Microstructures of dilute Ni-C alloys obtained from undercooled droplets

An electromagnetic levitation facility and a drop-tube have been utilized to containerlessly process liquid Ni-C alloy droplets (C content < 5 at%). The microstructures formed upon solidification are found to be either coarse-grained dendritic (at intermediate undercoolings) or fine-grained equiaxed (at low and high undercoolings). The results obtained by levitation are summarized in a microstructure-selection map with composition and undercooling as variables. The map is fairly well described by a dendrite-fragmentation model for grain refinement. The critical undercoolings for the onset of grain refinement increase with increasing carbon content. This behaviour is confirmed by the results obtained from drop-tube processing. The grain size of refined droplets decreases with increasing post-solidification cooling rate. Dendrite growth velocities have been measured as a function of undercooling for pure Ni and for alloys containing 0.6, 1.7, and 3.1 at% C. In contrast to the prediction of current dendrite growth theory, it is found that small additions of carbon to nickel do not enhance the growth velocity at low undercoolings.