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Why do grain boundaries form in a solid metal?

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Diagram of a zinc-lead blast furnace system showing ore and coke entering through charge hoppers, with labeled furnace, air preheater, blower, taphole for slag and lead bullion, and a connected lead-splash condenser using rotors to separate and cool lead and zinc, with pathways for molten metals and off-gas. — metallurgy A zinc-lead blast furnace and lead-splash condenser. This technology is utilized mainly for producing zinc while managing both lead and zinc from raw materials, a technique that was highly influential in 20th-century smelting.

In metallurgy, grain boundaries form during the solidification of a metal or during a phase transformation from one crystal structure to another.1

When a metal transitions from a liquid to a solid state, the process begins with the nucleation and growth of numerous small crystals, known as grains. While these grains share the same internal atomic structure, they develop with randomly distributed, varying orientations. As these individual crystals grow and eventually meet, the interface where they contact one another creates a grain boundary.12

These boundaries are essential to the material's performance, as the size and structure of the grains significantly influence the physical properties of the solid metal. For instance, grain boundaries can strengthen a metal at room temperature, though they may become sites of weakness or localized corrosion at high temperatures.21