Structural reorganisation governs atomic transport while preserving mobility hierarchy in a nanopolycrystalline Al₀.₈NbTiVNi₀.₂ lightweight refractory high-entropy alloy
Abstract
Understanding atomic transport mechanisms in lightweight refractory high-entropy alloys (LW-RHEAs) is essential for predicting their long-term high-temperature stability and service performance. Although diffusion in high-entropy alloys has been widely investigated, the individual contributions of bulk and grain-boundary-associated regions remain insufficiently understood, particularly in nanopolycrystalline systems. Moreover, atomistic diffusion mechanisms have not yet been elucidated for the experimentally relevant Al₀.₈NbTiVNi₀.₂ LWHEA. In this study, molecular dynamics simulations combined with Polyhedral Template Matching (PTM) were employed to investigate diffusion behavior in a nanopolycrystalline Al₀.₈NbTiVNi₀.₂ LW-HEA containing 20 Voronoi grains and 207,288 atoms between 600 and 1300 K. PTM was used to partition the alloy into bulk-core (BCC) and grain-boundary-distorted (GB-distorted) environments, enabling independent analysis of their transport characteristics. The results revealed a progressive temperature-driven loss of the BCC bulk-core network accompanied by transport pathway redistribution. GB-distorted regions exhibited markedly higher mobility than bulk-core regions at low temperatures, with a GB/Bulk MSD ratio of 5.7 at 600 K, which progressively decreased and approached unity at 1300 K, indicating increasingly homogeneous diffusion behavior. Arrhenius analysis (600–1200 K) yielded activation energies of 0.635 eV for bulk-core atoms and 0.533 eV for GB-distorted atoms, consistent with lower effective activation barriers in structurally distorted environments. Element-resolved analyses revealed a persistent mobility hierarchy, with Al remaining the fastest and Nb the slowest diffusing species throughout the investigated temperature range. Overall, increasing temperature reorganizes the structural transport network while preserving an intrinsic elemental mobility hierarchy, establishing a physically interpretable microstructure.