Types of dislocations

In our definition, we did not specify the direction of the circuit, but it has to be chosen consistently. One simple way of doing this is to use a right-hand rule oriented on the line vector t of the dislocation line as shown in figure 6.3. 

The choice for the direction of the line vector is arbitrary as weU. If it is reversed, the orientation of the Burgers vector reverses as well. 

The second basic type of a dislocation, the screw dislocation is shown in figure 6.2(b). It can be visualised by imagining that the crystal has slipped by one atomic distance on a half plane ending at the dislocation line. The screw dislocation can also be characterised by its line vector and Burgers vector. The figure shows that both are parallel. If we move along a crystal plane around the dislocation, the resulting path is helical and thus looks like a screw, which explains the name of this dislocation type. 

Dislocation lines are always either closed or end at the surface of the crystal, but they can never end within the crystal. Why this is so can be seen from figure 6.4. Imagine that a dislocation would end somewhere within the crystal. The crystal is distorted in the vicinity of the dislocation line, but is perfect at a sufficient distance away from the dislocation. We now walk on a 

Burgers circuit around the dislocation line and find a non-vanishing Burgers vector. If we take an identical circuit somewhere far away from the dislocation line, the Burgers vector would vanish. As both paths lie completely within the undistorted perfect region of the crystal, it should be possible to create one from the other by a parallel shift, but then, both should have the same Burgers vector. Therefore, getting from one path to the other is only possible if we intersect the dislocation line somewhere. 

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One type of dislocation is the edge dislocation, illustrated in Fig. VII-7. We imagine that the upper half of the crystal is pushed relative to the lower half, and the sequence shown is that of successive positions of the dislocation. An extra plane, marked as full circles, moves through the crystal until it emerges at the left. The process is much like moving a rug by pushing a crease in it. 

Owing to the tetragonality of the Llo structure, both ordinary dislocations and super-dislocations may by present on (HR type plants. For example, for the (111) plane (see Fig.l), slip along [101] or [Oil] requires super-dislocations but only ordinary dislocations are needed for slip along [110]. As described in the Introduction both these types of dislocations may contribute to the plastic flow, together with twinning in... 

Dislocations Dislocations are stoichiometric line defects. A dislocation marks the boundary between the slipped and unslipped parts of crystal. The simplest type of dislocation is an edge dislocation, involving an extra layer of atoms in a crystal (Fig. 25.2). The atoms in the layers above and below the half-plane distort beyond its edge and are no longer planar. The direction of the edge of the half-plane into the crystal is know as the line of dislocation. Another form of dislocation, known as a screw dislocation, occurs when an extra step is formed at the surface of a crystal, causing a mismatch that extends spirally through the crystal. 

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. 

Dislocations are line defects that occur in crystals. There are many types of dislocation. The easiest to visualize are the edge dislocation, which consists of an extra half-plane of atoms inserted into a crystal and the screw dislocation that resembles... 

Once the parallel mismatch is determined, some information about average dislocation density in the interface may be obtained. It is not possible unambiguously to determine the types of dislocation present, since different... 

Dislocations. Screw dislocations are the most important defects when crystal growth is considered, since they produce steps on the crystal surface. These steps are crystal growth sites. Another type of dislocation of interest for metal deposition is the edge dislocation. Screw and edge dislocations are shown in Figure 3.4. 

The primary consideration we are missing is that of crystal imperfections. Recall from Section 1.1.4 that virtually all crystals contain some concentration of defects. In particular, the presence of dislocations causes the actual critical shear stress to be much smaller than that predicted by Eq. (5.17). Recall also that there are three primary types of dislocations edge, screw, and mixed. Althongh all three types of dislocations can propagate through a crystal and result in plastic deformation, we concentrate here on the most common and conceptually most simple of the dislocations, the edge dislocation. 

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. 

Different scale features give different scale properties. At the smallest level, the lattice parameter is a key length scale parameter for atomistic simulations. Since atomic rearrangement is intimately related to various types of dislocations, Orowan [88], Taylor [89], Polyani [90], and Nabarro [91] developed a relationship for dislocations that related stress to the inverse of a length scale parameter, the... 

What are dislocations What are the different types of dislocations ... 

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.

The aim of this chapter is to illustrate and interpret the TEM images of the various types of dislocations and dislocation microstructures that are... 

The large vacancy clusters are called voids. At higher temperatures these voids may collapse and form loops. These loops may be regarded as a special type of dislocation. Dislocations are present in every non-ideal material and determine its mechanical properties. The two main types are the edge and the screw dislocations. Defects are called edge dislocations when one plane of atoms in the lattice is missing or supernumerary screw dislocations are formed when a part of the crystal is displaced by an atomic layer. Fig. 14 illustrates the two types of dislocation. 

A dislocation is an imperfection which extends across many lattice units and represents a distortion of the lattice framework. Two distinct types of dislocation have been recognized. An edge dislocation (Figure 1.2.(a)) can be conveniently portrayed as the insertion of a plane of atoms part of the way through the crystal, requiring distortion of the adjoining planes, which bend inwards within a zone of strain beyond the "additional" plane to accommodate the standard spacing. A screw... 

The second type of dislocation (ii) corresponds to any segment of a vacancy or interstitial loop. Normally loops of this kind (see Fig. 40) do not occur in thermally treated graphite except under special conditions 164). They are common in annealed irradiated graphite (165), but they need not concern us here. 

The third type of dislocation (iii) is, by definition, a nonbasal screw. If one of these were of unit strength, i.e.. Burgers vector equal to c [0001], the schematic picture would be as in Fig. 41 and, on the atomic scale, the effect would be as depicted in Figs. 42a and 42b. Note the... 

Table III reflects again the very large difference in kinetic anisotropy between ideal sites and dislocation sites. But even though the difference is close to 10 decades, it is dangerous to relate all of it to the total energy of the dislocation. As with graphite, there is hardly any difference between the Rc/a values for emergent edges and emergent screws, so it is conceivable that both types of dislocation are housing...

The imperfections in crystals discussed so far are called point defects because they involve a single unit of the crystal structure, that is an atom or molecule. Another type of imperfection is known as a line defect or dislocation. There are two types of dislocations known as edge dislocations and screw dislocations. 

Burton-Cabrera-Frank (BCF) Model. The models discussed in the previous section all require two-dimensional nucleation events for a new layer to start. These models fail to account for observed crystal growth rates at low supersaturations and are unsatisfying in the sense that they make crystal growth a noncontinuous process with the formation of a critical size two-dimensional nucleus the rate-determining step. A basis for a model in which the steps are self-perpetrating was put forward by Frank (1949). Frank s idea was that dislocations in the crystal are the source of new steps and that a type of dislocation known as a screw dislocation could... 

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