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How do grain boundaries form in polycrystalline materials?

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A rough, translucent yellow crystal of elemental sulfur displayed against a solid black background. — crystal A crystal form of elemental sulfur. Sulfur has an atomic weight of about 32.06 atomic mass units (amu), which means that its molar mass is 32.06 grams.

In polycrystalline materials, grain boundaries form during the solidification process when a substance crystallizes at multiple sites simultaneously.12

The process begins with nucleation, where small, stable clusters of atoms or molecules form within a medium. As these clusters grow into individual crystals—often called grains or crystallites—they expand outward in different, random orientations. Because these grains grow independently from various starting points, they eventually meet and intersect. The surfaces where these randomly oriented crystals come into contact are known as grain boundaries.3412

These boundaries are significant because they define the structure of the polycrystal and heavily influence the material's physical and mechanical properties, such as strength, ductility, and susceptibility to corrosion.41