Geology Reference
In-Depth Information
with triple points or turning points in the grain boundaries.
Some of the etch tracks have one end at a grain boundary
and the other end terminating abruptly at a point inside
the grain (particularly in the central area of the micro-
graph). Scanning electron micrography of a replica, made
during the climbing processes, showed that such etch
tracks seem to start at the boundary and end inside the
grain. The endpoints of the climbing dislocation are
revealed by the formation of whiskers, indicating that for
some reason some dislocations came to a stop while the
etching process continued along the core of the disloca-
tion. In the replica it appears as a whisker. It may also be
seen here that some of the etch tracks have ends without
whiskers. Figure 6.37 shows a bent whisker at the end of
a track and a series of etch tracks with different widths
corresponding to different velocities. As the whiskers are
very delicate, they are often not stiff enough to stand on
their own. Instead, they fall back on the surface, as may
be seen in this micrograph, which also shows many elon-
gated etch pits that indicate the direction of the
c
axis.
If one looks intensely at the micrograph in Figure 6.37,
one will notice innumerable tiny elongated etch pits in the
spaces between the visible tracks. They indicate that only
a fraction of the dislocations were mobile dislocations.
Figure 6.37
Bent whisker at the end of an etch track, and
tracks of different widths showing different velocities of dislo-
cations [SEM by
N. K. Sinha
, 1987b].
