Experimental observation of solidification of undercooled single phase alloys

The solidification process of undercooled Cu70/Ni30/ alloy melt in a cylindrical crucible was investigated by high speed cinematography. In the experiment, nucleation location was controlled at one end of the sample by means of artificial triggering. The result shows that the equiaxed dendritic crystals obtained at low undercooling form in sequential solidification, rather than simultaneously in the entire sample, and the morphology of the recalescence front frequently changes with time. If increment of undercooling results in that the dendrite growth is controlled by thermal diffusion, the primary dendrite arms propagate parallel to each other in '100', and eventually directional solidification microstructure is obtained. At higher undercooling, dendrite growth is instable due to solidification contraction, and the microstructure develops into fine quasi-spherical grains through recrystallization, and however, the massive nucleation is not found.