How do grains form when molten metal solidifies?

In materials science, the formation of grains during the solidification of molten metal is a two-stage process involving nucleation and growth.12

Nucleation

Solidification begins with nucleation, where atoms in the liquid metal arrange themselves into a stable, ordered crystalline pattern. This occurs in two ways:12
* Heterogeneous nucleation: The most common form, where atoms cluster around impurities, dust particles, or the walls of the mold.12
* Homogeneous nucleation: Occurs within the bulk of the liquid as atoms randomly collide and form clusters, typically under conditions of significant supercooling.12

Growth

Once a stable nucleus forms, it acts as a seed for further growth as additional atoms join the cluster. As the metal cools, these crystals expand until they encounter neighboring crystals. The regions where these differently oriented crystals meet are called grain boundaries.234

The morphology of this growth depends on the cooling rate:
* Planar growth: At slow rates, the interface between the solid and liquid remains smooth.5
* Dendritic growth: At faster rates, heat dissipation leads to instabilities, causing the crystal to grow in a branching, tree-like "finger" structure known as a dendrite.5

Ultimately, most solid metals are polycrystalline, meaning they are composed of millions of these randomly oriented grains packed together.4