Chemistry Reference
In-Depth Information
slip plane
Figure 2.6
A dislocation in a cubic lattice. As a shearing force is applied in the
direction of the arrows, motion can most easily occur along the shaded
plane with the dislocation moving from left to right as the bonds ''flip''
from one atom to the next.
relatively easy for motion to occur across the end of the dislocation where there
is a mismatch in the lattice planes. Of course, the basic structure of the crystal is
not changed and so when we pause the experiment and start again we find the
same modulus. Figure 2.6 illustrates the process with a cubic lattice.
Work hardening of our crystal system occurs as the dislocations move and
eventually meet and lock. However, these are still potential weaknesses and
increasing applied stress or strain will again produce motion. Eventually,
catastrophic failure occurs as the material fractures. Here, dislocations can
come together to produce a crack.
As a solid is produced by crystallisation from the melt, a space filling
multicrystalline structure is produced. Each crystallite or grain has a different
orientation from its neighbour and the mismatch of the lattices at the crystallite
edges produces a region of poorer packing known as grain boundaries. Impu-
rities can also be concentrated in these regions. During creep, the dislocations
generated by plastic flow of the crystallites can move through the grains to the
grain boundaries at the same rate as they are being formed, i.e. a steady state is
achieved. At higher deformation rates the dislocations build and the grains
become shattered and are reduced in size. At temperatures that are a significant
fraction of the melting point, i.e. T40.5T
m
, the creep rate, which in shear
would mean the shear rate and in extension the extension rate, is governed by
the diffusion of the atoms in the structure that enables the dislocation to remain
unblocked.
6
The creep rate is then given by:
g
¼
As
n
exp
E
D
k
B
T
ð
2
:
29
Þ
Where E
D
is the activation energy for self-diffusion. Now the diffusion of an
atom requires that there must firstly be an adjacent vacancy, so that the atom
