a dendrite
/ DEN-drite /
When most metals and alloys freeze, they do not grow as smooth blocks — they grow as tiny trees. A dendrite (from the Greek dendron, tree) is a branched, tree-like crystal that forms during solidification: a primary trunk shoots out, side-arms sprout from it at regular spacings, and smaller arms sprout from those, all sharing one crystal orientation. Frost ferns on a cold window are the same phenomenon in ice.
Dendrites form because a smooth (planar) solid-liquid front is unstable when the liquid ahead is supercooled — any small bump pokes into cooler liquid, freezes faster, and runs ahead, so fingers develop and branch. In alloys this is driven by constitutional supercooling: the growing solid rejects solute into the liquid, lowering its local freezing point ahead of the front. The trunk and arms grow along preferred easy-growth crystal directions (for cubic metals, the <100> directions), so a dendrite's geometry encodes its crystal orientation. The spacing between side-arms (the dendrite arm spacing) is set by cooling rate — faster cooling gives finer arms.
Dendrites set the as-cast structure and much of its quality. The last liquid to freeze, trapped between the arms, is enriched in solute, so a cast alloy comes out chemically uneven (microsegregation, or coring) that must often be homogenized by later annealing; shrinkage voids also tend to hide between arms. Each solidification grain is typically one dendrite (or a cluster sharing an orientation), which links dendrites to the columnar-versus-equiaxed grain structure of a casting. Honest note: a dendrite is a growth shape, not a phase or a crystal structure — the atoms inside are an ordinary crystal; the tree shape is simply how that crystal grew into a supercooled melt.
A sectioned aluminium casting reveals dendrites with primary arms tens of micrometres apart and finer secondary arms; measuring the secondary arm spacing (say 30 micrometres) reveals the local cooling rate, while the darker solute-rich regions between arms betray the microsegregation that a later homogenizing anneal must erase.
A tree-like solidification crystal: trunk and side-arms grow along easy-growth crystal directions, with solute-rich liquid freezing last between the arms.
A dendrite is a growth morphology, not a phase — the tree-shaped region is an ordinary single crystal. And the composition between dendrite arms differs from the arm centers (microsegregation), so an as-cast dendritic structure is chemically inhomogeneous until homogenized.