Surface effect on dislocation mobility in multi-principal element alloys

Understanding dislocation mobility is a powerful lever for engineering high-performance alloys because it directly governs mechanical response. Surface-mediated effects are established as decisive regulators of dislocation dynamics, arising from free-surface image forces and locally perturbed stress fields. Yet the mechanisms of surface-affected dislocation motion in multi-principal element alloys (MEAs) remain sparsely explored. Here, atomistic simulations quantify the correlation between edge-dislocation mobility and shear stress in NiCoCr and FeNiCoCr MEAs under periodic boundary condition (PBC) and free-surface boundary condition (FBC). We determine stress–velocity relations and demonstrate a pronounced dependence of mobility on surface state. Under FBCs, intersections with the free surface impose a drag that suppresses mobility and strengthens with increasing velocity. Under PBCs, edge-character mobility partitions into multiple regimes of stable, uniform motion that are separated by bands of forbidden velocity, revealing a non-single-valued dependence of v on τ. Building on these observations, we formulate a dislocation-mobility model that explicitly couples surface effects and recovers the measured trends. This model furnishes a mechanistic basis for predictive theory in NiCoCr and FeNiCoCr MEAs across varied surface states, and it supplies more accurate and reliable inputs to higher-length-scale simulations. The approach clarifies how surface intersections reconfigure local fields to impede glide.