Screw dislocation curvature

Figure 6.9 Curvature of a screw dislocation near a free surface a. screw dislocation that has moved about halfway thru the specimen and is emerging from the surface, b. by becoming curved the screw dislocation reduces its length and hence its energy.

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... 

Frank (1949) suggested that screw dislocations having a displacement vector parallel to the surface would provide a source of steps which would persist and not grow out. In this case, the lateral growth can take place continuously without formation of new steps. If a step is not flat but has a curvature p, the velocity of movement of a straight step (eq. 5) can be replaced by... 

Figure 4.6 (a) Schematic representation of a dislocation that has edge, screw, and mixed character, (b) Top view, where open circles denote atom positions above the slip plane, and sohd circles, atom positions below. At point A, the dislocation is pure screw, while at point B, it is pure edge. For regions in between where there is curvature in the dislocation Une, the character is mixed edge and screw, pigure (/>) from W. T. Read,... 

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