Types of grain boundaries

The following is a description of grain boundaries that is based primarily on the degree of misorientation between the adjacent grams. 

Small-angle boundaries are ones across which the difference in orientation is small. They are also called subgrain boundaries. There are two general classes of small-angle houndaries tilt boundaries, which comprise an array of edge dislocations, and twist boundaries, which comprise two or more arrays of screw dislocations. (The reader who is unfamiliar with the line defects known as dislocations will find a clear and readable description in Reference [6].) 

The geometry of the non-symmetric tilt boundaries which form when the two crystals are not mirror images of each other is more complex and must be described in terms of two arrays of edge dislocations [6]. 

High-resolution transmission electron microscopy (HRTEM) images of the atomic positions in several tilt grain boxmdaries are provided in Reference [7]. 

Weertman, Elementary Dislocation Theory (Oxford Oxford University Press, 1992) and is reproduced by permission of Oxford University Press.) 

---

Another simple type of grain boundary is the twist boundary, where the lattice planes of the grains are rotated relative to each other. In this case the interface consists of a cross grid of screw dislocations. In the more general case, combinations of these two simple types of dislocation will occur. 

There are several contradictions in the concept of pressure solution. For instance, it is widely believed that stylolites are the result of this mechanism. From a solid state point of view they are most likely associated with the formation of the broken-sinusoidal type of grain boundaries, probably due to their migration. Grain boundaries with a crystallographic disorientation, due to a grain s tendency to regular orientation, can locally bend (Poirier 1983). 

When the epilayer contains grain boundaries, the thin-film approach allows us to examine how the nucleation rate depends on the type of grain boundary (misorienta-tion, grain-boundary plane, etc.) intersecting the phase boundary. The growth of the product will depend on the nucleation site. 

A special type of grain boundary, known as crystal twinning, occurs when two crystals of the same type intergrow, so that only a shght misoiientation exists between them. Twinned crystals may form by inducing alteratirms in the lattice... 

A low angle tilt boundary is a special type of grain boundary in which a portion of the grain is tilted as a result of a periodic array of edge dislocations as shown in Figure 8.11. Similarly, a low angle twist boimdary can be formed by an ordered array of screw dislocations. 

A twin boundary is a special type of grain boundary across which there is a specific mirror lattice symmetry that is, atoms on one side of the boundary are located in mirror-image positions to those of the atoms on the other side (Figure 4.10). The region of... 

The existence of an additional relaxation mechanism has been noted by several authors for the heterogeneous system of block and graft copolymers and been assigned to a type of grain-boundary phenomena(20), i.e., interfacial... 

---