Atomic origin of sluggish diffusion in NbMoTaW multi-principal element alloys

The sluggish diffusion in multi-principal element alloys (MPEAs) remains debated, particularly regarding its underlying atomic mechanism. This study investigates vacancy-mediated diffusion in NbMoTaW MPEA using a machine-learned interatomic potential, enabling direct atomic-scale simulations of vacancy behavior. Molecular dynamics simulations reveal that vacancy diffusion of NbMoTaW in bulk is significantly slower than its constituent elements, manifesting as sluggish diffusion. This sluggishness originates from the rugged energy landscape formed by local chemical complexity, where we observe that vacancy is confined in low-energy traps constituted by surrounding atoms—ultimately resulting in an anomalously high rate of backward jumps and a significant reduction in the diffusion coefficient. In contrast, grain boundary (GB) diffusion does not exhibit sluggish behavior, as the inherent structural disorder of GBs flattens the rugged energy landscape induced by chemical complexity. This work identifies vacancy trapping as an important mechanism for sluggish bulk diffusion, while grain boundary diffusion is mainly governed by structural factors, providing crucial atomistic insights for alloy design.