Segregation in materials

In materials science, segregation refers to the enrichment of atoms, ions, or molecules at a microscopic region in a materials system. While the terms segregation and adsorption are essentially synonymous, in practice, segregation is often used to describe the partitioning of molecular constituents to defects from solid solutions, whereas adsorption is generally used to describe such partitioning from liquids and gases to surfaces. The molecular-level segregation discussed in this article is distinct from other types of materials phenomena that are often called segregation, such as particle segregation in granular materials, and phase separation or precipitation, wherein molecules are segregated in to macroscopic regions of different compositions. Segregation has many practical consequences, ranging from the formation of soap bubbles, to microstructural engineering in materials science, to the stabilization of colloidal suspensions.

Segregation can occur in various materials classes. In polycrystalline solids, segregation occurs at defects, such as dislocations, grain boundaries, stacking faults, or the interface between two phases. In liquid solutions, chemical gradients exist near second phases and surfaces due to combinations of chemical and electrical effects.

Segregation which occurs in well-equilibrated systems due to the instrinsic to the chemical properties of the system is termed equilibrium segregation. Segregation that occurs due to the processing history of the sample (but that would disappear at long times) is termed non-equilibrium segregation.

Equilibrium segregation is associated with the lattice disorder at interfaces, where there are sites of energy different from those within the lattice at which the solute atoms can deposit themselves. The equilibrium segregation is so termed because the solute atoms segregate themselves to the interface or surface in accordance with the statistics of thermodynamics in order to minimize the overall free energy of the system. This sort of partitioning of solute atoms between the grain boundary and the lattice was predicted by McLean in 1957.

Non-equilibrium segregation, first theorized by Westbrook in 1964, occurs as a result of solutes coupling to vacancies which are moving to grain boundary sources or sinks during quenching or application of stress. It can also occur as a result of solute pile-up at a moving interface.

There are two main features of non-equilibrium segregation, by which it is most easily distinguished from equilibrium segregation. In the non-equilibrium effect, the magnitude of the segregation increases with increasing temperature and the alloy can be homogenized without further quenching because its lowest energy state corresponds to a uniform solute distribution. In contrast, the equilibrium segregated state, by definition, is the lowest energy state in a system that exhibits equilibrium segregation, and the extent of the segregation effect decreases with increasing temperature. The details of non-equilibrium segregation are not going to be discussed here, but can be found in the review by Harries and Marwick.

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