Grain refinement in solidification of undercooled Ni-Cu melts

The microstructures of bulk Ni-Cu samples solidified from the undercooled melt are investigated. High levels of undercoolings were attained by containerless processing in an electromagnetic levitation facility. At both high and low undercoolings reached prior to nucleation, a grain-refined equiaxed microstructure is formed, whereas for intermediate undercoolings a coarse dendritic microstructure is observed. The transitions between the microstructures take place quite abruptly at critical values Delta T* of the undercooling. The dependence of the values of Delta T* on the composition is studied, leading to the construction of a microstructure-selection map for the system Ni-Cu. The results are discussed within the scope of a new model for grain refinement featuring the fragmentation of the primary dendrites.