Dislocation types

Dislocation Type Burger s Vector Propagation Direction... 

As seen from Fig. 2a, the van der Waals gap width is modulated periodically in positions of In atoms it is larger than in positions of Se atoms. This gap can be described as a layer of closely packed parallelepipeds, at the both ends of which pyramids are placed. The volume of this body (cavity) is equal to 50.5 A3, and for an ideal crystal, when defects of the dislocation type are absent the relative gap volume comprises 43% of the crystal bulk. It is obviously larger than the lower estimate 37% obtained using the ratio Q/(Q+Ci). 

The closest-packed planes are planes with minimum surface energy, and for this reason they are most commonly either faceting planes or slide planes. It has been observed that closest-packed layers are typical of intergrown crystals, or twins (Belov 1976). As mentioned above, one dislocation type is the closest-packing defect ... 

Fig. 37. Structural model of the Si(100)2xw -Bi surface as revealed by STM [94P2]. Bi atoms forming Bi dimers (black circles), the missing Bi dimer rows (MR) and the dislocation-type defects (in the middle of the upper three rows) are shown. A similar structure is formed in Bi/Ge(100) system.

This section discusses the generation mechanisms for type-I and type-II dislocations. Although the lattice mismatch for GaAs/Si and GaAs/GaP are almost the same, the types of dislocations are different. Therefore, the type of dislocation cannot be explained alone by lattice mismatch. Furthermore, the thermal stress in the epitaxial layer cannot explain the difference of the dislocation type because the thermal stress for GaAs/Si and GaP/Si is almost the same although the types of dislocations are different. 

There may be dislocation of the facet joints in lap belt injuries. This can be seen as widening, superior subluxation, perching or locking of the facets. Facet malalignment is best appreciated on CT reconstructions. Rotatory stability of the thoracolumbar spine is provided by the facet joints and these are injured in significant fracture dislocations caused by combined flexion and rotation. The major hallmark of fracture dislocation type of injuries is intervertebral subluxation or dislocation. 

After the above conceptual discussion of dislocation and some of its features, the following presents relevant experimental observations regarding dislocation types... 

The above model based on dislocation-type defects in polymer glasses is attractive in that it allows one to put some order and physical interpretation to... 

Quantitative images based on dynamic theory can be computer simulated by using the methods developed in [162]. A comparison of simulated and observed images allows determination of all the relevant parameters of various dislocation types (Fig, 34). More details are given in [163]. 

The second basic type of a dislocation, the screw dislocation is shown in figure 6.2(b). It can be visualised by imagining that the crystal has slipped by one atomic distance on a half plane ending at the dislocation line. The screw dislocation can also be characterised by its line vector and Burgers vector. The figure shows that both are parallel. If we move along a crystal plane around the dislocation, the resulting path is helical and thus looks like a screw, which explains the name of this dislocation type. 

The smectic analog of a vortex is a dislocation. Type II smectics would therefore develop dislocations when submitted to a bending or twisting stress whereas type I would resist until a critical stress eventually induces the nematic state. 

HiU and Rowcliffe reported dislocations features of the high-stress type after indentation at 300 °C, as can be seen in Fig. 6 of Ref. [53]. These dislocations were not thought to be associated to the deformation process itself they were assumed to be geometrically required at the ends of block shp displacements of the material under large stresses, close to the theoretical shear stress value. Similar dislocation types were also found by another group after room temperature indentation in the 1970s, but they were assumed to be irrelevant (V.G. Eremenko, private communication). 

Figure 11.11 shows the resulting hydrostatic pressure fields for a FS partial dislocation l/6[2-203], running in the [1120] direction (which was found as major dislocation type in the films) in comparison with the isotropic approximation. The effect of the anisotropy becomes clearly visible in this figure by a strong elongation of the pressure field in the (1100) directions. 

Most dislocations found in crystalline materials are probably neither pnre edge nor pure screw but exhibit components of both types these are termed mixed dislocations. All three dislocation types are represented schematically in Fignre 4.6 the lattice distortion that is produced away from the two faces is mixed, having varying degrees of screw and edge character. 

The motion of a screw dislocation in response to the applied shear stress is shown in Figure 7.26 the direction of movement is perpendicular to the stress direction. For an edge, motion is parallel to the shear stress. However, the net plastic deformation for the motion of both dislocation types is the same (see Figure 7.2). The direction of motion of the mixed dislocation line is neither perpendicnlar nor parallel to the apphed stress, bnt lies somewhere in between. 

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