Dislocation bending

Once the precipitates grow beyond a critical size they lose coherency and then, in order for deformation to continue, dislocations must avoid the particles by a process known as Orowan bowing(23). This mechanism appHes also to alloys strengthened by inert dispersoids. In this case a dislocation bends between adjacent particles until the loop becomes unstable, at which point it is released for further plastic deformation, leaving a portion behind, looped around the particles. The smaller the interparticle spacing, the greater the strengthening. 

Figure 5.2 Schematic illustration of two defect reduction mechanisms for a strained film (dark gray) grown on a porous substrate (light gray), (a) Dislocations bending toward the open surface at the tube walls, (b) Formation of relatively dislocation-free regions in the GaN film where the film has laterally grown over the pores. Reproduced from C. K. Inoki etal., Pbys. Stat. Soli, (a) 200, 44. Copyright (2003), with permission from John Wiley Sons, Ltd...

Glide of an edge dislocation occurs when a half-plane of atoms is moved over the atoms below the glide plane. The movement occurs by the nucleation and movement of kinks. Remember that the reason that dislocations are so important in plasticity is because it is easier to move one block of material over another (shear the crystal) one halfplane of atoms at a time. Similarly, it is easier to move a dislocation by moving a kink along it one atom at a time. In fee metals, the Peierls valleys are not deep, so the energy required to form a kink is small and dislocations bend (create kinks) quite easily. 

Solid metals obtained upon solidification of the molten metal exhibit grain structure. They consist of fine crystallites randomly oriented in space. The size of the individual crystallites (grains) is between 10 m (fine-grained structure) and 10 m (coarse-grained structure). The crystal stracture of the individual grains as a rule is not ideal. It contains various types of defects vacant sites, interstitial atoms or ions, and dislocations (lattice shearing or bending). Microcracks sometimes evolve in the zones between crystallites. 

Another special factor in ionic crystals is that dislocation cores in them acquire net charge. As a result, plastic bending of an ionic crystal causes the top and bottom regions to become charged relative to the middle. This is easily demonstrated because such specimens preferentially attract fine insulating powders. It has been studied in some detail by Li (2000). 

An elastic continuum model, which takes into account the energy of bending, the dislocation energy, and the surface energy, was used as a first approximation to describe the mechanical properties of multilayer cage structures (94). A first-order phase transition from an evenly curved (quasi-spherical) structure into a... 

Note 1 The twist and bend distortions can be stabilised by an array of screw or edge dislocations. 

Under the action of a local shear stress, a, a straight dislocation line that is fixed at two points will bend out. The bending radius is inversely proportional to a. The dislocation becomes unstable if the bending radius is <1/2, where / is the distance between the anchor points (Fig. 3-3). Dislocation loops can be formed and macroscopic plastic deformation can continuously occur under stress if... 

Polanyi, My time with x-rays and crystals, in Fifty Years of X-Ray Diffraction, 636. G.I. Taylor, The mechanism of plastic deformation of crystals, Proceedings of the Royal Society A145 (1934) 362-415 E. Orowan, "Zur Kristallplastizitat, Zeitschriftfur Physik 89 (1934) 605-659. On recent studies of dislocation, see R. F. Service, Materials scientists view hot wires and bends by the bay, Science 272 (1996) 484-485. 

This process is obviously a natural scattering process in polycrystalline materials, since polycrystalline films exhibit a high concentration of crystallographic defects, especially dislocations [133,134]. However, this process is rarely used to explain experimental data of carrier transport in polycrystalline semiconductors and especially transparent conducting oxides [88], which is mainly due to the fact that in most works on transport properties of polycrystalline films the density of defects was not determined. Podor [135] investigated bended n-type Ge crystals with a dislocation density around 107 cm 2... 

The regions of coalescence of the stripes were either without observable dislocations or had very few dislocations which originated from the LEO-GaN/Si02 interface, propagated vertically and terminated within one-third to one-half of the LEO-GaN Jhickness, as shown in FIGURE 4. Occasional 90° bending of fee dislocations occurred at fee 1101 planes, which effectively reduced the density of... 

Figure 5.3. Schematic diagram of an edge dislocation of Burgers vector b. Away from the dislocation core, the beam is incident on the planes at a glancing angle fl, =ffe +A . Thus, due to the bending of the planes near the core, the angle of incidence on the right-hand side is 02< i> nd on the left-hand side it is 03<0,.

Figure 5.4. Schematic diagram showing the displacement across the bend contour of the image of a dislocation from the position of the dislocation itself.

For n = 1, the image is displaced from x = 0 (the position of the dislocation) for all conditions except DF at s = 0. Thus, the image changes from one side to the other of the dislocation on crossing a bend contour (see also Figure 5.4). 

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