Dual enhancement in strength and ductility of Al_1.25CoCrFeNi₃ eutectic high entropy alloy by directional solidification

Directional solidification is an effective strategy to overcome the strength-ductility trade-off of high-entropy alloys (HEAs). Revealing the strengthening mechanism and deformation mechanism is significant for further exploration of directionally solidified eutectic HEAs (DSHEAs). In this work, we prepared Al_1.25CoCrFeNi₃ DSHEAs at different withdrawal rates, and the microstructure and mechanical properties were further explored. Results show that Al_1.25CoCrFeNi₃ DSHEAs with herringbone microstructure consist of L1₂ and B2 phases. The solid-liquid interface exhibits a cellular morphology at a slow withdrawal rate. The grain size and lamellar spacing refine with the increase of the withdrawal rate. Al_1.25CoCrFeNi₃ DSHEA with a withdrawal rate of 150 μm/s achieves an excellent combination of strength and plasticity. Compared with the traditional as-cast alloy, the fracture strain is increased by 40% without sacrificing strength. During the tensile process, the crack initiates in the B2 phase, and the eutectic lamellar morphology grown parallel to the heat flow direction delays the crack propagation. The irregular eutectic zone of adjacent grains accommodates the incompatibility of plastic deformation and avoids the initiation of cracks at grain boundaries. Theoretical calculations show that L1₂-B2 phase interface strengthening is the main strengthening mechanism.