Dislocations movement

If a sufficiently large shear stress acts on a dislocation, the dislocation moves through the crystal. How this happens is shown in figure 6.8 for an edge dislocation Near the dislocation line, the atoms are displaced from their equilibrium positions, stretching and compressing the atomic bonds. If an external shear stress is applied, trying to shift the upper crystal plane relative to the lower. 

The slip of a mixed dislocation follows from the cases already discussed. If we consider the example of a dislocation loop (figure 6.10), the loop increases or decreases its diameter when a shear stress is applied because the edge dislocation moves in the direction of the shear stress and the screw dislocation moves perpendicular to it. A dislocation loop changes its shape uniformly if both types of dislocation have the same mobility. 

on the other hand, one type of dislocation moves less easily than the other, dislocation movement is at first dominated by the more mobile type as sketched in figure 6.11. This increases the length of the less mobile type. In 

When the two vectors are parallel, the crystal planes perpendicular to the line form a helix, and the dislocation is said to be of the screw type. In a nearly isotropic crystal structure, the dislocation is no longer associated with a distinct glide plane. It has nearly cylindrical symmetry, so in the case of the figure it can move either vertically or horizontally with equal ease. 

Being the edge of a sheared area, a dislocation is a line, but does not, in general, lie on one plane, so its motion is usually three-dimensional. Since shear has two signs (plus and minus) so do dislocations and dislocations of like signs repel, while those of opposite signs attract. In some structures, the Burgers vector is an axial vector, so plus shear differs from minus shear (like a ratchet). 

A key feature of the motion of dislocation lines is that the motion is rarely concerted. One consequence is that the lines tend not to be straight, or smoothly curved. They contain perturbations ranging from small curvatures to cusps, and kinks. In covalent crystals where there are distinct bonds between the top 

When there are no distinct bonds crossing a glide plane, there are no distinct kinks. This is the case for pure simple metals, for pure ionic crystals, and for molecular crystals. However, the local region of a dislocation s core still controls the mobility in a pure material because this is where the deformation rate is greatest (Gilman, 1968). 

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Dislocation movement in copper is described by a slip plane 111 and a slip direction, the direction of dislocation movement, [110]. Each 111 plane can be depicted as a hexagonal array of copper atoms (Fig. 3.11a). The stacking of these planes is represented by the sequence. .. ABC. .. where the first layer is labeled A, the second layer, which fits into the dimples in layer A is labeled B... 

Precipitates have important effects on the mechanical, electronic, and optical properties of solids. Precipitation hardening is an important process used to strengthen metal alloys. In this technique, precipitates are induced to form in the alloy matrix by carefully controlled heat treatment. These precipitates interfere with dislocation movement and have the effect of hardening the alloy significantly. 

Beside dislocation density, dislocation orientation is the primary factor in determining the critical shear stress required for plastic deformation. Dislocations do not move with the same degree of ease in all crystallographic directions or in all crystallographic planes. There is usually a preferred direction for slip dislocation movement. The combination of slip direction and slip plane is called the slip system, and it depends on the crystal structure of the metal. The slip plane is usually that plane having the most dense atomic packing (cf. Section 1.1.1.2). In face-centered cubic structures, this plane is the (111) plane, and the slip direction is the [110] direction. Each slip plane may contain more than one possible slip direction, so several slip systems may exist for a particular crystal structure. Eor FCC, there are a total of 12 possible slip systems four different (111) planes and three independent [110] directions for each plane. The... 

Figure 3-3. Representation of dislocation movement in a Frank-Read dislocation source under stress a. Multiplication of dislocation pinned at a distance l.

The a-SiA10N has twice the unit-cell parameter in the c-direction compared to P-SiAlON, and the doubling of the Burgers vector for c[0001] dislocations means that dislocation movement is more difficult and the hardness is enhanced. 

When GBS is accommodated by some of the mechanisms involving dislocation movement or diffusion of point defects, the grains retain almost the original size and shape even after large deformations. This GBS, as the primary mechanism for deformation, is the basis for the high ductility exhibited... 

An important modification of this model was performed by Wakai.33 The main assumptions are that the solution and precipitation reactions take place at line defects as kinks in steps formed at the grain boundaries (Fig. 16.4), and the spacing between kinks is small enough for the step to be considered as an ideal source or sink of solute particles. Thus, the solution and precipitation of crystalline materials at these steps produces their movement, and consequently strain and strain rate will have an expression analogous to Orowan s equation for dislocation movement ... 

A7.3 Defects in GaN and related materials perfect dislocations, partial dislocations, dislocation movement and cracks... 

Dispersion hardening or strengthening of a material means an increased resistance to deformation. The movement of dislocations in the metal facilitates metal deformation. Incorporated particles block the dislocation movement and thus strengthen the metal.4,11 12,21 Grain refinement of the metal due to the codeposition of particles has also been thought to contribute to the hardening effect, but this is not supported by experimental evidence. For several composites it was found that the grain structure of the metal matrix was not altered by the codeposition of particles. 

Normally, dislocation-based plastic deformation is irreversible, that is, it is not possible to return the material to its original microstructural state. Remarkably, fully reversible dislocation-based compressive deformation was recently observed at room temperature in the layered ternary carbide Ti3SiC2 (Barsoum and El-Raghy, 1996). This compound has a hexagonal stmcture with a large cja ratio and it is believed that the dominant deformation mechanism involves dislocation movement in the basal plane. 

Dislocation movement requires only a small stress compared with that required for the simultaneous movement of one atomic plane over another because only a few atoms are directly involved in the slip process at any instant (see Figure 9.2). However, at higher temperatures, edge dislocations can move out of their slip planes by a process called climb, in which atoms (or vacancies) diffuse to, or away from, the dislocation core (Figure 9.3). The climb of dislocations is, therefore, an important process in high-temperature deformation. In some materials, deformation twinning may be important, especially at low temperatures. 

It has been shown that several polymers exhibit instabilities in their plastic deformation process. It should finally be mentioned that instabilities may also occur during the plastic deformation of metals This phenomenon which is called the Portevin-Le Chatelier effect, is generally interpreted in terms of different modes of dislocation movement depending on whether or not dislocations move by dragging along their atmosphere of impurities behind them. 

Foreman A. J. E. and Makin M. J., Dislocation Movement Through Random Arrays of Obstacles, Canadian J. Phys. 45, 511 (1967). 

For coarse-grained metals, dislocation movement and twinning are well known primary deformation mechanisms. Ultrafine, equiaxed grains with high-angle grain boundaries impede the motion of dislocations and... 

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