Abstract Rapid solidification of undercooled Fe–Co–Cu alloys was investigated by means of fluxing purification and cyclic superheating technique. A transition in microstructure from dendrites to phase-separation occurred above a phase-separation undercooling Δ T sep. When Δ T<Δ T sep, dendrite was observed, the trunks were rich in Fe and Co, while Cu was rich at inter-dendrites. However, the phase-separated microstructure was obtained once Δ T>Δ T sep, with a large sphere of L1 phase located almost at the center of the sample and enwrapped by L2 phase. Δ T sep was 222, 88 and 45 K for Fe 50Co 30Cu 20, Fe 25Co 25Cu 50 and Fe 15Co 15Cu 70 alloys in this work, respectively. It was investigated that L1 phase solidified before L2 phase after liquid separation and followed different ways.