polykrystal

Polykrystal, or polycrystalline material, is a solid composed of many crystallites or grains that are separated by grain boundaries. The crystallites are typically randomly oriented, though textures can occur, leading to anisotropic properties at the macro scale. In contrast, a single crystal lacks grain boundaries and has a continuous lattice orientation.

Grain boundaries influence mechanical, thermal, electrical, and chemical properties. They can impede dislocation motion, increasing strength and hardness as grain size decreases (the Hall-Petch effect). Boundaries also diffuse atoms, affecting diffusion, corrosion resistance and creep. In polycrystals, the overall properties are an average over grain orientations, often with reduced anisotropy compared to single crystals.

Polycrystals form during solidification, casting, powder metallurgy, sintering, annealing and recrystallization of deformed materials. Control of grain size and grain boundary character through processing leads to tailored properties. Grain growth occurs at elevated temperatures as small grains shrink and large grains grow, reducing total boundary area.

Characterization techniques include X-ray diffraction, where peak broadening relates to average grain size via Scherrer equation; microscopy and electron backscatter diffraction (EBSD) map grain structure and orientation.

Common examples include metallic polycrystals such as steel and aluminum alloys, as well as polycrystalline ceramics and polycrystalline silicon used in photovoltaic cells and electronics.