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Structural Description of Materials
Published in Snehanshu Pal, Bankim Chandra Ray, Molecular Dynamics Simulation of Nanostructured Materials, 2020
Snehanshu Pal, Bankim Chandra Ray
Dislocation loop is a phenomenon in which the dislocation lines end on it. The dislocation line is a closed curved loop line, where the dislocation changes its character from point to point, as shown in Figure 1.58. As we follow the dislocation line, the angular relationship between the tangent vector and the Burgers vector keeps on changing. One special case of dislocation loop is prismatic dislocation loop. In this type of dislocation loop, the Burgers vector is perpendicular to the plane of the loop. The Burgers vector is perpendicular to the tangent vector for all tangent vectors of the loop, and the entire loop has the edge character. More specifically, the plane of the loop is not the slip plane; it is rather the plane surface, which is cylindrical. Here, the Burgers vector does not lie on this plane; rather, it lies on the periphery of the cylindrical surface, as shown in Figure 1.59. Here, the slip surface is the prismatic surface containing both the Burgers vector and the tangent vector, having dislocation line as its base. Vacancy loop is an example of prismatic dislocation loop. This kind of situation occurs in metals, which are quenched from a higher temperature because of the presence of a large number of vacancies available and the metal wanting to get rid of them.
Cholesteric dislocations in mica wedges
Published in Liquid Crystals Reviews, 2021
Let us consider a prismatic surface, shown in Figure 25(e), with the base having the shape of the double line loop with one cusp shown in Figure 25(b). Its lateral size is such that it is located in the area where the cholesteric texture is made of N = 4 layers of thickness p/2. For the sake of readability, its height is exaggerated by a large factor. In the first approximation we will suppose that the cusp angle is negligibly small. Let us cut this surface along the vertical line passing through the tip of the cusp and unroll it (see Figure 25(f)) into the rectangle shown in Figure 25(g).