Geology Reference
In-Depth Information
0.5 mm
Figure 6.36
Optical micrograph of a replica exhibiting a cross
section of a few grains in freshwater S2 ice and etch tracks in
the form of thin lines corresponding to moving dislocations
(micrograph by
N. K. Sinha
, 1987b].
Figure 6.35
SEM of etch features, such as formation of a cell,
pileups of basal dislocations, and short tilt boundaries, inside
a grain of mechanically deformed ice;
c
-axis is indicated by
the long direction of the tiny elongated etch pits (SEM by N. K.
Sinha, unpublished).
this mechanism was theoretically acceptable, no direct
evidence for such a mechanism was available. Etching
and replicating in conjunction with the solid‐state (no
heat is applied) microtoming technique for surface prepa-
ration was used to provide evidence for climbing of basal
dislocations on planes parallel to the
c
axis in polycrystal-
line ice [
Sinha
, 1987b].
The climbing dislocations should produce etch tracks
parallel to the long axis of the etch pits or the <0001 > axis
while etching prismatic surfaces under a load. Both slip
and climb usually occur, producing etch track at a right
angle to each other so that the replica resembles a fabric.
Occasionally, the author (N. K. Sinha) has noticed only
etch tracks parallel to the <0001 > axis, corresponding to
the motion (climbing) at right angles to the direction of
slip. An optical micrograph of such a replica is shown in
Figure 6.36. In this case the ice was transversely isotropic
S2 type, with the <0001 > axis of the grains randomly ori-
ented in the plane of isotropy and the replicated surface.
These observations of the author provides probably the
best experimental evidence of climbing as the rate con-
trolling mechanism envisioned by
Weertman
[1968].
Most of the etch tracks in Figure 6.36 extend from one
grain boundary to the other, indicating that the grain
boundaries act as sources and sinks for the dislocations,
and that dislocations, generated at the boundaries, travel
to other boundaries through the matrix. Nonuniformity
of the stress field is also evident in this micrograph in the
form of patches of high‐density etch tracks. It can be
seen that these high‐density patches are closely associated
in the field of materials science. For most studies it is suf-
ficient to develop only the centrally depressed, elongated
etch pits by choice of the etching conditions because
whiskers often obstruct the view.
Movements of basal dislocations can be detected by
etching and replicating the required surface while the
material is under load, as shown earlier for nonbasal
dislocation. Only the slowly moving dislocations can be
detected by the formation of etch track, along the paths
traversed by the line defects. Mobility of nonbasal (
Sinha
,
1977b) and basal (
Sinha
, 1978b) dislocations has been
demonstrated. If the dislocations are blocked, then they
pileup, like marching soldiers. One prominent pileup and
several smaller pileups may be seen in Figure 6.35 for
basal dislocations in a previously deformed ice specimen.
Note that the pileup direction (located in the central area
of the micrograph) is normal to the direction of the long
axis of the etch pits corresponding the orientation of the
c
axis. The linear density of dislocations along a pileup is
an indication of the localized stresses involved.
Deformation in polycrystalline ice also involves dislo-
cation climb in addition to slip along the basal plane.
Because of the high temperatures involved in ice, diffu-
sion‐controlled climb of dislocations on planes normal to
the basal plane may even be the rate controlling process
during steady state creep [
Weertman
, 1968]. Although
