Geology Reference
In-Depth Information
A schematic diagram for the formation of an elon-
gated diamond‐shaped etch pit at the point of intersec-
tion of a basal dislocation and a prismatic plane is
shown in Figure 6.34a. The illustration helps one com-
prehend how a replica (reverse image) of such emergence
points of the core of basal dislocation and the etch pit
at the surface level would appear. The processes of
chemical etching along the core of basal dislocations,
producing whisker‐like replication of the line defects
and elongated pits at the surface are described in detail
in
Sinha
[1978b]. The scanning electron micrograph in
Figure 6.34b shows such elongated pits parallel to [0001]
directionsigu
r
e 6.36 and cores of dislocations on the
prismatic 1010 surface.
A basal dislocation that lies parallel to the glide plane
can be detected readily by the formation of a centrally
depressed, elongated etch pit at the point where it inter-
sects other planes. If the intersecting surface is normal to
the basal plane (0001), then the etch pit is symmetrical
with respect to the central depression and its long axis is
parallel to the
c
axis or <0001 > axis of the crystal.
E
xa
mples of tiny elongated dislocation etch pits on the
1120
surface may be seen. In Figure 6.22 (from sublima-
tion) large evaporation pits with their major plane paral-
lel to the basal plane clearly show the direction of the
crystallographic axis of symmetry. The central depres-
sion corresponds to the core of the dislocation. The depth
of the central depression can be controlled by suitable
choice of etchant concentration, thickness of etching
layer, and vapor pressure (evaporation time). Etch pits
with depressions as long as 200
μ
m along the core may be
produced in this way, corresponding to a stationary dislo-
cation [
Sinha
, 1977b, 1978b). When observed by SEM,
these etch pits give the appearance of “whiskers” in the
replica and have, in fact, been used to remove the ambi-
guity sometimes associated with the correspondence
between etch pits and dislocations. In fact, establishing
this one-to-one correspondence between dislocation etch
pits on the surface and the core of dislocations, and the
characteristics of these lines defects in crystals is unique
Figure 6.33
Scanning electron micrograph showing many
shallow etch tracts for rapidly moving dislocations and deeper
tracks for slow dislocations, ending in hexagonal pits of differ-
ent sizes at their rest positions (SEM by N. K. Sinha,1977b)
Figure 6.34
Schematic diagram of a chemically etched elongated pit on prism surface at (a) the intersection
of a basal dislocation and (b) scanning electron micrograph (SEM) of replica of five such pits and associated cores
(like shaved whiskers) of dislocations; whiskers broke when the replica was peeled off [SEM by N. K. Sinha,
unpublished, see more illustrations in
Sinha
, 1978a].
