Have you ever heard a Metallurgist or a Welding Engineer talk about a dendritic structure, or secondary dendrite arm spacing, and wondered what those terms meant? When metals solidify, the solid/liquid interface in most cases is not planar; rather ‘fingers’ of solid jut forward ahead of the solidifying interface. These fingers are dendrites, which in turn become the subgrain structure in the solidified metal.
The term dendrite is derived from the Greek word dendron, meaning ‘tree’. It is easy to see how this term is appropriate in the SEM image below (“Welding: Solidification and Microstructure” by David, Babu, and Vitek (JOM 6/03)), where the dendrites of a Ni-based superalloy, growing towards the viewer, resemble the birds eye view of a pine forest.
To see actual dendrite growth, have a look at this amazing YouTube video of dendritic solidification. This video shows an ingenious experiment where researchers used a low melting crystalline organic compound, which they melted with a resistively heated wire that was dragged ahead of the field of view of a microscope focused on the solid-liquid interface. Aside from a beautiful dendritic solidification pattern, this video also shows ‘competitive growth’, where dendrites growing in directions less favorably oriented to the heat flow are impinged by more favorably oriented dendrites (the winners!). Also, check out this YouTube video of simulated dendritic solidification, which gives more of a real time feel. Enjoy!