Dendrite (metal)

  A dendrite in metallurgy is a characteristic tree-like structure of crystals growing as molten metal freezes, the shape produced by faster growth along energetically favourable crystallographic directions. This dendritic growth has large consequences in regards to material properties.

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Dendrites usually form in multiphase alloys. The requirement is that the molten metal is supercooled below the freezing point of the metal. At slow cooling rates, the solidification front will be planar and stable. But, at increased cooling rates, the solidification may be so rapid that the alloy concentration at the solidification front will be different from the overall concentration. The increased concentration results in an increased melting point impeding solidification near the front. Solidification also releases energy, thus impeding solidification even more. A small distance away from the solidification front, the concentration is more favourable for solidification as well as the temperature is lower. This fact increases the solidification rate at the most protruding points, thus resulting in dendrite formation. Note also that a curved interface is less energetically favourable, thus limiting the sharpness of the dendrites.

If the metal is cooled slowly, nucleation of new crystals will be less than at large undercooling. The dendritic growth will result in dendrites of a large size. Conversely, a rapid cooling cycle with a large undercooling will increase the number of nuclei and thus reduce the size of the resulting dendrites(and often lead to small grains).

Smaller dendrites generally lead to higher ductility of the product. One application where dendritic growth and resulting material properties can be seen is the process of welding. The dendrites are also common in cast products, where they may become visible by etching of a polished specimen.

Dendrites also form during the freezing of many nonmetallic substances such as ice.

Dendrites usually form under non-equilibrium conditions.

Common dendritic metal material is nickel carbonyl, where the particles have a classical "spiky" morphology.