policristalline

Policristalline, or polycrystalline, describes materials that are composed of many small crystallites, or grains, separated by grain boundaries. Within each grain, atoms are arranged in a well-ordered crystal lattice, but neighboring grains have different orientations, so the material as a whole lacks long-range crystalline order. The overall properties of a policristalline material are the result of the collective behavior of many grains and their boundaries.

Policristalline structures form during solidification, recrystallization, or subsequent processing such as annealing. The cooling rate, temperature, and deformation history influence the average grain size: rapid cooling tends to produce finer grains, while slow cooling yields coarser grains. Grain boundaries act as barriers to dislocation motion, which can increase strength and hardness through mechanisms like the Hall-Petch effect. At the same time, boundaries can impede electrical and thermal transport and may serve as sites for diffusion, corrosion, or crack initiation, depending on the material and environment. Because grain orientations are typically random, policristalline materials often exhibit more isotropic behavior than single crystals, though some textures can introduce anisotropy.

Common policristalline materials include metals (such as steel and aluminum), ceramics (like zirconia and alumina), and polycrystalline silicon used in solar cells and electronics. Processing methods—such as casting, rolling, annealing, and sintering—control grain size and boundary character. Grain boundary engineering aims to optimize boundary types and distributions to improve properties such as toughness, creep resistance, and diffusion barriers. Policristalline materials are widely used across engineering, energy, and electronics due to their manufacturability and balanced properties.