A crystalline material is one in which the atoms are situated in a repeating or periodic array over large atomic distances—that is, long-range order exists, such that upon solidification, the atoms will position themselves in a repetitive three-dimensional pattern, in which each atom is bonded to its nearest neighbor atoms. Not all solids are single crystals. For example, when liquid water starts freezing, the phase change begins with small ice crystals that grow until they fuse, forming a polycrystalline structure. In the final block of ice, each of the small crystals (called “grains“) is a true crystal with a periodic arrangement of atoms, but the whole polycrystal does not have a periodic arrangement of atoms, because the periodic pattern is broken at the grain boundaries.
Not all solids are single crystals (e.g. silicon semiconductors). Most crystalline solids are composed of a collection of many small crystals or grains of varying size and orientation. These have random crystallographic orientations. When a metal starts with crystallization, the phase change begins with small crystals that grow until they fuse, forming a polycrystalline structure. In the final block of solid material, each of the small crystals (called “grains“) is a true crystal with a periodic arrangement of atoms, but the whole polycrystal does not have a periodic arrangement of atoms, because the periodic pattern is broken at the grain boundaries. Grains and grain boundaries help determine the properties of a material.
- Grains, also known as crystallites, are small or even microscopic crystals which form, for example, during the cooling of many materials (crystallization). A very important feature of a metal is the average size of the grain. The size of the grain determines the properties of the metal. For example, smaller grain size increases tensile strength and tends to increase ductility. A larger grain size is preferred for improved high-temperature creep properties. Creep is the permanent deformation that increases with time under constant load or stress. Creep becomes progressively easier with increasing temperature.
- Grain Boundaries. The grain boundary refers to the outside area of a grain that separates it from the other grains. The grain boundaries separate variously-oriented crystal regions (polycrystalline) in which the crystal structures are identical. Grain boundaries are 2D defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material. Most grain boundaries are 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. On the other hand, grain boundaries disrupt the motion of dislocations through a material. Dislocation propagation is impeded because of the stress field of the grain boundary defect region and the lack of slip planes and slip directions and overall alignment across the boundaries. Therefore reducing crystallite size is a common way to improve mechanical strength, because the smaller grains create more obstacles per unit area of slip plane.