Nanoscale phase separation in a fcc-based CoCrCuFeNiAl_0.5 high-entropy alloy

Nano-scale phase separation is reported in a nominal single-phase, high-entropy alloy (HEA), which was characterized using scanning transmission electron microscopy (STEM) combined with atom probe tomography (APT). Despite the fact that X-ray diffraction exhibits a single face-centered-cubic (fcc) phase feature of the as-cast alloy prepared by melt spinning, selected area electron diffraction reveals weak L1₂ ordering in the as-spun alloy. High-resolution STEM shows the presence of two coherent nanophases with distinct L1₂ and fcc structures, coupling with compositional segregations. The ordering of the L1₂ domains is enhanced after annealing at 500 °C. Electron energy loss spectroscopy and APT analyses reveal that the L1₂ nano-phase is enriched with Fe, Co, Cr and Ni, while the fcc domains are a Cu-rich phase. The nano-scale phase separation can effectively minimize the lattice distortions caused by the atomic size difference in the constituent elements, which may offer structural insights into the unusual mechanical behavior and phase stability of fcc HEA.