Abstract: High-entropy alloys (HEAs) have attracted extensive attention ascribed to their unique physical and chemical properties induced by the cocktail effect. However, their oxidation behaviors, in particular at nanoscale, are still lack because of multi-element complexity, which could also be completely different from the bulk counterparts. In this work, we synthesized FeCoNiTiCu five-element HEA nanoparticles(NPs) with uniform elemental distribution by arc-discharging approach, and further investigated their oxidation behaviors at 250 °C, and 350 °C. The morphology, structure and element distribution of NPs were analyzed by transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). The surface oxidation in FeCoNiTiCu NPs during the high-temperature process can induce nanoscale pores at core/shell interfaces by Kirkendall effect, and even the eventual coalescence into a single cavity. Additionally, the oxidation states of NPs with diameters (d) varying from 60 to 350 nm were analyzed in detail, revealing two typical configurations: hollow (d < 150 nm) and yolk-shell structures (d > 150 nm). The experimental results were complemented by first-principles calculations to investigate the diffusion behaviors of five elements, evidencing that the surface oxidation strongly alters the surface segregation preferences: (1) in the initial stage, Cu and Ni appear to prefer segregating on the surface, while Co, Ti and Fe tend to stay in the bulk; (2) in the oxidation process, Cu prefers to stay in the center, while Ti segregates to the surface ascribed to the reduced potential energies. The study gives new insights into oxidation of nanoscale HEA, and also provides a way for fabrication of high-entropy oxides with controllable architectures.

Key words: High-entropy alloy, Nanoparticles, Oxidation, Kirkendall effect, Exchange bias effect