Phase-Resolved Defect Transport Mechanisms Governing Asynchronous Ordering in a Eutectic High-Entropy Alloy

Eutectic high-entropy alloys (EHEAs) exhibit asynchronous phase evolution under identical thermal conditions, yet the underlying mechanisms remain unclear. Here, we investigated the isothermal annealing of AlCoCrCuFeNi alloy using phase-resolved experiments and atomistic simulations. Electron microscopy revealed sharply contrasting structural pathways: the initially disordered BCC-FeCr phase progressively developed long-range order, whereas the B2-NiAl phase retained its macroscopically ordered framework while undergoing local structural relaxation. Molecular dynamics and Monte Carlo simulations traced this discrepancy to phase-dependent defect energetics. In B2-NiAl, low vacancy formation and migration energies enable efficient vacancy-mediated transport, promoting local structural refinement without disrupting the ordered lattice. In BCC-FeCr, constrained vacancy transport together with accessible low-energy interstitial configurations gives rise to a cooperative vacancy-interstitial transport mechanism required for the disorder-to-order transition. These heterogeneous defect transport mechanisms govern asynchronous ordering in EHEAs and provide a phase-resolved basis for designing thermally stable complex alloys.