With the increase in the growth rate of crystals, the dimensions of the cells decrease. With intensive heat removal through the solidified metal, when the growth rate of the crystals is high, the cellular structure does not arise. Thus, at the low cooling rates, the surface of the crystallization front is almost smooth. At medium – the cellular structure appears, the elements of which are crushed with the acceleration of cooling. At the high cooling rate, the branches appear in place of the cells, indicating the initiation of dendritic crystallization.
In the presence of a super cooled melt zone in front of the crystallization front, in which accelerated growth is possible, the stability of smooth and cellular surfaces is violated and the formation of protrusions is facilitated. The zone should be wide in order to provide the branching characteristic of the dendrites. In this case, the concept of the crystallization front as the flat interface is meaningless, since dendrites, growing into the interior of the liquid, leave behind an uncrystallized part of the melt. The dendritic growth contributes to the heat sink.
The decrease in the temperature gradient ΔT in the liquid promotes dendritic crystallization. The increase in the growth rate Up of the crystals also leads to this. With the small content of impurities, the decrease in the temperature gradient and an increase in the growth rate may not lead to the formation of cells and dendrites. The conditions for the transition from one form of growth to another depend on the density of the packing of faces by atoms. Since the crystals are faceted by different planes, there is a transition between the dendritic and cellular regions: cells and dendrites are observed on the surface of the crystal.
The effect of hypothermia is considered using the example of the technically clean metal, whose equilibrium crystals have the octahedral form. The planes of the octahedron are the most closely packed and have the minimum surface energy. Already in the early stages of growth, the octahedral crystal (Fig. 1a) discards six processes in three mutually perpendicular directions (Fig. 1b).
Expanding, the processes turn into branches of the first order (trunks) of the dendrite. Simultaneously with their elongation, the branches of the second order grow perpendicular to them, on which third-order branches grow, etc. The dendrite grows in the form of a lattice of branches (Fig. 1c). As a result of thickening, the branches grow together and the solid crystal is formed.