Slip direction system, dislocation - movement

The movement of dislocations is constrained by crystallography. Some planes allow movement to take place more easily than do others, and these preferred planes are called slip planes. Similarly, dislocation movement is easier in some directions than it is in others. These preferred directions are called slip directions. The combination of a slip plane and a slip direction is called a slip system. 

Beside dislocation density, dislocation orientation is the primary factor in determining the critical shear stress required for plastic deformation. Dislocations do not move with the same degree of ease in all crystallographic directions or in all crystallographic planes. There is usually a preferred direction for slip dislocation movement. The combination of slip direction and slip plane is called the slip system, and it depends on the crystal structure of the metal. The slip plane is usually that plane having the most dense atomic packing (cf. Section 1.1.1.2). In face-centered cubic structures, this plane is the (111) plane, and the slip direction is the [110] direction. Each slip plane may contain more than one possible slip direction, so several slip systems may exist for a particular crystal structure. Eor FCC, there are a total of 12 possible slip systems four different (111) planes and three independent [110] directions for each plane. The... 

During dislocation movement, parts of the crystal slip relative to each other. The slip direction and the amount of slip are determined by the Burgers vector b. For an edge dislocation, the slip direction is also the direction of the dislocation movement, for a screw dislocation, these directions are perpendicular. The plane separating the two slipped crystal parts is called the slip plane, and the combination of slip direction and slip plane is called a slip system. 

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. 

---