Dislocation lines edge type

Table 1 Summary of the calculated properties of the various dislocations in NiAl. Dislocations are grouped together for different glide planes. The dislocation character, edge (E), screw (S) or mixed type (M) is indicated together with Burgers vector and line direction. The Peierls stresses for the (111) dislocations on the 211 plane correspond to the asymmetry in twinning and antitwinning sense respectively.

The Burgers vector of a dislocation can lie at any angle to the dislocation line. Although there are many different types of dislocations, they can all be thought of as combinations of two fundamental types, edge dislocations, which have Burgers vectors perpendicular to the dislocation line, and screw dislocations, with Burgers vectors parallel to the dislocation line. 

The second type of line defect, the screw dislocation, occurs when the Burger s vector is parallel to the dislocation line (OC in Figure 1.33). This type of defect is called a screw dislocation because the atomic structure that results is similar to a screw. The Burger s vector for a screw dislocation is constructed in the same fashion as with the edge dislocation. When a line defect has both an edge and screw dislocation... 

Figure 10.8. The two extreme types of dislocations. In the edge dislocation (a), the Burgers vector is perpendicular to the dislocation line. In the screw dislocation (b), the Burgers vector is parallel to the dislocation line.

Line imperfections are called dislocations and occur in crystalline materials only. Dislocations can be an edge type, screw type, or mixed type, depending on how they distort the lattice, as shown in Figure 8. It is important to note that dislocations cannot end inside a crystal. They must end at a crystal edge or other dislocation, or they must close back on themselves. 

Line defects are dislocations around which some of the atoms of the crystal lattice are misaligned. There are two types of dislocations the edge dislocation and the screw dislocation. Dislocations are caused by the termination of a plane of atoms in the middle of a crystal. In such a case, the surrounding planes are not straight, but instead bend around the edge of the terminating plane so that the crystal structure is perfectly ordered on either side. 

For lattice mismatched III-V semiconductors on Si, two kinds of misfit dislocations are observed one is the pure-edge Lomer misfit dislocation, whose Burgers vector is parallel to the interface (type-I dislocation), and the other is the misfit dislocation, whose Burgers vector is 60° from the dislocation line (60° dislocation or type-II dislocation) [34]. Schematic illustrations of type-I and type-I I dislocations are shown in Fig. 7(a) and 7(b), respectively. 

These particularities are somewhat exotic. The topological defects related to the existence of layers are of a more classic type. In the case of edge dislocations, they correspond to addition or removal of a layer. In Fig. 9.14a, the layer is added along the normal to the plane of the page, called the dislocation line. Figure 9.14b shows the more subtle case of a screw dislocation, which resembles a renaissance staircase, connecting the various layers. Once again, far from... 

There are different types of dislocations, depending on the crystal structure and the Burgers vector. Another characteristic example is a screw dislocation, which has a Burgers vector parallel to its line, as shown in Fig. 10.2. Dislocations in which the Burgers vector lies between the two extremes (parallel or perpendicular to the dislocation line) are called mixed dislocations. A dislocation is characterized by the direction of the dislocation line, denoted by and its Burgers vector b. For the two extreme cases, edge and screw dislocation, the following relations hold between... 

It is found that in metal crystals slip can take place at stresses well below the theoretically-calculated shear stress. The movement of line defects known as dislocations was invoked to account for this observation and with the advent of electron microscopy the presence of such defects was finally proved. The two basic types of dislocations found in crystals, screw and edge, are shown in Fig. 4.23. The dislocation is characterized by its line and Burgers vector. If the Burgers vector is parallel to the line it is termed a screw dislocation and if it is perpendicular it is called an edge dislocation. In general the Burgers vector and dislocation line may be at any angle as... 

The motion of a screw dislocation in response to the applied shear stress is shown in Figure 7.26 the direction of movement is perpendicular to the stress direction. For an edge, motion is parallel to the shear stress. However, the net plastic deformation for the motion of both dislocation types is the same (see Figure 7.2). The direction of motion of the mixed dislocation line is neither perpendicnlar nor parallel to the apphed stress, bnt lies somewhere in between. 

The other major defects in solids occupy much more volume in the lattice of a crystal and are refeiTed to as line defects. There are two types of line defects, the edge and screw defects which are also known as dislocations. These play an important part, primarily, in the plastic non-Hookeian extension of metals under a tensile stress. This process causes the translation of dislocations in the direction of the plastic extension. Dislocations become mobile in solids at elevated temperamres due to the diffusive place exchange of atoms with vacancies at the core, a process described as dislocation climb. The direction of climb is such that the vacancies move along any stress gradient, such as that around an inclusion of oxide in a metal, or when a metal is placed under compression. 

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