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Deep undercooling rapid solidification experiments of Cu–Ni–Co alloys were carried out using the molten glass purification cycle superheat method. 130 K, 177 K and 277 K undercooling degrees were obtained and analysed for Cu55Ni43Co2 alloy. The crystallographic information obtained by EBSD and the dynamics of the solidification front taken by high speed photography were analysed and it was concluded that both the formation of relatively rounded equiaxed grains during the first refinement and the subsequent formation of finer subcrystals were due to the release of a large amount of solidification energy during the solidification process. The re-glow stage releases a large amount of latent heat of solidification resulting in the remelting of the dendrites. TEM tests were carried out on Cu55Ni43Co2 alloy with a maximum undercooling of 277 K. It was found that part of the substructure showed a high density dislocation network. By analysing the effect of increasing undercooling on the microstructure evolution, combined with the EBSD and TEM characterisation data, it was found that the stress-induced recrystallization mechanism was the dominant factor in the grain refinement mechanism at large undercooling.

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]