Dislocations screw, defined

Surface Defects, Dislocations that exist in the bulk of crystal can extend onto the surface. Dislocation density is defined as the number of dislocations that cut through a unit area. in metals is usually on the order of 10 cm . This dislocation density can be reduced by annealing. In a well-annealed crystal, is in the range 10 to 10 cm . Screw dislocation free surfaces (about 0.01 mm ) can be produced by... 

Dislocations are line defects. They bound slipped areas in a crystal and their motion produces plastic deformation. They are characterized by two geometrical parameters 1) the elementary slip displacement vector b (Burgers vector) and 2) the unit vector that defines the direction of the dislocation line at some point in the crystal, s. Figures 3-1 and 3-2 show the two limiting cases of a dislocation. If b is perpendicular to s, the dislocation is named an edge dislocation. The screw dislocation has b parallel to v. Often one Finds mixed dislocations. Dislocation lines close upon themselves or they end at inner or outer surfaces of a solid. 

Line defects in crystals such as screw or edge dislocations are quite common in solid substances. The typical length of all dislocation in 1 m3 of annealed material amounts to 1010 m (10 km in 1 cm3). The so-called dislocation density is defined as ... 

A screw dislocation model that predicts preferred growth along a defined dislocation. Here,... 

Screw dislocation. The simplest case to start with is that of a straight screw dislocation of Burgers vector b parallel to the surface of a thin parallel-sided crystal foil, as shown in Figure 5.10. Using the coordinate system defined there, the dislocation AB is parallel to y and at a depth z below the top surface. The dislocation causes a column CD of unit cells parallel to z in the perfect crystal to be deformed. If we assume that the atomic displacements around the dislocation are the same in the thin specimen as in an infinitely large, elastically isotropic crystal, then the components u, v, w of the deformation of the column along thex, y, and z directions will be... 

The values in Figure 13.13 for edge and screw dislocations define upper and lower bounds for As the average contrast factors for the limiting cases of... 

As indicated in Sect. 3.3.1, dislocations are line defects. The two basic types of dislocations are the edge and screw dislocations. A schematic three-dimensional (3D) illustration of an edge dislocation appears in Fig. 3.25. A (100) plane of Fig. 3.25 in a simple cubic crystal is illustrated schematically in Fig. 3.28. This illustration will help to define the Burgers vector later on. Figure 3.29 is a schematic view of edge and screw dislocations. 

Fig. 3.53 The jog, PP, and the screw dislocation line, AP PB, after intersection. This jog has an edge character and its glide plane (i.e., where the jog can glide) is defined by the shaded area. Movement of the screw dislocation to A B can occur only by climb [14]...

---