Crystallite
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A crystallite is a domain of solid-state matter that has the same structure as a single crystal.
Solid objects that are large enough to see and handle are rarely composed of a single crystal, except for a few cases (gems, silicon single crystals for the electronic industry, certain types of fiber, and single crystals of a nickel-based superalloy for turbojet engines). Most materials are polycrystalline, i.e. made of a large number of single crystals - crystallites - "stuck" together. The crystallite size can vary from a few nanometers to several millimeters.
If the individual crystallites are oriented randomly (that is, if they lack texture), a large enough volume of polycrystalline material will be approximately isotropic. This propery helps the simplifying assumptions of continuum mechanics to hold in real-world solids. However, most manufactured materials have some alignment to their crystallites, which must be taken into account for accurate predictions.
Metallurgists often refer to crystallites as "grains"; thus fracture can be an intergranular fracture or a transgranular fracture. But there is an ambiguity with powder grains: a powder grain can be made of several crystallites. Thus, the (powder) "grain size" found by laser granulometry can be different from the "grain size" (in fact: crystallite size) found by X-ray diffraction (e.g. Scherrer method), by optical microscopy under polarised light or by scanning electron microscopy (backscattered electrons).
Grain boundaries
Although the term "crystallite" is more precise, the boundary between two crystallites is traditionally known as a grain boundary. The term "crystallite boundary" is rarely used, and the fact that powder grains are not attached to one another and so do not form boundaries helps to remove ambiguity in this case.
Grain boundaries disrupt the motion of dislocations through a material; reducing crystallite size is therefore a common way to improve strength without much sacrifice in toughness. The high interfacial energy and relatively weak bonding in grain boundaries makes them preferred sites for the onset of corrosion and for the precipitation of new phases from the solid. They are also important to many of the mechanisms of creep.
Grain boundaries are generally only a few nanometers wide. In common materials, crystallites are large enough that grain boundaries account for a small fraction of the material. However, very small grain sizes are achievable. In nanocrystalline solids, grain boundaries become a significant volume fraction of the material, with profound effects on such properties as diffusion and plasticity. In the limit of small crystallites, the volume fraction of grain boundaries approaches 100%, the material ceases to have any crystalline character, and becomes an amorphous solid.
Generally, polycrystals cannot be superheated; they will melt promptly once they are brought to a high enough temperature. This is because grain boundaries are amorphous, and serve as a nucleation point for the liquid phase. By contrast, if no solid nucleus is present as a liquid cools, it tends to become supercooled. Since this is undesirable for mechanical materials, alloy designers often take steps against it. See grain refinement.