Polykrystallinske

Polykrystallinske, or polycrystalline, materials are solids composed of many small crystallites, or grains, each with its own crystallographic orientation. The grains are separated by grain boundaries, which are regions where the crystal lattice is disrupted. Although individual grains have long-range order, the material as a whole lacks a single, continuous crystal lattice.

These materials form through processes such as solidification from a melt, powder sintering, or recrystallization and grain growth during heat treatment. The resulting grain size distribution and boundary character depend on processing conditions, such as temperature, cooling rate, and mechanical work. Grain size can be controlled to tailor properties, and annealing can promote grain growth or refinement.

Mechanically, polycrystalline materials tend to be more isotropic than single crystals because the random grain orientations average out directional properties. However, grain boundaries influence performance: they impede dislocation motion, often increasing strength as grain size decreases (the Hall-Petch effect); but boundaries can also be sites of diffusion, corrosion, and crack initiation, especially at high temperatures or in aggressive environments. Fracture and creep behavior are closely linked to grain size, boundary character, and phase composition.

Common examples include metals such as steel, aluminum, and copper; ceramics like alumina and silicon nitride; and semiconductors such as polycrystalline silicon (poly-Si), widely used in photovoltaics and electronics. Characterization methods include X-ray diffraction, scanning and transmission electron microscopy, and electron backscatter diffraction (EBSD) to determine grain size, orientation distribution, and boundary types.

Polycrystalline materials contrast with single-crystal materials, which have a continuous lattice with uniform orientation, and with amorphous materials, which lack long-range order.