polycrystallic

Polycrystallic, more commonly written polycrystalline, describes materials that consist of many small crystals, or grains, each with its own lattice orientation. The grains are separated by grain boundaries. In contrast to single-crystal materials, polycrystallic substances exhibit properties that are averaged over multiple orientations, while still possessing long-range order unlike amorphous materials. Polycrystalline forms are widespread among metals, ceramics, and many polymers.

Grain size and boundary character strongly influence properties. Smaller grains generally increase strength and hardness by obstructing dislocation motion (the Hall-Petch effect), but a larger grain boundary area can enhance diffusion, diffusion-driven processes, and grain-boundary related corrosion or creep at elevated temperatures. Grain boundaries also affect electrical and thermal conductivity and, in some ceramics and semiconductors, optical properties. The overall behavior of a polycrystallic material arises from the collective response of many anisotropic grains.

Production and processing methods include solidification from the melt (casting and directional solidification), deformation processing (rolling, forging), annealing and recrystallization, powder metallurgy, and various forms of additive manufacturing. Microstructure is shaped by temperature, time, strain, and atmosphere during processing. Characterization techniques such as electron backscatter diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction are used to map grain size, orientation, and boundary types.

Applications span structural metals, ceramics, and polycrystalline silicon used in photovoltaics and electronics. Polycrystallic materials offer cost advantages and scalability but require careful control of grain structure to optimize mechanical, electrical, and diffusion-related properties.