Grain-size-driven competition between deformation carriers and Hall–Petch reversal in nanocrystalline Nb₄₅Ta₂₅Ti₁₅Hf₁₅ refractory medium-entropy alloy

The grain-size-driven competition between intragranular dislocation storage and grain-boundary-mediated plasticity underlying Hall–Petch reversal remains unclear in chemically complex refractory medium-entropy alloys. Molecular dynamics simulations were used to examine the tensile response and local plasticity of nanocrystalline Nb₄₅Ta₂₅Ti₁₅Hf₁₅ with grain sizes of 7.4–27.1 nm at 300 K. Three independent Voronoi realizations were examined for each grain size in the peak region. The mean flow stress increased from 2.02 GPa at 27.1 nm to 2.86 ± 0.02 GPa as the grain size decreased from 27.1 to 20.5 nm, then declined to 2.21 GPa at 7.4 nm; segmented fitting placed the transition at an apparent, condition-specific crossover size of 20.9 nm (95% confidence interval 20.6–21.3 nm). ½⟨111⟩ dislocations dominated at all grain sizes. At ~20% strain, the total dislocation density decreased from 3.11 × 10¹⁶ m⁻² at 23.5 nm to 1.62 × 10¹⁶ m⁻² at 7.4 nm, showing that refinement retained dislocation activity but suppressed sustained propagation, storage, and network formation. Concurrently, the high-shear-atom fraction increased from 53.3% to 59.9%, whereas the top-10% shear contribution decreased from 34.2% to 30.9%, revealing a transition from localized grain-boundary hotspots and intragranular slip bands to distributed grain-boundary-mediated deformation. Intragranular Warren–Cowley parameters remained mainly between −0.021 and 0.019 and showed no evidence of pronounced deformation-induced chemical short-range order. Thus, Hall–Petch reversal arises primarily from diminished sustained intragranular dislocation storage and enhanced distributed grain-boundary plasticity, rather than extensive structural transformation or chemical ordering.