributions that increase with decreasing grain size (i.e., increasing grain boundary volume fraction). Using molecular dynamics simulations, we find that the average flow stress of the MPEA increases with decreasing average grain size, but below a critical grain size of 23.2 nm the average flow stress decreases. This change in the deformation behavior is driven by the transition from dislocation slip to grain-boundary slip as the predominant mechanism. The crossover to inverse-Hall-Petch regime is correlated to dislocation stacking at the grain boundary when dislocation density reaches a maximum.