What are dendrites

What are dendrites anyway?

The term dendrite can be found in different areas. The best-known example of dendrites are snowflakes (Fig. 1a) with their innumerable elegantly shaped branches in hexagonal symmetry. They arise during the crystallization of a supersaturated gas phase when the crystallization environment is almost identical for all sides of a small crystallization nucleus. The growth of the six processes is almost the same.
The word dendrite is of Greek origin (dendron stands for "similar to", drys for "tree") and is often translated as fir tree. In general, we understand it to be a spatial structure with arms and branches (secondary arms).
In biology, cell processes of nerve cells that emerge from the cell body and primarily serve to absorb stimuli are referred to as dendrites. They should not be confused with dendritic cells of the immune system.
We also find dendrites in minerals, for example in a grossular from the garnet group in Fig. 1b. They often form on layer surfaces through deposition from penetrating solutions and are often incorrectly called "petrified moss".

Fig. 1 Various dendrites a) snowflake, b) dendrites in a grossular, c) copper dendrite
Dendrites play an important role in metallography and crystallography. These tree-like or shrub-like crystal structures arise when a melt solidifies when the crystal structure has preferred growth directions. The crystals never grow evenly, especially if new nuclei are constantly being formed during cooling and the crystallization is particularly rapid in certain crystal directions. Starting from internal or foreign nuclei, individual crystal needles grow in preferred directions, according to the heat dissipation in the melt. After short growth times, further crystals appear at right angles on the existing transcrystallites in all directions. This leads to the fir tree-like structures. A nice example of dendrites made of copper is shown in Fig. 1c. Slow cooling rates (about 102 K / s) or long local solidification times lead to coarse dendrite structures with large dendrite arm distances, which can also be seen in Fig. 1c.
Dendritic cast structures often have poorer mechanical properties than those with spherical grains. In addition, the spaces between the dendrite arms are preferred locations for the accumulation of impurities or the formation of micropores due to the volume contraction when the melt solidifies. For this reason, deformation processes (e.g. forging, hot rolling) are often followed in order to at least partially convert the dendritic structure into a normal grain structure.

Fig. 2 Blowholes with dendrites in an excavator shovel (source: IWS Hamburg)
In casting technology, dendrites are one of the undesirable phenomena. Every foundry strives to optimize the casting process in such a way that defective cast products are created. The period in which the hot, liquid metal is still slowly cooling down and begins to solidify is particularly critical. Although dendrites form only a small amount of crystallized metal at the beginning, they are of decisive importance for the mechanical properties of the later workpiece. Sometimes, however, the casting process can go very badly, which can be illustrated with the help of Fig. 2. During casting, a giant blowhole with many large dendrites formed in the fork of an excavator shovel made of unalloyed cast steel, which immediately led to the shovel breaking. These dendrites could already be seen with the naked eye. So the cause of the damage was immediately clear. In practice, however, such extreme cases are an exception. And that is a good thing.<<