Misorientation dislocations

Low-angle tilt boundaries are the most easily visualized. Two regions of crystal separated by a slight misorientation can be drawn as a set of interlocking steps (Fig. 3.21a)- The edge dislocations coincide with the steps. The separate parts can be linked to make the edge dislocation array clearer (Fig. 3.21b). In the situation in which the misorientation between the two parts of the crystal is 0, the distance between the steps A and C is given by twice the dislocation separation, 2d, where... 

Figure 3.1 A schematic representation of the defects common in epilayer structures, (a) mismatch, (b) misorientation, (c) dislocation content, (d) mosaic spread, (e) curvature, (f) relaxation, (g) inhomogeneity...

The width of the image can be deduced using this simple idea of contrast being formed when the misorientation around the defect exceeds the perfect crystal reflecting range. We consider the case of a screw dislocation nmning normal to the Bragg planes, where the line direction / coincides with the diffraction vector g. The effective misorientation at distance r from the core is =bH r (8.41)... 

Miltat and Bowen showed that direct images can be synthesised from the cylinders of misorientation drawn aroimd a dislocation line using continuum elasticity theory. The image full width can be calculated from the projected width circumscribed by the contour where ( ) is equal to times the reflecting range,... 

As seen in the last chapter, the image width is easy to quantify for a screw dislocation, where the diffraction vector is parallel to the dislocation line. Around a screw dislocation, the misorientation at a distance r from the core is given by... 

A, B, and C in vicinal (001) twist grain boundary in Au. Static array of screw dislocations in background accommodates the twist deviation of the vicinal boundary shown from the crystal misorientation of the nearby singular twist boundary to which it is vicinal. Excess selfinterstitial defects were produced m the specimen by fast-ion irradiation and were destroyed at the grain-boundary dislocations by climb, causing the boundary to act as a defect sink, (a) Prior to irradiation, (b) Same area as in (a) after irradiation, (c) Diagram showing the extent of the climb. From Komer et al. [24],... 

The line defects which are either dislocations or dislocation/ledges may be further classified as intrinsic or extrinsic. So far, only intrinsic line defects have been considered. These line defects are arranged in uniform arrays and accommodate deviations of interface misorientation and/or inclination from certain reference structures. As part of the minimum-energy equilibrium structure of the interfaces, they are termed intrinsic. On the other hand, similar line defects can be present in interfaces in a more or less random fashion, so that their Burgers vectors cancel. In... 

At low angles, y is proportional to 9, but at higher angles, ytO decreases, as illustrated in Figure 12.8. Screw dislocations onaplane form twist boundaries. The misorientation across a low-angle twist boundary and its energy are proportional to the number of dislocations. 

We begin our geometric discussion with the case of pure tilt boundaries on the grounds that the connection between dislocations and the boundary is most evident visually in this case. As was discussed in some detail in chap. 9, a tilt grain boundary is characterized by a simple misorientation between the two grains as was discussed in chap. 9. The intent of this section is to illustrate that a simple superposition of the elastic displacements implied by the Volterra solution for straight dislocations discussed in chap. 8 can lead to exactly the same type of misorientation. 

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