Polycrystal

A polycrystal is a solid material composed of many small crystallites, or grains, each with its own distinct crystallographic orientation. The grains are separated by grain boundaries, which are interfaces where crystal lattices are misaligned. This structure contrasts with a monocrystal, which has a continuous lattice throughout with no grain boundaries.

Polycrystalline structures form during solidification from a melt, through recrystallization or annealing of metals and ceramics, or by sintering powdered materials. Grain size and distribution are influenced by cooling rate, composition, impurities, and heat treatment. Processing conditions can also induce textures, where certain orientations are more common, leading to anisotropy in some properties.

The properties of polycrystals are governed by grain boundaries and grain size. Smaller grains generally increase strength and hardness via the Hall-Petch effect, while grain boundaries can impede dislocation motion, diffusion, and crack propagation. On a macroscopic scale, polycrystals often exhibit more isotropic behavior than single crystals when grain orientations are random; processing-induced textures can introduce directional dependence of properties.

Common examples include metals such as iron and aluminum alloys, ceramics like alumina and silicon carbide, and polycrystalline silicon used in electronics. Polycrystalline materials are routinely examined with microscopy and orientation mapping techniques (such as electron backscatter diffraction) to determine grain size, shape, and misorientation. They are widely used in structural, electronic, and industrial applications due to their favorable combination of properties and processing versatility.