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A polycrystalline solid.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



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.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
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3Y-TZP: yttria partially stabilized tetragonal zirconia polycrystal (3y-TZP) is the most popular and frequently used form of zirconia commercially avail- able for dental applications.
Ikuhara, "Grain-boundary structure and microstructure development mechanism in 2-8 mol% yttria-stabilized zirconia polycrystals," Acta Materialia, vol.
(2011) Deformation in the lowermost mantle: From polycrystal plasticity to seismicanisotropy.
As mentioned above, nucleation of a transformed region in polycrystals does often take place at grain boundaries, edges and vertices.
Among the topics are enhancing simulation in complex systems, a numerical investigation of deployable drag surfaces used for recovery systems, liquid crystal thermography for determining the heat transfer coefficient in rectangular ducts, evaluating crack growth rate and the growth model of ultrafine-grained copper, the compression property of a waste polyurethane rubber/unsaturated polyester composite cube, a non-contact breath sensor based on a Doppler detector, new solid-state organic membrane based lead-selective micro-electrode, simulating intergranular crack nucleation and evolution in polycrystal metals on mesoscale, a coupled numerical-experimental study of armor perforation by armor piercing projectiles, and lessons from airlines for railway disruption recovery.
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