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If metallic melt is allowed to solidify, a large number of particles (crystallites) is formed that will eventually grow radially and join up to complete the crystallization process. The transitions between adjacent grains are called grain boundaries. Metals are also referred to as multi- crystals. The figure illustrates this crystal growth. As the last grain boundaries solidify, impurities and weak points in the structure of the metal gather there. Fractures will usually occur t here as well. This phenomenon is exploited by the zone melting method.