Grain-Size Effects on the Deformation in Nanocrystalline Multi-Principal Element Alloy

Multi-principal element alloys (MPEAs) continue to garner interest due to their remarkable mechanical properties, especially at elevated temperatures. Here, we examine a representative nanocrystalline 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 phase distributions that increase with decreasing grain size (i.e., increasing grain boundary volume fraction). Using molecular dynamics simulations, we find that the yield strength of the MPEA increases with decreasing average grain size, but below a critical grain size < 23.2 nm the yield strength decreases. This change in the deformation behavior is driven by the transition from dislocation slip to grain-boundary slip as the predominant mechanism. Our results reveal that the change from Hall-Petch to inverse-Hall-Petch regime is correlated to dislocation stacking at the grain boundary when dislocation density reaches a maximum.