A chef pours hot sugar syrup onto a cold table, and it sets into crystals. That's roughly how engineers pictured metal solidification: the liquid instantly becomes a scatter of crystalline grains. But close-up imaging revealed an extra stage wedged between liquid and solid.
In three grades of steel, X-ray imaging captured a strange phase during cooling: they named it superite. It's neither liquid nor the usual crystal, but something like scaffolding made of tiny oriented particles. First, the entire melt turns into such 'scaffolding,' and only then, as if on cue, real austenite grains (the basis of stainless steel) start growing inside it. Growth proceeds from the edges of dendrite branches toward the center, pushing excess atoms into the remaining superite: this creates the chemical inhomogeneity metallurgists call segregation.
Then the leftover superite, enriched with impurities, separates into several different solid phases, like congealing broth splitting into fat and jelly. This gives rise to inclusions, eutectic (a fine mixture of crystallites), and that dendritic pattern you see under a microscope. The discovery flips textbooks: dendritic segregation is not a side effect of crystal growth but the work of superite.
Now metallurgists don't have to guess why one corner of an ingot is stronger steel while another is brittle. By controlling cooling rate, they can adjust the thickness of the 'scaffolding' and the composition of the leftovers, thus tuning the properties of the finished metal without expensive remelting.
Dendrites in metal really do look like tiny Christmas trees: branches come off a central trunk, and smaller twigs come off those. Such a 'tree' ranges from fractions of a millimeter to several centimeters.