Edge and screw dislocations

The other major defects in solids occupy much more volume in the lattice of a crystal and are referred 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 temperatures 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. 

Dislocations are characterized by the Burgers vector, which is the extra distance covered in traversing a closed loop around the core of the dislocation, compared with the corresponding distance traversed in a normal lattice, and is equal to about one lattice spacing. This circuit is made at right angles to the dislocation core of an edge dislocation, but parallel to the core of a screw dislocation. 

Interface mismatch between two solids compensated with an edge dislocation 

As mentioned earlier, if there is a large disparity in structure at the film-substrate interface, such as a crystalline phase growing on an amorphous, glassy, substrate, the film may detach and grow a separate morphology. 

It is generally observed that the interface between two solids which results from the growth of one phase by condensation to form a film on the other is one in which the number of nearest neighbour bonds between the two phases is maximized. The close-packed planes tend to be found at the interface, which is consequently usually nearly atomically flat and this minimizes the interfacial 

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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... 

R.M. Thomson and R.W. Balluffi. Kinetic theory of dislocation climb I. General models for edge and screw dislocations. J. Appl Phys., 33(3) 803—817, 1962. 

Poisson s ratio v is approximately 0.3 for many solids, so to a fair approximation, the energy difference between edge and screw dislocations can be ignored. Following Cahn,... 

Edge and screw dislocations are abrupt changes in the regular ordering of atoms along an axis in the crystal (Figure 2.29). These line defects are introduced during the crystallization process, and may not always be detrimental. As we will see in Chapter 3, it is the dislocation interactions within a metal that are responsible for... 

The structure and stability of ceria films grown on mismatched substrates have been studied computationally. A 36% lattice mismatch in growth of CeO on YSZ is found to be accommodated by the formation of islands and dislocation arrays such as edge and screw dislocations. A configuration obtained by simulation is shown in Fig. 9.2 and demonstrates the complexity of defect structures that might... 

Other, nonfocal defects also occur in smectics. These include walls, such as the tilt wall depicted in Fig. 10-34a, as well as dislocations such as the edge and screw dislocations depicted in Fig. 10-34b and 10-34c. The most common defects in small-molecule smectics are those that maintain a constant lamellar spacing, such as focal domains, screw dislocations, and walls. Edge dislocations seem to be more common in lamellar block copolymers, which also have smectic symmetry (see Chapter 13). 

OD point inhomogeneities, such as atomic disorder, vacancies, interstitials, emergence points of edge and screw dislocations,... 

The values in Figure 13.13 for edge and screw dislocations define upper and lower bounds for As the average contrast factors for the limiting cases of... 

The superscripts e and 5 refer to edge and screw and serve as an instruction to use the isotropic linear elastic stress fields for the edge and screw dislocation, respectively, but with the Burgers vector adjusted to account for relevant trigonometric weighting factors. 

Recent advances in applying transition state theory to geochemical kinetics (SQ, SD have emphasized the interaction of the activated complex with specific surface reaction sites. The rate of reaction is assumed to be a function of the surface reaction site density. A correspondence is also observed between surface dissolution features such as etch pits, and crystallographically controlled extended defect features such as edge and screw dislocations (S2). Based on these lines of evidence, the reactive surface area has been proposed to be proportional to the defect density within minerals... 

Indicate whether the Burgers vector is parallel or perpendicular to the edge and screw dislocation lines. 

The two main types of line defect which can play an important role in the model of crystal growth are the edge and screw dislocations. Both of these are responsible for slip or shearing in crystals. Large numbers of dislocations occur in most crystals they form readily during the growth process under the influence of surface and internal stresses. 

In Section 9.4 we present analyses of nucleation of both edge- and screw-dislocation half loops from edges and surfaces of lamellae in spherulites under stress in PE samples with a wide range of lamella thicknesses and compare the predictions with the plastic deformation rate. 

As shown in Fig. 20.36b, an edge dislocation intersecting a free surface does not cause a step. However, the atoms adjacent to the core of the dislocation, being in non-equilibrium positions, have a higher energy than other atoms in the surface. The emergent ends of both edge and screw dislocations thus represent potential sites for preferential dissolution. Finally, there may be vacancies in the surface and ad-atoms on the surface (Fig. 20.36b). 

Just as in the smectic phases and in solids, both edge and screw dislocations can occur. Three typical examples are illustrated in Fig. 9.20. In fact, big deformations do not involve dislocations (or only slightly), but simply the column curvature, which costs far less in energy terms. This feature is easy to understand by considering an example. Let us try to construct a columnar system from a cylinder of revolution of radius Tq with the boundary condition that the columns should be orthogonal to the surface of the cylinder where they meet it. Requiring the columns to be straight, either the space will not... 

As indicated in Sect. 3.3.1, dislocations are line defects. The two basic types of dislocations are the edge and screw dislocations. A schematic three-dimensional (3D) illustration of an edge dislocation appears in Fig. 3.25. A (100) plane of Fig. 3.25 in a simple cubic crystal is illustrated schematically in Fig. 3.28. This illustration will help to define the Burgers vector later on. Figure 3.29 is a schematic view of edge and screw dislocations. 

Fig. 3.51 The intersection of edge and screw dislocations a before intersection b after intersection [14]...

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