  Refractory multi-principal element alloys (MPEAs) have exceptional mechanical
properties, including high strength-to-weight ratio and fracture toughness, at
high temperatures. Here, we elucidate the complex interplay between
segregation, short range order and strengthening in the NbMoTaW MPEA through
atomistic simulations with a highly accurate machine learning interatomic
potential. In the single crystal MPEA, we find greatly reduced anisotropy in
the critically resolved shear stress between screw and edge dislocations
compared to the elemental metals. In the polycrystalline MPEA, we demonstrate
that thermodynamically-driven Nb segregation to the grain boundaries (GBs) and
W enrichment within the grains intensifies the observed short range order
(SRO). The increased GB stability due to Nb enrichment reduces the von Mises
strain, resulting in higher strength than a random solid-solution MPEA. These
results highlight the need to simultaneously tune GB composition and bulk SRO
to tailor the mechanical properties of MPEAs.
