Grain Refinement during Melt-spinning of Dilute Cu-base and Ni-base Alloys

Abstract

In order to obtain a fine and equiaxed grain structure in a casting, grain formation has been examined in past work in terms of undercooling of the melt, promotion of heterogeneous nucleation and remelting or fragmentation of dendrites. Some techniques to obtain grain refinement are commercially used, but the mechanism is not fully understood. The addition of a second element to a pure metal is one of the possible ways to obtain grain refinement in bulk samples. In the present work, this approach was taken with a melt-spinning method, which allows for an extremely directional and rapid heat extraction.
Dilute Cu-base and Ni-base alloys with selected solute additions were studied to examine the operating mechanism of the grain refinement in melt-spun alloys. In melt-spun Cu-base and Ni-base binary alloys, it was found that the type of solidification structure (columnar or equiaxed) can be predicted by the constitutional undercooling parameter, P, in most alloy systems as well as the case of bulk samples. There were some cases in which the melt-spun grain structure was not consistent with the P parameter prediction. In all cases, however, the final solidification temperatures of the alloys appear to be correlated to the development of grain refinement. As a result, to explain all the results in the present work, a model has been proposed which considers the effects of the parameter P, dendrite fragmentation and final solidification temperature concurrently and offers useful guidance in alloy design for optimum grain refinement.