Dislocations, partial

A7.3 Defects in GaN and related materials perfect dislocations, partial dislocations, dislocation movement and cracks... 

The sequence just outlined provides a salutary lesson in the nature of explanation in materials science. At first the process was a pure mystery. Then the relationship to the shape of the solid-solubility curve was uncovered that was a partial explanation. Next it was found that the microstructural process that leads to age-hardening involves a succession of intermediate phases, none of them in equilibrium (a very common situation in materials science as we now know). An understanding of how these intermediate phases interact with dislocations was a further stage in explanation. Then came an nnderstanding of the shape of the GP zones (planar in some alloys, globniar in others). Next, the kinetics of the hardening needed to be... 

In spite of the very high APB energy, most of the dislocations split into two 1/2(111) super-partials as shown in Figure 2. This is true for all dislocation characters studied, except for the pure screw dislocation which is displayed in Figure 3. 

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

Figure 5.9 Plan view of the (111) plane of the diamond structure. A—Normal structure with open circles in the plane of the paper, and crossed circles in the plane above. Each pair is connected by a covalent bond. B—Partial shear of the upper plane over the lower one on the right-hand side creating a screw dislocation line with a kink in it (dashed line). C—Upper plane sheared down-ward by the displacement, b.

The sigma phases are hard and brittle at below their Debye temperatures, but have some plasticity at higher temperatures. Thus there is some covalent bonding in them, and their glide planes are puckered, making it difficult for dislocations to move in them until they become partially disordered. Their structures are too complex to allow realistic hardness values to be calculated for them. Their shear moduli indicate their relative hardnesses. 

The structure of Ni3Al is the Ll2 (Cu3Au) structure (Figure 8.5). It is fee with the corners occupied by A1 atoms, and the face-centers by Ni atoms. The primary glide planes are (111) and the glide directipns are (110). Therefore, the shears in the cores of dislocations in these crystals are broken into four parts as illustrated in Figures 8.6, 8.7, and 8.8. Each unit dislocation in the structure is split into four partial dislocations. 

These extended dislocations cannot move concertedly, so kinks must form on the partials and these do the moving, causing consequent movement of the... 

Since there is no good physical framework in which the measured hardness versus temperature data can be discussed, descriptions of it are mostly empirical in the opinion of the present author. Partial exceptions are the elemental semiconductors (Sn, Ge, Si, SIC, and C). At temperatures above their Debye temperatures, they soften and the behavior can be described, in part, in terms of thermal activation. The reason is that the chemical bonding is atomically localized in these cases so that localized kinks form along dislocation lines. These kinks are quasi-particles and are affected by local atomic vibrations. 

Fig. 9. Arrhenius plots of the free hole concentration p (log p versus 1000/T) in two samples cut from a partially dislocated slice of ultra-pure germanium. The dislocation-free sample contains an acceptor with Ev + 80 meV. The shallow level net-concentration is the same in both samples.

Figure 3.12 Partial dislocations in copper (a) a unit dislocation, Burgers vector bl (b) initially slip is easier in the direction represented by the Burgers vector of the partial dislocation b2 than bl (c) the result of the movement in (h) is to generate a stacking fault and (d) the combined effect of displacements by the two partial dislocations b2 and b3 is identical to that of the unit dislocation, but the partials are separated by a stacking fault.

If both these partial dislocations exist in the crystal, they will be linked by an antiphase boundary (Fig. 3.12d). 

The partial dislocations described in copper and similar materials are not the only ones that can be described, and a number of other types are well known, including Frank partial dislocations, which mediate in a different slip process. 

Figure 3.17 Layer of the corundum (AI2O3) structure, projected down the c axis. The unit cell is marked. A unit dislocation, Burgers vector b, can be decomposed into four partial dislocations bl-b4.

Figure 3.18 Cadmium iodide, Cdl2, structure (a) perspective view of the structure as layers of Cdlg octahedra (b) one layer of the structure, with the lower I- anion (A) layer and the middle Cd2+ (c) layer shown complete and just three anions of the upper (B) layer indicated. The vector b represents the Burgers vector of one of three unit dislocations and bl and b2 represent the Burgers vectors of the two equivalent partial dislocations. The unit cell dimensions are a, b, and c.

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