Polycrystals

Polycrystals are solids made of many crystallites, or grains, each with its own lattice orientation. Grains are separated by grain boundaries, defective interfaces that disrupt the continuous lattice. The size distribution, orientation, and area of grain boundaries determine the material’s macroscopic properties.

In polycrystals there is no single long-range order, so properties are often averages over many grains and can be isotropic or display texture if grain orientations are biased. Grain sizes typically range from nanometers to millimeters, and the grain boundary network affects diffusion, phase stability, and fracture behavior.

Strength usually increases as grain size decreases—a relation known as the Hall-Petch effect—because grain boundaries impede dislocation motion. At very small sizes, toughness may suffer, and high-temperature creep can involve grain-boundary sliding. Boundaries also influence electrical and thermal transport and diffusion.

Grain size and texture are controlled by processing: solidification, hot or cold working, annealing, recrystallization, and sintering. Thermal treatments promote grain growth or boundary relaxation, altering properties accordingly.

Polycrystalline materials are ubiquitous in metals, ceramics, and minerals. While many exhibit random grain orientations, processing can develop texture, making properties direction dependent. Understanding and engineering grain boundaries—grain boundary engineering—aims to optimize strength, toughness, and durability.