ABSTRACT
This chapter focuses on a less well understood aspect of grain-structure control in solidification, namely control of grain size in the usually preferred equiaxed regime. The grains grow from the initial nucleation site or sites, and clearly show the direction of solidification. However, it is also commonly found that alloys with solidification triggered in particular ranges of undercooling show fine equiaxed grains. Solidification shrinkage could indeed generate the conditions for cavitation ahead of the solid, while collapse of a cavity would generate a significant pressure pulse. In any case, the progression of solidification across the drop rules out the possibility that the grain refinement is the result of copious nucleation throughout the melt at a critical temperature. Rapid solidification can be achieved by prior undercooling of the bulk melt, or by rapid heat extraction. The combination of slow growth rate and rapid external heat extraction ensures that a high undercooling can be maintained even as solidification progresses.
