Polykrystaller

Polykrystaller, or polycrystals, are solids composed of many small crystalline grains, each with its own crystallographic orientation. The grains are separated by grain boundaries, which influence mechanical, thermal, and electrical properties. Polykrystaller form when a material solidifies from a melt at multiple nucleation sites or when processes such as deformation and recrystallization create new grains. The resulting microstructure can vary from uniform to broad grain-size distributions depending on cooling rate, composition, and post-processing.

The structure of polykrystaller is characterized by its grain boundaries and the random or oriented arrangement of grains. In many cases, processing can induce a preferred texture, where certain crystallographic directions are more common, affecting anisotropy in properties.

Properties of polycrystals are strongly influenced by grain size. The Hall–Petch relation describes how reducing grain size typically increases yield strength and hardness due to impeded dislocation motion at boundaries. However, very small grains can reduce ductility. Grain boundaries also affect diffusion, creep, and electrical or thermal transport, often increasing scattering or resistance. Overall, polycrystals tend to be more isotropic than single crystals, though texture can introduce directional behavior.

Common examples include metals such as steel and aluminum alloys, and ceramics like alumina. Many geological materials, including most rocks and minerals such as quartz and feldspar, are polycrystalline. Characterization relies on optical or electron microscopy and X-ray diffraction to assess grain size, orientation, and boundary character. Control of grain structure through processing is central to materials design and performance. See also: single crystal, grain boundary, recrystallization, texture.