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The mystery of low-angle grain boundaries and high-angle grain boundaries: What are the surprising differences between them?
In materials science, a grain boundary is the interface between two grains or crystals in a polycrystalline material. Grain boundaries are two-dimensional defects that significantly reduce the electrical and thermal conductivities of materials. Many grain boundaries are preferred locations for corrosion, and they are also prime sites for the precipitation of new phases in solids. In addition, grain boundaries are involved in many creep mechanisms. However, grain boundaries can also disrupt the movement of dislocations in materials, so reducing grain size is a common way to improve mechanical strength. This relationship is called the Hall-Petch relationship.
There is a fundamental difference between low-angle grain boundaries (LAGB) and high-angle grain boundaries (HAGB) in terms of the degree of lattice dislocation.
According to the degree of dislocation between grains, grain boundaries can be divided into low-angle grain boundaries (dislocation less than about 15 degrees) and high-angle grain boundaries (dislocation greater than about 15 degrees). Generally, low-angle grain boundaries are composed of dislocation arrays, and their properties and structure are related to the degree of dislocation. The properties of high-angle grain boundaries are usually independent of the degree of dislocation, which is related to the type of material. The relevant "special interface" has an interface energy significantly lower than that of ordinary high-angle grain boundaries.
The formation of a wave, in simple terms, involves a boundary that may be a sloped boundary, where the axis of rotation is parallel to the boundary surface. This can be viewed as continuous grains that are bent due to some external force. Inserting dislocations can reduce the energy associated with the elastic bending of the crystal lattice, allowing these dislocations to maintain persistent dislocations on both sides. As the grains bend further, more and more dislocations are introduced to accommodate the deformation, eventually forming low-angle grain boundaries.
Compared with low-angle grain boundaries, high-angle grain boundaries are more disordered, have larger mismatch areas and a relatively open structure.
In fact, early scientists thought that high-angle grain boundaries might be an amorphous or liquid layer, but later with the development of electron microscopy, directly observed grain boundary structures overturned this assumption. It is now generally accepted that the boundary is composed of structural units, which depends on the degree of misalignment of the two grains and the nature of the interface plane.
The energy of grain boundaries and their behavior are very complex. The energy of the low-angle boundary varies with the degree of dislocation between adjacent grains, and when the dislocation reaches the high-angle boundary state, the energy will be higher. For high-angle bounds, it depends more on the atomic structure and its physical and chemical binding properties. Some results suggest that simple relationships at low Σ may mislead people's understanding of grain boundary energies, and that a more comprehensive approach should be taken to explore the structural and chemical context in which these changes arise.
Grain boundaries are preferred locations for contaminants to accumulate, forming thick films that can significantly change the properties of materials.
Like the energy of grain boundaries, the excess volume of grain boundaries is another important feature. Excess volume refers to the degree of expansion due to the presence of grain boundaries, which is closely related to the distribution of contaminants and the nature of the grain boundaries. With the advancement of science and technology, especially the observation and analysis of nanomaterials, researchers have found that the relationship between excess volume and various properties of the substrate (such as mechanical properties and electrical properties) is very complex.
The migration process of grain boundaries has an important impact on recrystallization and grain growth. In particular, the movement of low-angle grain boundaries (LAGB) will significantly affect the recovery process. This process is usually affected by pressure, and the speed at which the boundary moves is usually proportional to the pressure. Overall, the moving speed of low-angle grain boundaries shows lower mobility than high-angle grain boundaries, mainly based on structures containing dislocations. As deformation proceeds, this structure must withstand pressure from the surroundings.
The structure of high-angle grain boundaries determines the ability of atoms to transfer, which depends on the chemical composition and temperature between the grains.
Research in recent years has shown that the movement of low-angle and high-angle grain boundaries can be affected by particles, such as the Zener locking phenomenon, which is often exploited in commercial alloys during heat treatment to minimize or prevent recrystallization or Grain growth technology. Combined with the current understanding of grain boundary composite structures, grain boundaries are no longer regarded as simple interfaces, but as a variable layer that can affect material properties, providing many new research directions.
Finally, the properties of grain boundaries and their impact on material properties are gradually being explored and exhibit complex behaviors, which makes people think: How will future materials science profoundly change our understanding and application of grain boundaries?
