Complete solute trapping of rapidly growing nickel dendrites within liquid Ni72Mo28 hypoeutectic alloy

Abstract

The rapid dendrite growth in highly undercooled liquid Ni72Mo28 hypoeutectic alloy was accomplished by containerless processing via electromagnetic levitation and drop tube techniques. The (Ni) dendrites achieved a high growth velocity of 26 m/s at the maximum undercooling of 226 K (0.14 TL) under levitated state. Remarkable dendritic structure refinement and Mo solubility extension were observed with the increase in undercooling. For freely falling alloy droplets, the largest undercooling was enhanced to 246 K (0.15 TL), which resulted in thorough solute trapping and almost segregationless solidification. A microstructure transition from columnar dendrite to equiaxed dendrite took place once alloy undercooling exceeded a threshold about 74–79 K. In addition, the Vickers hardness of primary (Ni) dendrite was significantly improved, which was caused by the extension of Mo solubility and microstructure refinement.