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 sizes and orientations, with random crystallographic orientations. When 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. Still, 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 microscopic crystals that form, for example, during the cooling of many materials (crystallization). A very important feature of a metal is the average size of the grain, and the size of the grain determines the properties of the metal. For example, smaller grain size increases tensile strength and tends to increase ductility. 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, and 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 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 grain boundary defect region’s stress field, 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 the slip plane.
The orientation of crystallites can be random with no preferred direction, called random texture, or preferred, possibly due to growth and processing conditions. Random orientation can be obtained by cross-rolling the material. If such a sample were rolled sufficiently in one direction, it might develop a grain-oriented structure in the rolling direction, which is called preferred orientation. In many cases, preferred orientation is desirable, but it can be most harmful in other instances. For example, preferred orientation in uranium fuel elements can result in catastrophic changes in dimensions during use in a nuclear reactor.
Special reference: U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.