Effects of niobium on the microstructure, corrosion, and mechanical behavior of ultra-fine lamellar Al_0.3CrFeCoNiNb_x eutectic high-entropy alloys

This study investigates the effects of the element, Nb, on the microstructure, corrosion, and mechanical behavior of the Al_0.3CrFeCoNiNb_x (in molar ratio) high-entropy alloys (HEAs). It is found that the Nb addition facilitates the formation of the Nb-rich Laves phase for the HEAs. The HEAs show a hypoeutectic structure [the primary face-centered-cubic (FCC) phase] at x = 0.25–0.4, eutectic structure at x = 0.45, and hypereutectic structure (the primary Laves phase) at x = 0.5–0.75. In the 3.5 mass% NaCl solution, the Al_0.3CrFeCoNiNb_x HEAs are spontaneously passivated and exhibit a low corrosion rate of less than 10⁻³ mm/year as a result of the formation of the protective passive films enriched in the chemically-stable Cr- and Nb-oxides. Compared with the FCC phase, the Laves phase displays a better corrosion resistance because of the higher concentration of Nb-oxides in its passive film. The Al_0.3CrFeCoNiNb_0.45 eutectic HEA possesses superior corrosion resistance among the HEAs, which is ascribed to a high concentration, and more homogeneous distribution of Nb-oxides in the passive film, as well as higher electrochemical impedance and thicker passive film. Furthermore, the Al_0.3CrFeCoNiNb_x HEAs with larger fractions of the Laves phase and ultra-fine eutectic structure present higher strength and hardness. Nevertheless, the plastic strain of the HEAs sharply decreases with increasing the amount of the brittle Laves phase. The results propose an effective approach for developing the HEAs with good corrosion resistance, high strength, and hardness.