Dislocation motion/movement

An analogy to sHp dislocation is the movement of a caterpillar where a hump started at one end moves toward the other end until the entire caterpillar moves forward. Another analogy is the displacement of a mg by forming a hump at one end and moving it toward the other end. Strain hardening occurs because the dislocation density increases from about 10 dislocations/cm to as high as 10 /cm. This makes dislocation motion more difficult because dislocations interact with each other and become entangled. SHp tends to occur on more closely packed planes in close-packed directions. 

T13AI has an ordered DOig structirre that contains three independent slip systems that account for dislocation motion on the hasal 0001, prism 1010, and pyramidal 0221 planes ( f 1, 2). Prism shp requires only a single dislocation without creating a near-neighbor antiphase boundaiy, and additional shp requires movement of two dislocations (superdislocations) (Ref 3). In addition, two independent shp systems involving (c + a) shp occur to satisfy the Von Mises criterion for viniform deformation. 

Phason lines are linear defects with a [010] line direction, which can move along the [0 01] direction. It is unlikely, however, that phason lines move as a whole, that is, that the complete line performs a vertex jump in one single step. Although this has not been investigated in detail, it is a plausible assumption that phason lines move by a mechanism involving sequential jumps of small portions of the line, that is, by the formation of kinks and their subsequent movement along the line. This is in full analogy to the Peierls model, which describes dislocation motion by a kink-pair mechanism [36]. 

Answer While an analysis of the actual stresses and dislocation movement in the complicated powder compact is not practical, we do know that in powder compacts of the particle size we have the stresses created by the surface tension can easily exceed the macroscopic yield stress of the material as the temperature is raised. There is a large literature (for example, the work by Gilman and Johnston) showing that the movement of dislocations is extremely sensitive to stress and temperature. The densification kinetics which we observe are in qualitative agreement with the kinetics of dislocation motion. 

The process by which plastic deformation is produced by dislocation motion is termed slip the crystallographic plane along which the dislocation line traverses is the slip plane, as indicated in Figure 7.1. Macroscopic plastic deformation simply corresponds to permanent deformation that results from the movement of dislocations, or slip, in response to an applied shear stress, as represented in Figure 7.2a. 

Dislocations do not move with the same degree of ease on all erystallographie planes of atoms and in all crystallographic directions. Typically, there is a preferred plane, and in that plane there are specific directions along which dislocation motion occurs. This plane is called the slip plane it follows that the direction of movement is called the slip direction. This combination of the slip plane and the slip direction is termed the slip system. The slip system depends on the crystal structure of the metal and is such that the atomic distortion that accompanies the motion of a dislocation is a minimum. For a particular crystal structure, the slip plane is the plane that has the densest atomic packing—that is, has the greatest planar density. The slip direction corresponds to the direction in this plane that is most closely packed with atoms—that is, has the highest linear density. Planar and linear atomic densities were discussed in Section 3.11. 

In addition to movement of lattice members within a crystal, it is also possible for there to be motion of members along the surface. Consequently, this type of diffusion is known as surface diffusion. Because crystals often have grain boundaries, cracks, dislocations, and pores, there can be motion of lattice members along and within these extended defects. 

An edge dislocation is confined to move on its slip plane (conservative motion), and the slip due to the motion of the dislocation is also confined to the slip plane. Movement of a screw dislocation can capture on the plane where it started or else move to any other, parallel to the dislocation line (cross slip). If an edge dislocation were to move... 

The host of interesting kinetic processes associated with the movement of dislocations through materials containing various obstacles to their motion is far too large to be described in this book. The reader is therefore referred to specialized texts [2, 7-9]. 

This is a type of dislocalion motion, differing fundamentally front slip, associated with the edge components of dislocations. In climb, an edge dislocalion moves in a direction perpendicular to iis slip plane as atoms are either added lo or taken away from the extra plane of the dislocation. The motion of atoms to and from the dislocalion is accomplished by vacancy movements. If an atom in the plane next to the edge jumps out of its position and attaches itself to the extra plane, as indicated in the... 

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