dislocations are impossible. Therefore, dislocation-generating systems, which are
pinned at nodes, must be discussed in view of plastic deformation. The most
common mechanism to generate dislocations is the Frank–Reed source that, to
operate within a slip plane of one grain, requires a dislocation to be anchored at two
nodes. A Frank–Reed source in its temporal sequence is shown in Figure 11.11.
A Frank–Reed source of dislocations starts at a dislocation that is pinned at two
fixed nodes within one grain (an interior source) or at one node and a point at the
grain boundary (a surface source). Owing to their high elastic energy, dislocations
are connected to a line tension and, therefore, any dislocation has the tendency to
shorten. The applied stress bows the dislocation out and this leads to a decrease in
the radius of curvature until the line tension is in equilibrium with the applied
stress. Increasing the line stress beyond a point where the dislocation is semicircular
creates a situation where the dislocation no longer has an equilibrium position.
Consequently, the dislocation expands rapidly and rotates around the nodes until the
Figure 11.11 Frank–Reed source of
dislocations in its temporal sequence. Note the
extension of a dislocation fixed at two pinning
points under the influence of a stress and,
finally, the separation from the pinning points.
This leads to the formation of the next
dislocation, and the cycle then restarts. For the
indicated times t 1 < t 2 < t 3 is valid.
Figure 11.10 Influence of dislocation gliding and climbing on the shape of a single crystalline
specimen.
11.2 Bulk Metallic and Ceramic Materials j307
pinned at nodes, must be discussed in view of plastic deformation. The most
common mechanism to generate dislocations is the Frank–Reed source that, to
operate within a slip plane of one grain, requires a dislocation to be anchored at two
nodes. A Frank–Reed source in its temporal sequence is shown in Figure 11.11.
A Frank–Reed source of dislocations starts at a dislocation that is pinned at two
fixed nodes within one grain (an interior source) or at one node and a point at the
grain boundary (a surface source). Owing to their high elastic energy, dislocations
are connected to a line tension and, therefore, any dislocation has the tendency to
shorten. The applied stress bows the dislocation out and this leads to a decrease in
the radius of curvature until the line tension is in equilibrium with the applied
stress. Increasing the line stress beyond a point where the dislocation is semicircular
creates a situation where the dislocation no longer has an equilibrium position.
Consequently, the dislocation expands rapidly and rotates around the nodes until the
Figure 11.11 Frank–Reed source of
dislocations in its temporal sequence. Note the
extension of a dislocation fixed at two pinning
points under the influence of a stress and,
finally, the separation from the pinning points.
This leads to the formation of the next
dislocation, and the cycle then restarts. For the
indicated times t 1 < t 2 < t 3 is valid.
Figure 11.10 Influence of dislocation gliding and climbing on the shape of a single crystalline
specimen.
11.2 Bulk Metallic and Ceramic Materials j307