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Applying cycle superheating and molten glass purification technologies, Cu55Ni45 and Cu55Ni43Co2 alloys were deeply undercooled to maximum undercoolings of 284 K and 250 K respectively. The microstructure characteristics and its evolution varying with bulk undercooling during the rapidly solidified process were analyzed. It was found that, after minor addition of element Co, refinement mechanisms of dendritic grain of the undercooled Cu-Ni-Co alloys was the same as those of the Cu Ni alloys. Dendritic grain refinement occurring at low undercooling range was induced by dendrite breakup due to remelting, while the dominant factor of the grain refinement at high undercooling range was the recrystallization process. The driving force (i.e. stress induced strain energy) for recrystallization could be divided into two parts, one was the thermal stress generated by the releasing of solidification latent heat during the recalescence process, and the other was the accumulated stress and plastic strain caused by the interaction of liquid flow and primary dendrites caused by rapid solidification. • The maximum undercoolings were achieved respectively by using the deep undercooling technology. • The recalescence behavior and microstructure refinement were systematically studied. • The relationship between solute composition and solidification process was established. • High-angle grain boundaries and twin boundaries exist in the microstructure at high undercooling. [ABSTRACT FROM AUTHOR]