Shuffle dislocation

If we left the half-plane 1234 where it is but removed the other half, we would create a dislocation with the opposite b, but instead of the last atom being R(S) it would be T(U) Zn instead of S The core of the dislocation is fundamentally different it is still a shuffle dislocation. These two dislocations are fundamentally different because the zinc blende structure does not have a center of symmetry. Similar considerations will hold for materials such as AIN and GaN, which also lack a center of symmetry but have the wurtzite structure crystal structure. The stacking fault in the diamond-cubic structure is described as AbBbCcBbCcAaBb the pair of planes Aa behaves just as if it were one fee plane in this case. Two possible SFs are shown in Figure 12.11d. 

Although the dissociation of a shuffle dislocation is unlikely stricto sensu, a dissociation involving shuffle dislocations was studied from a theoretical viewpoint by different authors [10-12,15]. This dissociation leads to a stacking fault between type II planes, bounded at one end by a glissile Shockley dislocation and at the... 

Other end by a sessile partial dislocation. The motion of this last partial reqnires a nonconservative atomic rearrangement. This type of rearrangement is called a shuffling and, for that reason, Hirth and Lothe [12] called the set of type I planes the shuffle set and the dislocations lying in these planes shuffle dislocations. 

The dissociated shuffle dislocation, called extended shuffle dislocation by Alexander [1], can be described in two ways either as a stacking-fault ribbon bounded by two Shockley dislocations of opposite sign associated with a shuffle dislocation, or as a dissociated glide (DG) dislocation that has emitted or absorbed a line of vacancies or interstitials in the core of one of its partials. 

Since the PL spectra do not show any evidence of specific emissions associated to shuffle dislocations, Pizzini et al. [81] concluded that perfect dislocations present a reconstructed core, and their generation is accompanied by the introduction of point defects and point defect clusters. A broad band around 1 eV is the only PL... 

Fig. 20. Possible reconstruction at kinks, (a) Perfect 60° shuffle dislocation with dangling bonds in 1 and 2. (b) Perfect 60° shuffle with a ti bond. Dangling bonds have been exchanged between 2 and 3 positions, and pairing of bonds has occurred between 2 and 1. This reconstruction is expected to be feasible at kink...

A point to be clarified is whether the nucleation of partial glide dislocations is assisted when perfect shuffle dislocations preexist. Then, one would have to consider possible mechanisms for core transformation. 

Fig. 32. Transformation from a perfect shuffle dislocation into a dissociated glide dislocation. The perfect dislocation has moved by one interplanar spacing into the ghde set by (a) stress-assisted (shear stress <7 ) cross shp and (b) climb under the action of a chemical force (fch)- After Rabier [86].

Cross slip is a conservative mechanism that occurs under stress with the help of thermal activation. Climb reqnires point defect annihilations at the dislocation cores and can also be stress-assisted, bnt to a lesser extent. This indicates that cross shp can be operative at lower temperatures than climb, and points at the core structure of perfect screw dislocations as an important factor determining the stabUity of perfect shuffle dislocations. 

Regarding the issue of ghde versus shuffle dislocation cores, experiments are needed to determine the exact location of the perfect dislocations and to establish a correlation between their actual core structures and their physical properties. The investigation of the core structure of dislocations has to be performed using corrected HREM. Clearly, the main difficulty arises from the preparation of an adequate specimen with edge-on dislocations, whose core is not affected by kinks, impurities, or any defect that may blur the structure determination. As far as electron microscopy is concerned, in situ nanoindentation experiments can be useful provided this technique can be extended down to atomic scale. This is a technical challenge. 

Heterophase Interfaces. In certain cases, sharp heterophase interfaces are able to move in military fashion by the glissile motion of line defects possessing dislocation character. Interfaces of this type occur in martensitic displacive transformations, which are described in Chapter 24. The interface between the parent phase and the newly formed martensitic phase is a semicoherent interface that has no long-range stress field. The array of interfacial dislocations can move in glissile fashion and shuffle atoms across the interface. This advancing interface will transform... 

General features of perfect dislocations in the shuffle plane a geometrical analysis 78... 

Fig. 4. 60° dislocation in the type I glide plane (shuffle set), after Amelinckx [16]. (a) Undissodated and (b) dissociated with formation of an intrinsic stacking fault. In that case, the stacking fault is of type II and is associated to a dislocation dipole at its right end (see Ref. [16]). 

Deformation substructure in Si indented at low temperature Asaoka et al. [66] recently revisited the indentation of silicon with the aim of deforming it plastically below room temperature. These authors indented silicon at 77 K and showed that it can be deformed plastically. TEM observations of the microstructure showed dislocations aligned along the <110> and <112> directions. Weak-beam dark field showed these dislocations were perfect ones and had a/2<110> Burgers vectors. A HREM observation was also performed on a dislocation seen edge-on, which was shown to have an undissociated core. The exact location of this core, in a glide plane or a shuffle plane, could not be determined. 

The various authors who observed perfect dislocations assumed that they were located in the shuffle set, as follows from the calculations of Duesbery and Jobs [45]. However, there is yet no direct experimental evidence about the nature of the plane, ghde, or shuffle, in which the core of these perfect dislocations is located. 

The physical properties of perfect dislocations, which are assumed to be shuffle ones, were mainly probed through photoluminescence (PL) measurements. Pizzini et al. [81] performed PL measurements on samples that were deformed following the procedures reported in Section 2.2.1. 

Fig. 19. (111) glide plane viewed in the shuffle set and showing dangling bonds. The edge of the supplementary plane of a dislocation is indicated, (a) <112>/30° dislocation and (b) <112>/30° dislocation with <110) kinks this allows for bond pairing and resnlts in a < 12 3 >/41° dislocation. After... 

Fig. 21. Possible core configurations for a non-dissociated screw dislocation shuffle (A), mixed shuffle/glide (B), simple period glide (Ci), and double-period glide (C2). Thicker dark gray bonds show a Burgers circuit the dislocation line along <110> is marked by a dashed line in each case. Atoms located in the immediate vicinity of the core are represented by black spheres. After Pizzagalli et al. [90]. The A, B, C positions for a screw dislocation are shown in the insert on the left side...

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