Design of a novel (CoCrNi)84.5Al15.5 eutectic medium-entropy alloy with hierarchically nano-lamellar microstructure through additively manufactured compositionally graded materials

Abstract

Eutectic medium/high-entropy alloys (E-M/HEAs) exhibit excellent castability and mechanical properties. This present work developed a novel CoCrNiAl based E-MEAs through a high-throughput method based on compositionally graded materials by laser directed energy deposition. The microstructure in the CoCrNiAlx (x ≈ 0−0.8) compositionally graded alloy changed from columnar to equiaxed with increasing Al content, and the fraction of body-centered cubic (BCC) phase increased simultaneously. Especially, a eutectic lamellar microstructure with face-centered cubic (FCC) and ordered BCC (B2) was obtained when the Al content reached 15.5 at%, and it showed a better strength-ductility combination than other reported cast eutectic alloys. The mechanical-property improvement of this novel E-MEA is related to its hierarchically nano-lamellar microstructure, in which the orientation relationship (OR) between FCC and B2 lamellae conforms to the Kurdjumov-Sachs OR, and spherical BCC and ordered FCC phases precipitate from B2 and FCC lamellae, respectively. Furthermore, the fracture mechanisms of this novel E-MEA were also investigated by in-situ tensile test and transmission electron microscope, indicating that the crack modes were closely related to the eutectic lamellar direction relative to the tensile axis, and the considerable ductility stemmed from the cooperative deformation between FCC and B2 phases caused by various deformation mechanisms.