Defects extra-planes

A dislocation is generally subjected to another type of force if nonequilibrium point defects are present (see Fig. 11.2). If the point defects are supersaturated vacancies, they can diffuse to the dislocation and be destroyed there by dislocation climb. A diffusion flux of excess vacancies to the dislocation is equivalent to an opposite flux of atoms taken from the extra plane associated with the edge dislocation. This causes the extra plane to shrink, the dislocation to climb in the +y direction, and the dislocation to act as a vacancy sink. In this situation, an effective osmotic force is exerted on the dislocation in the +y direction, since the destruction of the excess vacancies which occurs when the dislocation climbs a distance Sy causes the free energy of the system to decrease by 8Q. The osmotic force is then given by... 

Jogs are formed by climb. They are favorable sites for the absorption and emission of point defects. In thermal equilibrium, the atoms at jog sites are in dynamic equilibrium and arrive and leave the jog at equal rates. If there is an increase in vacancies, for example, in the vicinity of a dislocation line above the thermal equilibrium value, the probability of atomic exchange at a jog with a vacancy increases, climb occurs and the extra plane (defining the dislocation line) shrinks. Therefore, excess vacancies promote the process of climb. Similarly, an excess of interstitial atoms adds atoms to the existing jog, which causes it to grow. In summary, when atoms are removed from an extra plane, the crystal collapses... 

Lattice defects are holes within the lattice due to missing ions. The missing ions leave the lattice with unbalanced charges. Charge imbalances can also arise in crystal structures due to dislocations. There are two types of dislocations. In the screw dislocation a section of a crystal is skewed one atom spacing. In the edge dislocation an extra plane of atoms has been inserted into a section of a crystal. The charge imbalances arise at the sites of the dislocations. 

The simplest type of line defect is the edge dislocation, which consists of an extra half plane of atoms in the crystal, as illustrated schematically in Fig. 20.30a edge dislocations are often denoted by 1 if the extra half plane ab is above the plane sp, or by T if it is below. 

To a good approximation, only atoms within the dotted circles in Figs. 20.30a and b are displaced from their equilibrium position in a real, three-dimensional crystal the diameter d of these circles would be very much less than the length / of the dislocation, i.e. the length, perpendicular to the page, of the extra half plane of atoms ab in Fig. 20.30a, or of the line cd in Fig. 20.306. Dislocations strictly, therefore, are cylindrical defects of diameter d and length / however, since I d they are referred to as line defects. 

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. 

Figure 2.4. Definition of a displacement (Burgers or shear) vector b (a) a Burgers vector around a dislocation (defect) A in a perfect crystal there is a closure failure unless completed by b (b) a schematic diagram of a screw dislocation—segments of crystals displace or shear relative to each other (c) a three-dimensional view of edge dislocation DC formed by inserting an extra half-plane of atoms in ABCD (d) a schematic diagram of a stacking fault. (Cottrell 1971 reproduced by the courtesy of Arnold Publishers.)...

Line defects in a crystalline material are known as dislocations. Dislocations are formed due to nonequilibrium conditions such as ion implantation and thermal processing. Under equilibrium conditions, there is no requirement for the presence of dislocations or any other defect (except native point defects) in the crystal. An edge dislocation may be viewed also as having an extra half-plane inserted into the crystal (see Fig. 9.9). 

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