Rapid growth of nickel dendrite in highly undercooled Ni-Mo alloys

Ni-39.3%Mo, Ni-45%Mo, hypoeutectic alloys and Ni-47.7%Mo eutectic alloy have been rapidly solidified with different droplet sizes by containerless processing in a drop tube. For Ni-39.3%Mo hypoeutectic alloy, which corresponds to the maximum solid solubility of nickel phase, the solidification microstructure is characterized by nickel dendrite plus (Ni+NiMo) eutectic structure. The undercooling of this alloy up to 182 K has been realized in the experiments. With an increase in undercooling, the dendritic microstructure is refined. The microstructural evolution of primary Ni phase in Ni-45%Mo hypoeutectic alloy evolves from remelted dendrite to equiaxed grains, whereas Ni-47.7%Mo eutectic alloy exhibits a structural transition from lamellar eutectic to anomalous eutectic. Theoretical analyses indicate that, for Ni-39.3%Mb, Ni-45%Mo and Ni-47.7%Mo alloys, the nickel phase shows a transition from solutal-diffusion-contrplled growth to thermal-diffusion-controlled growth at undercoolings of 66.6, 81.9 and 85.0 K. The critical transition temperature decreases with a reduction in the nickel content.