Dislocations defined

A unit, or perfect, dislocation is defined by a Burgers vector which regenerates the structure perfectly after passage along the slip plane. The dislocations defined above with respect to a simple cubic structure are perfect dislocations. Clearly, then, a unit dislocation is defined in terms of the crystal structure of the host crystal. Thus, there is no definition of a unit dislocation that applies across all structures, unlike the definitions of point defects, which generally can be given in terms of any structure. 

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... 

A crystal yields when the force xh (per unit length) exceeds /, the resistance (a force per unit length) opposing the motion of a dislocation. This defines the dislocation yield strength... 

Exactly the same procedure can be followed to define the free energy of a dislocation core. It should be surrounded by a box, the terminating planes of which can be dealt with exactly as above. Special attention has to be given to the atoms at the comers of the box, but this presents no particular problems their weights are simply a oroduct of the weights generated by the planar terminations which they share. 

Dislocation motion in covalent crystals is thermally activated at temperatures above the Einstein (Debye) temperature. The activation energies are well-defined, and the velocities are approximately proportional to the applied stresses (Sumino, 1989). These facts indicate that the rate determining process is localized to atomic dimensions. Dislocation lines do not move concertedly. Instead, sharp kinks form along their lengths, and as these kinks move so do the lines. The kinks are localized at individual chemical bonds that cross the glide plane (Figure 5.8). 

Figure 1. Calculated values of AG, free energy of formation of a pit at a dislocation on a quartz surface at 300°C, plotted vs. pit radius, r. Labels defined in text. Adapted with permission from Ref. 16. Copyright 1986 Pergamon Press.

The distribution of microstates may be defined as the distribution of spatial dislocations, orientations, and interactions of groups of the main chain and side groups with respect to their most probable values. 

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... 

In extended defects, the displacement vector b (or R) associated with them can be defined from the Burgers Circuit shown in figure 2.4(a), for a simple cubic system (Frank 1951, Cottrell 1971, Amelinckx et al 1978). In the defective crystal (A), a sequence of lattice vectors forms a clockwise ring around the dislocation precisely the same set of lattice vectors is then used to make a second... 

Define a positive direction along the dislocation line. This is usually done into the crystal. 

---