On the length-scale and location of channel nucleation in directional solidification

This work provides evidence that channels in directionally solidified hypoeutectic Pb-Sn alloy nucleate at the dendrite tips. Using a finite-element simulator, distinctive 'convective signatures' are shown to exist for stable and unstable cases, where the stability of a system is shown to be largely a function of the thickness of an inverted density layer that can exist ahead of the moving solidification front. The thickness of this layer, and therefore, the stability of the systems studied, is shown to largely be a function of the length scale D/V, where it is shown that channeling can be turned on or off simply by changing this length scale. [Application: turbine blades in jet engines.]