Geology Reference
In-Depth Information
Figure 6.29 Optical micrograph of thermally etched grain
boundary in the middle separating two grains in S3‐type sea ice
and subgrain boundaries inside the two grains; orientation of c
axis, parallel to the length of the elongated dislocation etch pits,
is shown by < c > (micrograph by N. K. Sinha, unpublished).
Subgrain
boundary
show how the elongated tiny sublimation pits directly
indicate the orientation of the c axis of the subgrains and
hence the average orientation of the grains. This optical
micrograph, with three-dimensional effect, clearly shows
a grain boundary going from the bottom of the picture
to the top in the middle of the photograph. This grain
boundary separates the grain in the left half from that in
the right half. The subgrain boundaries inside each grain
indicate mismatch between the a axis and probably slight
mismatch in the c axis. The orientation of the c axis is
indicated by the long dimension of the elongated etch
pits. These pits correspond to the intersections of basal
dislocations with prismatic surfaces and will be explained
later in detail. One can see here, as well as in Figure 6.28,
that it is difficult, if not impossible, for the untrained
eyes to distinguish between grain boundaries and sub-
grain boundaries in sea ice. In a direct way, these images
exemplify that grain boundaries in sea ice, unlike fresh-
water ice, are not the most important features in sea ice.
Due to the propensity of subgrains, it is the subgrain size
and subgrain boundaries that control the microstruc-
ture‐properties relationship in sea ice. It is the subgrain
boundaries where cracks are also nucleated, with con-
comitant generation of acoustic emission, under the
influence of thermally induced strains or externally
applied loads [ Sinha , 1996].
Figure  6.30 shows surface characteristics after 72 h of
thermal etching at −10 °C. It shows triple points of sub-
grains with little or no entrapped brine. Note the differ-
ences in the orientations of elongated pits inside the
subgrains. Note also the details of the shape of the pits.
They are wedge shaped with central depression, but reverse
looking in the micrograph due to the oblique transmittance
of light. The proof that these pits are due to the basal
1mm
Grain boundary
Figure 6.28 Micrographs of a horizontal thin section of FY S3‐
type sea ice, taken within 20 min after completion of the DMT
technique of thin sectioning (top) and after 96 h of thermal etch-
ing at −10 °C (bottom) (micrographs by N. K. Sinha, unpublished).
was taken less than 20 min after the completion of the
DMT process and placing the section inside a glass‐top
thermal etching box with crushed ice in it. Though the
top surface of the section was focused, the micrograph
provides three‐dimensional thickness views of the brine
pockets. It shows the boundary area between the two
adjacent grains as viewed (but not illustrated here) in
transmitted polarized light. The bottom image shows
thermally etched crystal boundaries and intracrystalline
(inside subgrains) features after 96 h. It illustrates several
important aspects of sea ice, such as the complex network
of dislocations within the subgrains and/or the orienta-
tion of the c axis of the subgrains and hence the grains. The
most obvious are the elongated etch pits and the fact that
grain boundary grooves are not distinguishable from the
grooves along subgrain boundaries. Grain boundaries are
the large‐angle boundary, whereas subgrain boundaries are
low‐angle lines between platelets. No discernible differ-
ences in depths of the grooves at the boundaries between
subgrains, belonging to two grains with large mismatch in
the orientation of their c axis and subgrains inside a grain
with little mismatch across their boundaries.
Some details of thermally etched dislocation pits (tiny
linear features parallel to each others) in S3 type of sea
ice are shown in Figure  6.29. The photographs clearly
Search WWH ::




Custom Search