Crystallites are small individual crystals which do not or only partly reflect the actual crystal form. In metallurgy, crystallites are referred to as grains.

Crystallites are formed when a large number of crystals solidify in a melt and adjacent grains interfere with their growth. The resulting solid – composed of many small crystallites – is called polycrystalline. The adjacent crystallites in a polycrystal only differ from each other in the orientation of their crystal lattice.

In micrographs of polished and chemically or electrochemically etched metal specimens (macrostructure etching, contrast etching, grain boundary etching, see Etching), the crystallites can be distinguished by different colors and the grain boundaries by the transitions from one crystallite to another by changes in color (Fig. 1) or dark lines (Fig. 2).In metallurgy, a grain boundary is a two-dimensional lattice defect. The grain boundary separates sections of different orientation but otherwise the same crystal structure within a crystal, the different sections being the crystallites or grains.

These boundaries are divided into low-angle and high-angle grain boundaries.

The high-angle grain boundary is the boundary range in which randomly oriented crystal sections are adjacent whose difference in orientation exceeds an angle of 15°. Such a grain boundary does not only indicate a defect within a grain, but the grain boundary to the adjacent crystallite.

  • Fig. 1: Crystallites (grains) in an aluminum casting alloy, electrochemical etching and image taken with polarizing filter, source: H. Rockenschaub, FT&E
  • Fig. 2:  Crystallites (grains), source: H. Rockenschaub, FT&E