Line of dislocation

Dislocations Dislocations are stoichiometric line defects. A dislocation marks the boundary between the slipped and unslipped parts of crystal. The simplest type of dislocation is an edge dislocation, involving an extra layer of atoms in a crystal (Fig. 25.2). The atoms in the layers above and below the half-plane distort beyond its edge and are no longer planar. The direction of the edge of the half-plane into the crystal is know as the line of dislocation. Another form of dislocation, known as a screw dislocation, occurs when an extra step is formed at the surface of a crystal, causing a mismatch that extends spirally through the crystal. 

A low-angle grain boundary in a single crystal is revealed by a line of dislocations with a separation of 7 nm. The angle between the grains is estimated to... 

The intensity of a dislocation, which is the vector addition of individual simple imperfections within a selected region, can be quantitatively expressed by the Burgers vector. This is the difference in distance and direction between a path traced by moving from one structural constituent to the next around the zone of the real crystal of interest and an exactly comparable path traced in a region of perfect lattice. The Burgers vector of an edge dislocation is normal to the line of dislocation, while that of the screw dislocation is parallel to the line of dislocation. 

Fig. 4.4 Schematic representation of a spiral dislocation. The line of dislocations A runs parallel to the shear strain. This line of dislocations is surrounded by distorted material.

Slip. (1) The mechanism by which shear stress causes plastic deformation, by driving lines of dislocation across certain crystal planes, the slip or glide planes. 

Fig. 4.23 Schematic representation of dislocations in crystals, (a) Screw dislocation, (b) Edge dislocation, (c) Mixed screw and edge dislocation. The line of dislocation (AB) and the Burgers vector (b) are indicated in each case (after Kelly and Groves).

Fig. 5.14 Gliding of step dislocation in the x direction along a glide plane spanned by the line of dislocation (s z axis, i.e. plane of the paper) and the Biu gers vector (b x axis) driven by mechanical stress (see arrows). FVom Ref. [132].

Fig. 5.15 The line of dislocation can migrate upwards formation of interstitial atoms or occupation of vacancies. (In the converse case, the core of the dislocation migrates downwards. FVom Ref. [132].)...

Dislocation is a linear crystal defect for which Burger s vector is not zero. This definition is common for rather large numbers of a different kind of linear defect. The considered dislocation is referred to as an edge disposition the extreme line (edge) of the induced plane is referred to as the line of dislocation (in Figure 9.24c this line in a point w is perpendicular to the plane of drawing). For such a dislocation. Burger s vector is perpendicular to the dislocation line. 

The density of dislocations is usually stated in terms of the number of dislocation lines intersecting unit area in the crystal it ranges from 10 cm for good crystals to 10 cm" in cold-worked metals. Thus, dislocations are separated by 10 -10 A, or every crystal grain larger than about 100 A will have dislocations on its surface one surface atom in a thousand is apt to be near a dislocation. By elastic theory, the increased potential energy of the lattice near... 

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. 

A mesoseale variable deseribing a eolleetion of atomie disloeations is the total length of dislocation line per unit volume in metallurgieal publieations... 

An electron microscope picture of dislocation lines in stainless steel. The picture was taken by firing electrons through a very thin slice of steel about lOOnm thick. The dislocation lines here ore only about 1000 atom diameters long because they have been chopped off where they meet the top and bottom surfaces of the thin slice. But a sugar-cube-sized piece of ony engineering alloy contains about 10 km of dislocation line. (Courtesy of Dr. Peter Southwick.)... 

Very recently, people who engage in computer simulation of crystals that contain dislocations have begun attempts to bridge the continuum/atomistic divide, now that extremely powerful computers have become available. It is now possible to model a variety of aspects of dislocation mechanics in terms of the atomic structure of the lattice around dislocations, instead of simply treating them as lines with macroscopic properties (Schiotz et al. 1998, Gumbsch 1998). What this amounts to is linking computational methods across different length scales (Bulatov et al. 1996). We will return to this briefly in Chapter 12. 

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.,... 

All large lumps or particles contain cracks, microcracks or lines of weakness. Any normal particle structure will contain imperfections, dislocations and... 

Dislocations are readily visible in thin-film transmission electron micrographs, as shown in Figs. 20.28 (top) and 20.33 (top). The slip step (Fig. 20.31c) produced by the passage of a single dislocation is not readily apparent. However, for a variety of reasons, a large number of dislocations often move on the same slip plane or on bands of closely adjacent slip planes this results in slip steps which are very easily seen in the light microscope, as shown by the slip lines in Fig. 20.33 (bottom). 

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