Recalescence Behavior, Solidification Characteristics and Microstructure Transformation of Rapidly Solidified Undercooled Cu-based Alloys

The undercooled solidification microstructures of Cu55Ni45, Cu55Ni43Co2, and Cu60Ni38Co2 Cu-base alloys were obtained by fluxing method. Using infrared temperature measuring device, the law of the change of the recalescence degree with the increase of the undercooling during rapid solidification was studied. At the same time, high-speed camera was used to capture and photograph the images of solid/liquid interface migration during rapid solidification of undercooled melt, and the morphology evolution of solidification front was discussed. Finally, the microstructure morphology and transformation process of the Cu-based alloys were systematically analyzed. It is found that the microstructure morphology of the alloys goes through the same evolution process and appeared two grain refinement phenomena, that is, “coarse dendrite-equiaxed grain — oriented fine dendrite — equiaxed grain”. But its characteristics undercooling ΔT1, ΔT2, and critical undercooling ΔT* varies. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to characterize the grain refinement structure with high undercooling. EBSD results show that the grain refinement structure with high undercooling presents a very high proportion of high angle grain boundaries, the grain orientation is random and there is no high strength texture, and a large number of annealing twins, which indicates that recrystallization occurs in the structure. TEM results show that dislocation network and stacking fault density are relatively low in most areas of grain refinement structure with high undercooling, which can confirm the theory that stress induces recrystallization of the structure.