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an aggregate of small crystals of a substance, which, because of their irregular form, are sometimes called crystallites or grains. Many materials of natural or artificial origin—such as minerals, metals, alloys, and ceramics—are po-lycrystals.
The properties of a polycrystal are due to the properties of the constituent crystalline grains; the average size of the grains, which ranges from 1–2 × 10-6 μ to a few millimeters, the crystallographic orientation of the grains; and the structure of the grain boundaries. If the grains are randomly oriented and their dimensions are small in comparison with the size of the polycrystal, the polycrystal does not exhibit anisotropy of physical properties, which is characteristic of single crystals. If the polycrystal has a texture, or preferred crystallographic orientation of its grains, it exhibits anisotropic properties. The presence of grain boundaries has a considerable effect on the physical, particularly the mechanical, properties of polycrystals, since such phenomena as the scattering of conduction electrons, the scattering of phonons, and the inhibition of dislocations occur at the boundaries.
Polycrystals are formed during crystallization, during polymorphic transformations, and as a result of the sintering of crystalline powders. A polycrystal is less stable than a single crystal. For this reason, during prolonged annealing of a polycrystal there occurs recrystallization, or the preferential growth of certain grains at the expense of others, which results in the formation of large crystalline blocks.
A. L. ROITBURD