Geology Reference
In-Depth Information
(
1975
, Chap. 7) on the geometrical aspects of kinks formed by intersection). A jog
is in effect a short segment of dislocation having the same Burgers vector as the
main part of the dislocation line but having, in general, a different slip plane. If this
second slip plane is not one for easy slip, the presence of the jog will have a
dragging effect on the main part of the dislocation. The dragging effect will be
especially marked if the jog has to undergo nonconservative motion in being
displaced with the main part of the dislocation, as in the case of a jog in a screw
dislocation when this displacement of the jog necessary leaves a trail of vacancies
or interstitials (in the case of compounds, the ''vacancies'' or ''interstitials'' would
need to consist of the whole repeating group of atoms, making jog dragging
especially difficult in screw dislocations in a crystal of complex structure and
suggesting that in such a case a precondition for the easy glide of a segment of
screw dislocation would be for it to be cleared of jogs by moving them conser-
vatively along the dislocation). Jogs are also thought to be important in the climb
of dislocations, providing sites for more ready attachment or detachment of the
diffusing atoms (for example, Nabarro
1967
, p. 351). At elevated temperatures the
jogs may be formed thermally. For the energetics of jogs, see Hirth and Lothe
(
1982
, pp. 495-497, 569-585).
6.2.5 Zonal, Extended and Partial Dislocations
The changes in linkage or interrelationship between neighboring atoms brought
about during the passage of a dislocation need not be concentrated on a single
structural site or confined to a single structural plane at any instant but can be
distributed over a small region. Correspondingly, during the shearing process
individual atoms may move in directions not parallel to the slip direction
(a movement called ''shuffling'') and different patterns of movement may occur on
adjacent structural layers, the only constraint being that the net bulk displacement
of one part of the crystal relative to the remainder corresponds to the Burgers
vector and that the original structure be left unchanged after the passage of the
dislocation. When the nonparallel atomic movements are confined to a single pair
of structural layers, the term ''synchro-shear'' is applied to the movement pattern
(Kronberg
1961
) and sometimes, by extension, to the dislocation itself (Amelinckx
1979
, p. 393). In more general cases where several layers are simultaneously
involved, the term ''zonal dislocation'' can be used (Amelinckx
1979
). In effect, in
a zonal dislocation the topological disruptions in the core region can be envisaged
as extending over several adjacent structural sites. Thus, in the dislocations
envisaged in aluminum oxide (corundum) two adjacent atomic layers are involved
in a synchro-shear, while in
½
1123
ð
1122
Þ
slip in zinc zonal dislocations involving
three layers are thought to be involved (Amelinckx
1979
). Similar enlarged core
regions may be expected to be not uncommon in minerals. However, in zonal
dislocations the long-range elastic stress field can still be regarded as being
determined by a sole singularity represented by the Burgers vector; it is the core
