Dislocations annihilation

C. B. Soh, H. Hartono, S. Y. Chow, S. J. Chua, and E. A. Fitzgerald, Dislocation annihilation in regrown GaN on nanoporous GaN template with optimization of buffer layer growth, Appl. Phys. Lett. 90, 0531121-0531123 (2007). 

As in time-independent plastic deformation, dislocations play an important role in the time-dependent plastic deformation of metals. At the onset of creep deformation, the number of dislocations in the material usually increases, causing hardening that can be experimentally observed by the reduction in the creep rate at constant stress. However, the dislocation density cannot increase arbitrarily since recovery occurs simultaneously (see section 6.2.8), with dislocations annihilating by climb. This process becomes the easier, the closer the dislocations are. Accordingly, after some transition time, an equilibrium between the generation of additional dislocation segments by plasticity and the annihilation of dislocations by recovery will be found. This equilibrium causes the creep rate to become constant in the secondary stage. 

J.L. Santailler, 2003, A visco-plastic model of the deformation of InP during LEG growth taking into account dislocation annihilation ,... 

Assume the edge dislocation density to be divided into positive and negative populations, N+ and N, moving only on slip planes at 45° (maximum shear stress) to the planar shock front. For a dislocation multiplication (annihilation) rate M, show that conservation of dislocations requires that... 

We first observe that because the evolution is reversible, dislocations cannot annihilate one another in a collision (if we were to reverse time in such a case, the forward collision point would become a point of spontaneous creation). Margolus [marg84] point out, however, that, at every iteration step t , the total number of blocks of the form... 

The influence of plastic deformation on the reaction kinetics is twofold. 1) Plastic deformation occurs mainly through the formation and motion of dislocations. Since dislocations provide one dimensional paths (pipes) of enhanced mobility, they may alter the transport coefficients of the structure elements, with respect to both magnitude and direction. 2) They may thereby decisively affect the nucleation rate of supersaturated components and thus determine the sites of precipitation. However, there is a further influence which plastic deformations have on the kinetics of reactions. If moving dislocations intersect each other, they release point defects into the bulk crystal. The resulting increase in point defect concentration changes the atomic mobility of the components. Let us remember that supersaturated point defects may be annihilated by the climb of edge dislocations (see Section 3.4). By and large, one expects that plasticity will noticeably affect the reactivity of solids. 

Creep and fracture in crystals are important mechanical processes which often determine the limits of materials application. Consequently, they have been widely studied and analyzed in physical metallurgy [J. Weertmann, J.R. Weertmann (1983) R.M. Thomson (1983)]. In solid state chemistry and outside the field of metallurgy, much less is known about these mechanical processes [F. Ernst (1995)]. This is true although the atomic mechanisms of creep and fracture are basically independent of the crystal type. Dislocation formation, annihilation, and motion play decisive roles in this context. We cannot give an exhaustive account of creep and fracture in this chapter. Rather, we intend to point out those aspects which strongly influence chemical reactivity and reaction kinetics. Illustrations are mainly from the field of metals and metal alloys. 

The (100) split-dumbbell defect in Fig. 8.5d, while having the lowest energy of all interstitial defects, still has a large formation energy (Ef = 2.2 eV) because of the large amount of distortion and ion-core repulsion required for its insertion into the close-packed Cu crystal. However, once the interstitial defect is present, it persists until it migrates to an interface or dislocation or annihilates with a vacancy. The... 

Solution. Before any growth, the two dislocations are associated with steps that may be as indicated in Fig. 12.9a. During growth, each dislocation rotates about its point of intersection to produce a spiral step, as in Fig. 12.5. However, the spirals will rotate in opposite directions, and sections will annihilate one another when they meet as in (a) and (b). The process will then continue as in (c)-(f) generating a potentially unlimited series of concentric steps. 

At the late stage of lamella orientation, classical topological defects (dislocations and disclinations) dominate [40, 41] (Fig. 8h and Fig. 9), and their movement and annihilation can be followed in Fig. 8h-i and Fig. 9. The latter presents an example of the apparent topological defect interactions and their transformations. Displayed are two dislocations of PMMA, which have an attractive interaction due to their opposite core sign. Therefore, in the next annealing step the dislocation is shifted... 

These results indicate that the radiation induced defects such as some point defects, dislocations and lattice distortions have no influence on the protonic conduction. However, the electronic conduction is modified by sub-band annihilation in gap between valence and conduction bands after neutron irradiation [2, 6, 7],... 

Figure 22 If a needle crystal of (TMTSF)2a04 is pressed with a force perpendicular to the needle axis, kink (K) and antikink (A) pairs are formed. The kinks can be annihilated, reproduced, and moved back and forth along the needle axis by applied stress. The reproducibility and mobility of the kinks indicate that the samples are of high crystallinity (few dislocations or defects). (From Ref. 169.)...

Dislocations of the same Burgers vector, but of opposite sign, climb toward one another and annihilate each other, thereby reducing the dislocation density. 

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