---

Slip, Slip Direction, Slip Plane

---

Quick
Slip is the process by which a dislocation moves and causes a material to deform.
The slip direction is the direction in which the dislocation moves, which is the direction of the Burgers vector for edge dislocations.
During slip, the edge dislocation sweeps out the plane formed by the Burgers vector and the dislocation; this plane is called the slip plane.
The combination of slip direction and slip plane is the slip system.


Equations

(Eq1)     τ = c e−kd/b

Peierls-Nabarro stress

Nomenclature

τ	Peierls-Nabarro stress, stress required to move the dislocation from one equilibrium location to another
c	constant for the material
k	constant for the material
d	interplanar spacing between adjacent slip planes
b	Burgers vector

Details

The Burgers vector could be translated from the loop to the edge dislocation, as shown:



After this translation, it is found that the Burgers vector and the edge dislocation define a plane in the lattice. The Burgers vector and the plane are helpful in explaining how materials deform.

When a shear force acting in the direction of the Burgers vector is applied to a crystal containing a dislocation, the dislocation can move by breaking the bonds between the atoms in one plane. The cut plane is shifted slightly to establish bonds with the original partial plane of atoms. This shift causes the dislocation to move one atom spacing to the side, as shown:



If this process continues, the dislocation moves through the crystal until a step is produced on the exterior of the crystal; the crystal has then been deformed. If dislocations could be continually introduced into one side of the crystal and moved along the same path through the crystal, the crystal would eventually be cut in half.

The process by which a dislocation moves and causes a material to deform is called slip. The direction in which the dislocation moves, the slip direction, is the direction of the Burgers vector for edge dislocations. During slip, the edge dislocation sweeps out the plane formed by the Burgers vector and the dislocation; this plane is called the slip plane. The combination of slip direction and slip plane is the slip system. A screw dislocation produces the same result; the dislocation moves in a direction perpendicular to the Burgers vector, although the crystal deforms in a direction parallel to the Burgers vector.

During slip, a dislocation moves from one set of surroundings to an identical set of surroundings. The Peierls-Nabarro stress is required to move the dislocation from one equilibrium location to another:

(Eq1)

τ = c e−kd/b

where τ is the shear stress required to move the dislocation, d is the interplanar spacing between adjacent slip planes, b is the Burgers vector, and both c and k are constants for the material. The dislocation moves in a slip system that requires the least expenditure of energy. Several important factors determine the most likely slip systems that will be active.

1. The stress required to cause the dislocation to move increases exponentially with the length of the Burgers vector. Thus, the slip direction should have a small repeat distance or high linear density. The close-packed directions in metals satisfy this criterion and are the usual slip directions.

2. The stess required to cause the dislocation to move decreases exponentially with the interplanar spacing of the slip planes. Slip occurs most easily between planes of atoms that are smooth (so there are smaller "hills and valleys" on the surface) and between planes that are far apart (or have a relatively large interplanar spacing). Planes with a high planar density fulfill this requirement. Therefore, the slip planes are typically close-packed planes or those as closely packed as possible.

3. Dislocations do not move easily in materials such as silicon or polymers, which have covalent bonds. Because of the strength and directionality of the bonds, the materials typically fail in a brittle manner before the force becomes high enough to cause appreciable slip.

4. Materials with an ionic bond, including many ceramics such as MgO, also are resistant to slip. Movement of a dislocation disrupts the charge balance around the anions and cations, requiring that bonds between anions and cations be broken. During slip, ions with a like charge must also pass close together, causing repulsion. Finally, the repeat distance along the slip direction, or the Burgers vector, is larger than in metals. Again, brittle failure of the material typically occurs before the dislocations move.