Screw dislocation core

Dislocations are characterized by the Burgers vector, which is the exua distance covered in traversing a closed loop around die core of the dislocation, compared with the conesponding 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. 

The core structure of the (100) screw dislocation is planar and widely spread w = 2.66) on the 011 plane. In consequence, the screw dislocation only moves on the 011 glide plane and does so at a low Peierls stress of about 60 MPa. 

Figure 3 Core configuration of the (111) screw dislocation. The Burgers vector distribution is calculated for a 211 cut and clearly shows a compact dislocation core.

Fig. 2. Schematic pictures of the core structure of the 1/2 [110] screw dislocation in TlAl. (a) Planar core spread in the (111) plane, (b) Non-planar core spread in both (111) and (111) planes.

Fig. 4. Schematic picture of the core structure of the )/ 112] screw dislocation.

Figure 5.10 Schematic dislocation core. Arrangement at kink on screw dislocation line.

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)... 

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... 

Next, let us compile some quantitative relations which concern the stress field and the energy of dislocations. Using elastic continuum theory and disregarding the dislocation core, the elastic energy, diS, of a screw dislocation per unit length for isotropic crystals is found to be... 

The possibility that the core of a dislocation could be empty was first recognised by Frank [1], If the strain energy density arising from a dislocation is sufficiently large, it may become energetically favourable to remove the material near the core and place it in an unstrained environment far from any dislocations. This process creates additional surface area around the core of the dislocation. The equilibrium radius of the hollow core of a screw dislocation is given by... 

From atomistic aspects, a metal can be considered as a fixed lattice of positive ions permeated by a gas of free electrons [1], Positive ions are the atomic cores, while the electrons are the valence electrons. Since there are about 1022 atoms in 1 cm3 of a metal, one can expect that some atoms are not exactly in their right place. Thus, one can expect that a real lattice will contain defects (imperfections). The most common defects are point defects (e.g. a vacancy, an interstitial) and dislocations (e.g. the edge dislocation, screw dislocation) [2]. 

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