Shockley dislocation

W.T. Read and W. Shockley. Dislocations models of grain boundaries. In Imperfections in Nearly Perfect Crystals. John Wiley Sons, New York, 1952. 

Figure 10.12. The removal (or insertion) of a portion of a close-packed layer introduces two Frank dislocations of opposite sign and stacking faults, as shown in (a). The Shockley dislocation (b) is a displacement of a portion of atoms in a close-packed layer into adjacent sites of a different type in the same plane.

Fig. 6b. Illustration of the formation of an extended dislocation by dissociation of a unit edge dislocation [as in (a)] into two partial (Shockley) dislocations, which are separated by a stacking fault.

A perfect dislocation lying in the basal plane of close-packed hexagonal solids (e.g., zinc, magnesium, or graphite) may dissociate, according to Eq. (1), to yield two partial (so-called Shockley) dislocations. The... 

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... 

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. 

Fig. 4. Reconstructed image of a (1 1 0) plane in an fee crystal (light shaded in the unit ceU schematic on the right) intersected by a stacking fault on the (111) plane (dark shaded on the right), which ends in a Shockley dislocation with Burgers vector l/6a[l 1 2] (along the intersection of the two planes).

Dislocations can be identified in a 3D reconstructed image by the standard tracing of a Burgers circuit. Fig. 4 gives an example. The end of the stacking fault is a Shockley dislocation, the Burgers vector of which is l/6a[l 1 2], as identified by the closure failure of the circuit in the figure. 

The early understanding of the geometry and dynamics of dislocations, as well as a detailed discussion of the role of vacancies in diffusion, is to be found in one of the early classics on crystal defects, a hard-to-find book entitled Imperfections in Nearly Perfect Crystals, based on a symposium held in the USA in 1950 (Shockley et al. 1952). Since in 1950, experimental evidence of dislocations was as yet very sparse, more emphasis was placed on a close study of slip lines (W.T. Read, Jr.,... 

The Shockley involved in this symposium was ihe same William Shockley who had participated in the invention of the transistor in 1947. Soon after that momentous event, he became very frustrated at Bell Laboratories (and virtually broke with his coinventors, Walter Brattain and John Bardeen), as depicted in detail in a rivetting history of the transistor (Riordan and Hoddeson 1997). For some years, while still working at Bell Laboratories, he became closely involved with dislocation geometry, clearly as a means of escaping from his career frustrations, before eventually turning fulltime to transistor manufacture. 

Koehler, J.S, Seitz. F., Read Jr., W.T., Shockley, W. and Orowan, E. (eds.) (1954) Dislocations in Metals (The Institute of Metals Division, American Institute of Mining and Metallurgical Engineers, New York). 

Figure 3, Area of clean gold (111) surface showing a surface Shockley partial dislocation (arrowed) - see 2. Atomic columns are black.

Accordingly, glide planes are those planes which have the shortest b vectors a/2 <110> for fee, a/2 <111> for bcc, and a/3 <211.0) for hep lattices. Dislocations can split into so-called Shockley partials b = bx +b2, if b2>b +b. Since b and b2 are not translational vectors of the crystal lattice, they induce a stacking fault. The partial dislocation therefore bounds the stacking fault. 

FIGURE 2 HREM image of partial dislocations and their schematic maps found in the low temperature GaN layer near the interface (a) interstitial Frank loop (the position of the extra half plane is indicated by arrows in the image) (b) isolated Frank partial dislocation (c) reaction between a dissociated edge dislocation and Shockley partial that is displaced by one basal plane (d) dislocation notation for hexagonal crystal used in (a) - (c). (From [3].)... 

In contrast to the imperfect dislocation discussed in the preceding paragraph, the Shockley partial dislocation (Fig. Q. 2b) is a mobile imperfect dislocation. A Shockley partial dislocation can be considered as a displacement of a portion of the atoms in one close-packed plane into a new set of positions. For example, in the close-packed layer sequence... ABCABC..., a portion of the atoms in, say, the B layer, are shifted to C sites. 

Consequently, as the dislocation cores get more closely spaced with increasing tilt angle, the well-coupled regions get smaller and smaller. Indeed, using the Read-Shockley formula together with the observed size of the cores, one... 

Fig. 39. Model of how a basal-plane, unit strength, dislocation of the type Ja has dissociated into two Shockley partials of Burgers vector Ja lOTO> and Jo .

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