Dislocations screw type

Fig. VII-8. (a) Screw dislocation (from Ref. 115). (b) The slip that produces a screw-type dislocation. Unit slip has occurred over ABCD. The screw dislocation AD is parallel to the slip vector. (From W. T. Read, Jr., Dislocations in Crystals, McGraw-Hill, New York, 1953, p. 15.)...

When the two vectors are parallel, the crystal planes perpendicular to the line form a helix, and the dislocation is said to be of the screw type. In a nearly isotropic crystal structure, the dislocation is no longer associated with a distinct glide plane. It has nearly cylindrical symmetry, so in the case of the figure it can move either vertically or horizontally with equal ease. 

Dislocations. Screw dislocations are the most important defects when crystal growth is considered, since they produce steps on the crystal surface. These steps are crystal growth sites. Another type of dislocation of interest for metal deposition is the edge dislocation. Screw and edge dislocations are shown in Figure 3.4. 

This result is easily generalized for mixed dislocations which are partly screw-type and partly edge-type, and also for cases having subsaturated vacancies. For a mixed dislocation, 6 must be replaced by the edge component of its Burgers vector... 

This is slip that occurs simultaneously on several slip planes having Ihe same slip direction. See Fig. 14. This type of plastic deformation is normally associated with the movement of screw dislocations. Screw dislocations can move on any slip plane that passes through the dislocation. This is a result of the fact that the slip plane of a dislocation is that plane which contains both the dislocation and its Burgers veclor, and the fact that the Burgers vector of a screw dislocation lies parallel io the dislocation itself,... 

The double cross-slip mechanism can then be considered as the most probable deformation process, complementary to the basal slip. Indeed, dislocation climb can hardly be invoked in this torsion loading conditions since most of the dislocations are of screw type. 

Figure 8.7 Plan-view transmission electron micrograph of the same lateral growth as in Figure 8.6, with the [0001] direction normal to the image plane, showing the bending of screw-type dislocations into edge-type dislocation arrays at (B) during the lateral growth [10]...

Line imperfections are called dislocations and occur in crystalline materials only. Dislocations can be an edge type, screw type, or mixed type, depending on how they distort the lattice, as shown in Figure 8. It is important to note that dislocations cannot end inside a crystal. They must end at a crystal edge or other dislocation, or they must close back on themselves. 

Three major types of line defects edge dislocations, screw dislocations, and antiphase defects) are summarized here. The Hne defects are generally important factors in the electrical behavior of semiconductor devices since their influence (as trapping centers, scattering sites, etc.) extends over distances substantially larger than atomic spacings. 

Sillimanite and mullite are sUicates with Si-Al tetrahedral chains. They occur in high-temperature metamorphic rocks and muUite is an important ceramic material. TEM analyses identify [001] as Burgers vector in sillimanite [350,351]. Dislocations are generally of screw type in the [0 01] direction [Fig. 14(b)] and sometimes are dissociated [352]. Dislocation-assisted high-temperature deformation has been documented in mullite [353]. The influence of dislocations and strain on the aluminosilicate phase transformations were investigated for kyanite [354]. 

Extended imperfections can also occur. An extra plane of atoms can be common in a crystal lattice. Extended imperfections of this type are called edge dislocations. Another common type of extended imperfection in crystals is the screw dislocation. This type of dislocation occurs when part of a crystal has slipped one atomic distance relative to its adjacent part. 

The other major defects in solids occupy much more volume in the lattice of a crystal and are refeiTed to as line defects. There are two types of line defects, the edge and screw defects which are also known as dislocations. These play an important part, primarily, in the plastic non-Hookeian extension of metals under a tensile stress. This process causes the translation of dislocations in the direction of the plastic extension. Dislocations become mobile in solids at elevated temperamres due to the diffusive place exchange of atoms with vacancies at the core, a process described as dislocation climb. The direction of climb is such that the vacancies move along any stress gradient, such as that around an inclusion of oxide in a metal, or when a metal is placed under compression. 

The other major defeets in erystalline solids oeeupy mueh more of the volume in the lattiee. They are known as line defeets. There are two types viz. edge dislocations and screw dislocations (Figure 1.4). Line defeets play an important role in determining erystal growth and seeondary nueleation proeess (Chapter 5). 

Table 1 Summary of the calculated properties of the various dislocations in NiAl. Dislocations are grouped together for different glide planes. The dislocation character, edge (E), screw (S) or mixed type (M) is indicated together with Burgers vector and line direction. The Peierls stresses for the (111) dislocations on the 211 plane correspond to the asymmetry in twinning and antitwinning sense respectively.

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