Most engineering metals and many ceramics are polycrystalline: they contain many crystalline regions called grains.

Within one grain, the crystal lattice has one orientation. A neighboring grain generally has a different orientation, and the transition region between them is a grain boundary.

Grain structure

Grain size, shape and orientation distribution are part of a material's microstructure. Processing can produce equiaxed grains, elongated grains, or preferred crystallographic orientations called texture.

A polycrystal can appear approximately isotropic when many randomly oriented grains average their directional behavior. Strong texture can instead make bulk properties direction dependent.

Grain boundaries are interfaces

Atoms at a grain boundary do not fit the perfect periodic arrangement of either adjoining grain. The boundary therefore has excess energy and altered atomic packing. It can act as:

• an obstacle to dislocation motion;
• a fast diffusion path;
• a preferred site for segregation, precipitation or nucleation;
• a region where cavities or corrosion can sometimes initiate.

Smaller grains create more grain-boundary area per unit volume. Changing grain size can therefore affect strength, diffusion, creep and phase-transformation kinetics in different—and sometimes opposing—ways.

Grains and grain boundaries are distinct from point defects and dislocations, which are treated separately because they control different material mechanisms.