Abstract:

A cost-effective Fe-enriched eutectic high-entropy alloy (EHEA), Fe₃₅Ni₂₅Cr₂₅Mo₁₅, was designed and prepared to avoid the use of expensive Co that is commonly used in HEAs. However, the as-cast Fe-enriched EHEA was associated with brittleness. The present work aims to evaluate the possibility and feasibility of spheroidization of the lamellar structure of the EHEA in order to improve the ductility. Due to the high cooling rate of arc-melting, the as-melted Fe₃₅Ni₂₅Cr₂₅Mo₁₅ EHEA was found to be a pseudo eutectic alloy comprised of alternant σ phase (Cr_0.22Mo_0.18Fe_0.6-type intermetallic) and face centred cubic (FCC) phase. The lamellar structure in the Fe-enriched EHEA remained stable up to 800 °C. The instability of the lamellar structure occurred at temperatures over 800 °C, which was resulted from migration of high-density faults (i.e. lamellar termination and ledges in the lamellae). However, the Fe₃₅Ni₂₅Cr₂₅Mo₁₅ EHEA still exhibited brittleness even after spheroidization at 1100 °C for 168 h due to the formation of the hard and brittle σ matrix in the pseudo Fe₃₅Ni₂₅Cr₂₅Mo₁₅ EHEA as a result of decomposition of the lamellar structure. Therefore, in contrast to the softening of traditional eutectic alloys, spheroidization treatment was considered as invalid to improve the ductility of pseudo-eutectic HEA with high fraction of intermetallic phase. The present work provides a valuable reference for those who aim to improve the ductility of brittle EHEA through spheroidization.

Key words: Eutectic high-entropy alloys, Instability, Lamellar structure, Spheroidization, Mechanical properties