Dendritic and cellular doublets: Morphologies of thin solid films growing along a substrate during the initial state of solidification of bulk melts

We present detailed experimental investigations of thin solid films growing along a borosilicate substrate for different dilute alloys in the succinonitrile-argon system. These thin solid films revealed a dendritic growth pattern with close-spaced sidebranches of different morphologies. We found surface dendrites with doublet tips as well as three different classes of sidebranches: cellular sidebranches, dendritic sidebranches, and an unsteady pattern, which we term “cauliflower structure.” Although the cellular sidebranches grew in a highly unstable manner, we found that cellular doublets build a stable substructure at low concentrations. Dendritic sidebranches with double tips divided by a straight groove in the plane of symmetry build a stable substructure at high concentrations. In the intermediate concentration regime cellular and dendritic doublets grew unstably with curved grooves forming irregular and a highly unsteady growth pattern. Proceeding from these thin solid films, the remaining bulk melt solidifies perpendicular to the substrate. For dilute alloys the appearance of surface dendrites can effect the final microstructure of the cast part, at least in the outer regions, whereas for higher concentrated alloys, where cells or even dendrites appear, no influence is expected.