Influence of grain boundaries and natural convection on microstructure formation in cellular directional solidification of dilute succinonitrile alloys in a cylinder

The influence of grain boundaries and natural convection on the birth and development of morphological instability is investigated during the directional solidification of succinonitrile-based alloys (succinonitrile-0.5 wt% acetone and succinonitrile-1.5 wt% water) in a cylindrical crucible. Continuous and in situ observation of the solid-liquid interface by optical methods is carried out through the melt. It is in particular shown that grain boundaries do not always cause morphological instability early and that a dynamical grooving of grains may occur by the propagation of channels, emanating among others from depressions in the solid ridges aside grain boundaries. Then, the accommodation of a grain boundary in the cellular array is presented. As convection in the melt is strong enough in the experiments for solute sweeping over the growth front to be significant, a delay of morphological instability is furthermore observed downstream in regions adjacent to well-oriented solid ridges. (C) 1998 Elsevier Science B.V. All rights reserved.