Microstructure Refinement in Solidification of a Deeply Undercooled Ternary Nickel Based Alloy

The experimental method employed the use of melt purification and cyclic superheating technique to achieve maximum undercooling of Ni65Cu31Co4 alloy at 300 K. Simultaneously, high-speed photography techniques were used to capture the process of alloy liquid phase interface migration, and analyze the relationship between the shape characteristics of the front end of alloy solidification and undercooling. The microstructure of the alloy was observed through metallographic microscopy, and the micro-morphological characteristics and evolution of the rapidly solidified microstructure were systematically studied. It was found that the grain refinement mechanism of Ni-Cu-Co ternary alloy is similar to that of Ni-Cu binary alloy. Grain refinement at low undercooling is caused by intense dendritic remelting, while grain refinement at high undercooling is attributed to recrystallization, driven by the stress and plastic strain accumulated from the interaction of liquid flow and primary dendrites caused by rapid solidification.