Crystallite

A crystallite is a microscopic or small crystal that forms during the cooling of many materials. You may also hear these referred to as grains. Most solid objects we interact with are not single crystals. Instead, they are polycrystalline, meaning they consist of many crystallites of different sizes and orientations. These crystallites are held together by thin layers of amorphous solid. An amorphous solid is a material that lacks an ordered internal structure.

Most inorganic solids are polycrystalline. This includes common metals, many ceramics, rocks, and even ice.

Materials exist on a spectrum of order. A single crystal is highly ordered with a continuous, unbroken lattice. On the other end, amorphous materials like glass or many polymers have no ordered arrangement at all. Polycrystalline structures and paracrystalline phases sit between these two extremes. Some materials, called oligocrystalline, consist of only a few coarse grains. These are often columnar and parallel to the axis of an ingot.

Crystallite sizes can vary greatly. They can range from just a few nanometers to several millimeters in length.

Where these individual crystallites meet, they form interfaces called grain boundaries. These boundaries are generally only a few nanometers wide. A grain boundary is a single-phase interface where crystals of different orientations meet. The atoms at these boundaries are often perturbed from their original lattice sites. They can also contain dislocations or impurities that have migrated to these lower energy areas. To define a grain boundary geometrically, scientists use five specific variables. These include a rotation axis, an angle of rotation, and the plane of the boundary.

Grain boundaries significantly impact the physical properties of a material. For example, they affect how a solid melts. Polycrystals usually melt promptly once they reach a high enough temperature. This happens because the amorphous grain boundaries serve as nucleation points. A nucleation point is a site where a liquid phase can begin to form. In contrast, if no solid nucleus is present while a liquid cools, the substance may become supercooled.

Grain boundaries also influence how materials break. Fractures can be intergranular, occurring along the boundaries, or transgranular, cutting through the crystals themselves.

In metallurgy, the size of the grains determines the strength of the metal. Reducing the grain size is a common way to improve strength. This is because smaller grains create more obstacles for dislocations to move through. A dislocation is a defect in the crystal lattice. This relationship between crystallite size and strength is known as the Hall–Petch relationship. However, grain boundaries also affect how materials deform over time through a process called creep. During grain boundary migration, shear stress causes grains to slide. Because fine-grained materials have more grain boundary sites, they may have poor resistance to creep at high temperatures.

Geology provides a natural look at how grain size relates to time. Coarse-grained rocks are formed very slowly on geological time scales. Fine-grained rocks are formed much more quickly. If lava from a volcano solidifies extremely fast, it may form no crystals at all. This process creates obsidian, which is an amorphous volcanic glass.

This shows how the speed of cooling directly dictates the internal structure of the resulting solid.

Modern technology relies heavily on controlling crystallites. Computer hard disks use ferromagnetic materials containing magnetic domains. These domains act like tiny grains that store data as bits, such as "1" or "0". Smaller grain sizes allow more data to be stored on a single disk. In aerospace engineering, grain boundaries can be a danger in superalloy turbine blades. To solve this, engineers use directional solidification. This process eliminates most grain boundaries by creating columnar grain structures. This results in blades that act like a single grain, making them much more reliable during flight.