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,[1] whereas adsorption is generally used to describe such partitioning from liquids and gases to surfaces.
Segregation has many practical consequences, ranging from the formation of soap bubbles, to microstructural engineering in materials science,[2] to the stabilization of colloidal suspensions.
In polycrystalline solids, segregation occurs at defects, such as dislocations, grain boundaries, stacking faults, or the interface between two phases.
This sort of partitioning of solute atoms between the grain boundary and the lattice was predicted by McLean in 1957.
[3] Non-equilibrium segregation, first theorized by Westbrook in 1964,[4] occurs as a result of solutes coupling to vacancies which are moving to grain boundary sources or sinks during quenching or application of stress.
The details of non-equilibrium segregation are not going to be discussed here, but can be found in the review by Harries and Marwick.
[6] Segregation of a solute to surfaces and grain boundaries in a solid produces a section of material with a discrete composition and its own set of properties that can have important (and often deleterious) effects on the overall properties of the material.
Segregation to grain boundaries, for example, can lead to grain boundary fracture as a result of temper brittleness, creep embrittlement, stress relief cracking of weldments, hydrogen embrittlement, environmentally assisted fatigue, grain boundary corrosion, and some kinds of intergranular stress corrosion cracking.
[7] A very interesting and important field of study of impurity segregation processes involves AES of grain boundaries of materials.
This technique includes tensile fracturing of special specimens directly inside the UHV chamber of the Auger Electron Spectrometer that was developed by Ilyin.
[10] Segregation to free surfaces also has important consequences involving the purity of metallurgical samples.
In applications where an ultra-pure surface is needed (for example, in some nanotechnology applications), the segregation of impurities to surfaces requires a much higher purity of bulk material than would be needed if segregation effects did not exist.
Thus, a greater understanding of all of the mechanisms surrounding segregation might lead to being able to control these effects in the future.
[11] Modeling potentials, experimental work, and related theories are still being developed to explain these segregation mechanisms for increasingly complex systems.
Several theories describe the equilibrium segregation activity in materials.
[12] This is the earliest theory specifically for grain boundaries, in which McLean[3] uses a model of P solute atoms distributed at random amongst N lattice sites and p solute atoms distributed at random amongst n independent grain boundary sites.
McLean used basic statistical mechanics to find the fractional monolayer of segregant,
, at which the system energy was minimized (at the equilibrium state), differentiating G with respect to p, noting that the sum of p and P is constant.
This method gives values correct to within a factor of two (as compared with experimental data for grain boundary segregation), but a greater accuracy is obtained using the method of Seah and Hondros,[10] described in the following section.
and the equation becomes This theory for grain boundary segregation, derived from truncated BET theory, provides excellent agreement with experimental data obtained by Auger electron spectroscopy and other techniques.
If, in a binary system, adjacent adsorbate atoms are allowed an interaction energy
Guttman, in 1975, extended the Fowler theory to allow for interactions between two co-segregating species in multicomponent systems.
This modification is vital to explaining the segregation behavior that results in the intergranular failures of engineering materials.
The surface segregation enrichment ratio increases when the solute atom size is larger than the matrix atom size and when the melting point of the solute is lower than that of the matrix.
In the presence of a coverage of a chemisorbed species theta, it is proposed that the Langmuir-McLean model is valid with the free energy of surface segregation given by
At high temperatures, evaporation from the surface can take place, causing a deviation from the McLean equation.
to be a constant, but in practice this is only true for dilute systems with low segregation levels.
[12] All metal castings experience segregation to some extent, and a distinction is made between macrosegregation and microsegregation.
Microsegregation refers to localized differences in composition between dendrite arms, and can be significantly reduced by a homogenizing heat treatment.
This is possible because the distances involved (typically on the order of 10 to 100 μm) are sufficiently small for diffusion to be a significant mechanism.