Dislocation character

Table 1 Summary of the calculated properties of the various dislocations in NiAl. Dislocations are grouped together for different glide planes. The dislocation character, edge (E), screw (S) or mixed type (M) is indicated together with Burgers vector and line direction. The Peierls stresses for the (111) dislocations on the 211 plane correspond to the asymmetry in twinning and antitwinning sense respectively.

In spite of the very high APB energy, most of the dislocations split into two 1/2(111) super-partials as shown in Figure 2. This is true for all dislocation characters studied, except for the pure screw dislocation which is displayed in Figure 3. 

Figure 3.9 Part of a dislocation loop (a) pure edge, pure screw, and mixed dislocation character (b) glide is perpendicular to the Burgers vector, b, for the edge component, parallel to b for the screw component and at an angle to b for the mixed component and (c) continued glide results in removal of the dislocation from the crystal, leaving a step of height b on the surface.

Heterophase Interfaces. In certain cases, sharp heterophase interfaces are able to move in military fashion by the glissile motion of line defects possessing dislocation character. Interfaces of this type occur in martensitic displacive transformations, which are described in Chapter 24. The interface between the parent phase and the newly formed martensitic phase is a semicoherent interface that has no long-range stress field. The array of interfacial dislocations can move in glissile fashion and shuffle atoms across the interface. This advancing interface will transform... 

The ledge has zero dislocation character. A detailed discussion of the topological basis of these different types of line defects is given by Sutton and Balluffi [2]. 

Fig. 6.5. Dislocation line in a crystal. The dislocation has the same constant Burgers vector b over its whole length. The positions with edge dislocation character differ by opposite line vectors t. Thus, the crystal half-planes are added from the top in the one case and from the bottom in the other case...

In its current implementation, the CADD method can only deal with 2D problems, i.e., problems where the dislocations all have a line direction perpendicular to the modeled plane. However, the method does not contain any limitation on the dislocation character (edge, screw, or mixed) because periodic boundary conditions along the z direction are used in the atomistic region and three degrees of freedom (displacements U , Uy, and u ) are considered in the two-dimensional (x-y) continuum region. The extension of the methodology to the 3D case is not trivial and is currently being explored. 

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