Geology Reference
In-Depth Information
The explanation of the discrepancy lies in a lattice
imperfection called a
screw dislocation
(Figure 8.7b).
Such defects are stacking 'mistakes' incorporated
randomly into crystals and perpetuated during
growth. A mismatch of layers on one side of the crystal
is not seen on the other side, and must disappear at a
line (dislocation) extending vertically through the mid-
dle of the crystal, where one layer actually twists up
into the next one. The crystal resembles the kind of
multi-storey car park in which the floors are part of a
large spiral leading to the top. The vital feature is that
the step affording favourable crystallization sites, like
B and C, is perpetuated as crystallization proceeds,
spiralling continuously upward as suggested by the
arrows and the crystallization 'fronts' in Figure 8.7c.
Such dislocations make continued crystallization prac-
ticable at modest degrees of supersaturation.
greater than the measured shear strength for the material
in question. In resolving this embarrassing discrepancy,
we have to recognize the role of another type of crystal
imperfection, the
edge dislocation
.
An edge dislocation (Figure 8.8a) marks the edge of
an extra half-layer (a-a-a) in a crystal lattice. This may
represent a layer overstepped during growth of the
crystal, or may be the result of deformation. Note that
bonds in the immediate vicinity of the dislocation are
either stretched or compressed, and owing to this
departure from equilibrium length are not as strong
(Box 7.1) as bonds in the undistorted lattice. The high
free energy associated with the edge dislocation makes
it a particularly susceptible site for initiating chemical
reactions: acid etching of freshly broken crystal sur-
faces leads to the formation of pits at points where dis-
locations intercept the surface, rendering them visible
under an electron microscope.
The mechanical significance of the dislocation becomes
clear if we consider the crystal being subjected to
shear
stress
as shown in Figure 8.8b. The crystal at first responds
with
elastic strain
, in which it is deformed to a minute and
reversible extent by the stretching and compression of
bonds, without any being broken. Permanent shear
deformation (
ductile deformation
) requires bonds to be
broken so that the top half of the crystal can be bodily
moved to a new position across the lower half.
Mechanical strength of crystals
Box 7.7 includes estimates of the strength of bonding in
several crystalline materials. From data such as these it
is not difficult to calculate values for the shear strength
of a number of structurally simple crystalline materials,
such as pure metals. However carefully such calculations
are carried out, the results are generally 100-1000 times
(a)
a
a
a
(b)
Shear
stress
a
c
S
S
b
Figure 8.8
(a) Edge dislocation, (b) Migration of edge dislocation in response to shear stress (arrows). The stippled portion of
the upper layer
slips
over the lower layer (s-s is the
slip plane
) as the edge dislocation (heavy stipple) jumps from row to row.
(Source: Bloss 1971. Reproduced with permission of Holt, Rinehart and Winston, Inc.)
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