Grain boundary

Grain boundaries are two-dimensional defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material.

[2] On the other hand, grain boundaries disrupt the motion of dislocations through a material, so reducing crystallite size is a common way to improve mechanical strength, as described by the Hall–Petch relationship.

This boundary can be conceived as forming from a single, contiguous crystallite or grain which is gradually bent by some external force.

The energy associated with the elastic bending of the lattice can be reduced by inserting a dislocation, which is essentially a half-plane of atoms that act like a wedge, that creates a permanent misorientation between the two sides.

It is now accepted that a boundary consists of structural units which depend on both the misorientation of the two grains and the plane of the interface.

Deviations from the ideal CSL orientation may be accommodated by local atomic relaxation or the inclusion of dislocations at the boundary.

Generally, the convenience of ignoring the boundary plane orientation, which is very difficult to determine, outweighs the reduced information.

The energy of a low-angle boundary is dependent on the degree of misorientation between the neighbouring grains up to the transition to high-angle status.

Although theory predicts that the energy will be a minimum for ideal CSL configurations, with deviations requiring dislocations and other energetic features, empirical measurements suggest the relationship is more complicated.

[6] It describes how much expansion is induced by the presence of a GB and is thought that the degree and susceptibility of segregation is directly proportional to this.

[7] Experimental techniques have been developed which directly probe the excess volume and have been used to explore the properties of nanocrystalline copper and nickel.

A key observation is that there is an inverse relationship with the bulk modulus meaning that the larger the bulk modulus (the ability to compress a material) the smaller the excess volume will be, there is also direct relationship with the lattice constant, this provides methodology to find materials with a desirable excess volume for a specific application.

Both low- and high-angle boundaries are retarded by the presence of particles via the so-called Zener pinning effect.

This effect is often exploited in commercial alloys to minimise or prevent recrystallization or grain growth during heat-treatment.

Grain boundaries are the preferential site for segregation of impurities, which may form a thin layer with a different composition from the bulk and a variety of atomic structures that are distinct from the abutting crystalline phases.

Grain boundary complexions were introduced by Ming Tang, Rowland Cannon, and W. Craig Carter in 2006.

[14] Grain boundaries can be analyzed using equilibrium thermodynamics but cannot be considered as phases, because they do not satisfy Gibbs' definition: they are inhomogeneous, may have a gradient of structure, composition or properties.

For this reasons they are defined as complexion: an interfacial material or stata that is in thermodynamic equilibrium with its abutting phases, with a finite and stable thickness (that is typically 2–20 Å).

[16] Grain boundaries can cause failure mechanically by embrittlement through solute segregation (see Hinkley Point A nuclear power station) but they also can detrimentally affect the electronic properties.

In metal oxides it has been shown theoretically that at the grain boundaries in Al2O3 and MgO the insulating properties can be significantly diminished.

[17] Using density functional theory computer simulations of grain boundaries have shown that the band gap can be reduced by up to 45%.

[20][21][22] Interesting examples of the complications of how point defects behave has been manifested in the temperature dependence of the Seebeck effect.

[23] In addition the dielectric and piezoelectric response can be altered by the distribution of point defects near grain boundaries.

[25][26] It has also been found that the Kondo effect within graphene can be tuned due to a complex relationship between grain boundaries and point defects.

[27] Recent theoretical calculations have revealed that point defects can be extremely favourable near certain grain boundary types and significantly affect the electronic properties with a reduction in the band gap.

[28] There has been a significant amount of work experimentally to observe both the structure and measure the properties of grain boundaries but the five dimensional degrees of freedom of grain boundaries within complex polycrystalline networks has not yet been fully understood and thus there is currently no method to control the structure and properties of most metals and alloys with atomic precision.