Shockley partials

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. 

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 .

The usual schematic presentation of the splitting of a perfect dislocation into Shockley partials with a fault between them is shown in Fig. 3.57c. 

The extended dislocation, discussed hitherto, consists of Shockley partials and a SF, which can glide within its own glide plane therefore, the accepted notation is glissUe dislocation. 

AHq is the energy for cross-slip, cr is the critical resolved shear stress, a is the applied stress and C and c are constants. A model of creep controlled by cross-slip from the 111 to the 100 plane in the temperature range of 530-680 °C over the stress range of 360-600 MN was found to be in good agreement with the experimental results. The energy to form a restriction between the partials, namely to recombine the Shockley partials, was evaluated on the basis of Dorn s expression [8]. (See also Hemker et al. [51] for the creep mechanism at intermediate temperatures in Ni3Al). 

Values of SFE from 20 to 60 mjm determine intensive mechanical twinning related to TWIP effect. At SFE values higher than about 60 mJm-2, the partition of dislocations into Shockley partial dislocations is difficult, and therefore the glide of perfect dislocations is the dominant deformation mechanism (Hamada, 2007). In TRIPLEX steels with a structure of austenite, ferrite and K-carbides ((Fe,Mn)3AlC) and for SFE > 100 mJm-2, the SIP (Shear Band Induced... 

It has been observed experimentally that the formation of Shockley partial dislocations separated by stacking faults is much more common in diamond cubic epitaxial films in some crystallographic orientations than it is in other orientations... 

Sieradzki 1989), the resistance due to creation or annihilation of a surface ledge in the wake of the threading dislocation (Matthews 1975), the presence of a deposition flux during dislocation motion, and the possibility of dissociation of the dislocations into Shockley partial dislocations (Alexander 1986). Thus, the critical thickness condition could be restated a G = 71. where 7Z is a resisting force representing any or all of these additional effects. 

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