Interfacial structure governs room-temperature ductility in additively manufactured FeCoNiAl alloy

Additively manufactured (AM) FeCoNiAl eutectic high-entropy alloys (EHEAs) exhibit excellent mechanical performance but remain highly sensitive to post-processing. Here, we systematically investigate how post-AM heat treatments affect the room-temperature tensile behavior of FeCoNi1.8Al0.8 EHEAs fabricated by selective laser melting. The tensile ductility shows a non-monotonic dependence on annealing temperature, and the response does not follow the conventional strength–ductility trade-off. Microstructural analyses reveal that this anomaly originates from the evolution of B2 precipitate morphology and interfacial coherency. Aging below 550 °C produces coarse, block-like B2 precipitates with incoherent interfaces, causing severe embrittlement. At higher temperatures, the B2 phase becomes more uniformly distributed, and semi-coherent FCC/B2 interfaces together with abundant high-angle grain boundaries effectively suppress crack propagation. Consequently, the alloy aged at 740 °C for 2 h achieves an optimal strength–ductility balance, with a tensile strength of 1.21 GPa and an elongation of 22 %. These results establish a “morphology–coherency–toughness” relationship for designing damage-tolerant dual-phase high-entropy alloys.