GRAIN-REFINEMENT DURING MELT-SPINNING OF DILATE CU-BASE AND NI-BASE ALLOYS

In order to obtain a fine and equiaxed grain structure in a casting, g rain formation has been examined in past work in terms of undercooling of the melt, promotion of heterogeneous nucleation and remelting or f ragmentation 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 wa ys to obtain grain refinement in bulk samples. In the present work, th is approach was taken with a melt-spinning method, which allows for an extremely directional and rapid heat extraction. Dilute Cu-base and N i-base alloys with selected solute additions were studied to examine t he 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 ty pe of solidification structure (columnar or equiaxed) can be predicted by the constitutional undercooling parameter, P, in most allay system s 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 temperatur es of the alloys appear to be correlated to the development of grain r efinement. As a result, to explain all the results in the present work , a model has been proposed which considers the effects of the paramet er P, dendrite fragmentation and final solidification temperature conc urrently and offers useful guidance in alloy design for optimum grain refinement.