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4.5: Polycrystals

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    Single crystals form only in special conditions. The normal solid form of an element or compound is polycrystalline. As the name suggests, a polycrystalline solid or polycrystal is made up of many crystals. The properties of a polycrystal are notably different from those of a single crystal. The individual component crystallites are often referred to as grains and the junctions between these grains are known as grain boundaries .

    The size of a grain varies according to the conditions under which it formed. Galvanised steel has a zinc coating with visibly large grains. Other materials have much finer grains, and require the use of optical microscopy.

    Photograph of galvanised steel

    In galvanised steel, the grains are big enough to be seen unaided. The plate measures 5 cm across.

    In many other metals, such as this hypoeutectoid iron-carbon alloy, the grains may only be seen under a microscope.

    These photographs show a polycrystalline sample of quartz mixed with feldspar in which the grains all have optically anisotropic properties. Between the crossed polarisers, each grain allows transmission of light at a slightly different point in the rotation. This gives the strange effect seen here.