Geology Reference
In-Depth Information
(a)
(b)
5
6
5
P
0
P
6
P
1
0
4
2
1
3
4
2
5
6
3
Fig. 6.16 a Dislocation source initiated as a dislocation 0 pinned at point P; the subsequent
positions 1, 2, 3,…of the dislocation line represent increasing length of the dislocation,
augmenting the dislocation density of the crystal. b A ''two-armed'' source, known as a Frank-
Read source
boundaries may form for reasons of energy minimization, their formation being a
process that occurs more readily at higher temperatures and probably usually
involves some climb of the dislocations. However, although an energy minimi-
zation argument for cell formation can be given (Weertman and Weertman
1983a
,
p. 1292), cell structure may also form for kinetic rather than energetic reasons
(Kocks
1985a
). Geometric constraint may also be an influential factor in dislo-
cation distribution, especially in requiring an excess of dislocations of given sign
in order to accommodate imposed bending; such dislocations are sometimes called
geometrically necessary dislocations.
When there is a well-defined substructure, the dislocations in the cell walls or
subgrain boundaries are sometimes referred to as ''bound'' dislocations and those
lying within the cells or subgrains, often forming themselves a three-dimensional
network, as ''free'' dislocations. However, it cannot be concluded without further
evidence that these two sets of dislocation are immobile and mobile, respectively.
In fact, the reverse may be the case, as shown for example, by Martin and coworkers
in careful TEM studies on various metals undergoing creep, using techniques both
of pinning the dislocations before releasing the applied stress and of direct obser-
vation during deformation in the electron microscope (Caillard and Martin
1982a
,
b
,
1983
; Clément et al.
1984
; Kubin and Martin
1980
; Morris and Martin
1984a
,
b
).
They showed that the mobile dislocations in these materials traverse the subgrains
very rapidly after breaking away from subboundaries in which they reside for most
of the time (t
0
t
g
in expressions (
6.17
)). The breakaway process probably
