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Crystalline solids have a very
regular atomic structure: that is,
the local positions of atoms with
respect to each other are repeated
at the atomic scale. These
arrangements are called
crystal structures, and their
study is called
crystallography. However, most
crystalline materials are not
perfect: the regular pattern of
atomic arrangement is interrupted
by crystal defects. The
various types of defects are
enumerated here.
Point defects
- Vacancies are sites which
are usually occupied by an atom
but which are unoccupied. If a
neighboring atom moves to occupy
the vacant site, the vacancy
moves in the opposite direction
to the site which used to be
occupied by the moving atom. The
stability of the surrounding
crystal structure guarantees
that the neighboring atoms will
not simply collapse around the
vacancy. In some materials,
neighboring atoms actually move
away from a vacancy, because
they can better form bonds with
atoms in the other directions.
-
Interstitials are atoms
which occupy a site in the
crystal structure at which there
is usually not an atom. They are
generally high energy
configurations. Small atoms in
some crystals can occupy
interstices without high energy,
such as
hydrogen in
palladium.
- Impurities occur because
material are never 100% pure. In
the case of an impurity, the
atom is often incorporated at a
regular atomic site in the
crystal structure. This is
neither a vacant site nor is the
atom on an interstitial site and
it is called a substitutional
defect. The atom is not supposed
to be anywhere in the crystal,
and is thus an impurity.
- Anti-site defects occur in
an ordered alloy. For example,
some alloys have a regular
structure in which every other
atom is a different species, for
illustration assume that type A
atoms sit on the cube corners of
a cubic lattice, and type B
atoms sit in center of the
cubes. If one cube has an A atom
at its center, the atom is on a
site usually occupied by an
atom, but it is not the correct
type. This is neither a vacancy
nor an interstitial, nor an
impurity.
- Complexes can form between
different kinds of point
defects. For example, if a
vacancy encounters an impurity,
the two may bind together if the
impurity is too large for the
lattice. Interstitials can form
'split interstitial' or
'dumbbell' structures where two
atoms effectively share an
atomic site, resulting in
neither atom actually occupying
the site.
Line defects
Line defects can be described
by gauge theories.
-
Dislocations are linear
defects around which some of the
atoms of the crystal lattice are
misaligned. There are two basic
types of dislocations, the EDGE
dislocation and the SCREW
dislocation. ("MIXED"
dislocations combining aspects
of both types are also common).
Edge dislocations are caused by
the termination of a plane of
atoms in the middle of a crystal.
In such a case, the adjacent
planes are not straight, but
instead bend around the edge of
the terminating plane so that the
crystal structure is perfectly
ordered on either side. The
analogy with a stack of paper is
apt: if a half a piece of paper is
inserted in a stack of paper, the
defect in the stack is only
noticeable at the edge of the half
sheet.
The screw dislocation is more
difficult to visualise, but
basically comprises a structure in
which a helical path is traced
around the linear defect
(dislocation line) by the atomic
planes of atoms in the crystal
lattice.
The presence of dislocations
results in lattice strain
(distortion). The direction and
magnitude of such distortion is
expressed in terms of a
Burgers vector (b). For an
edge type, b is perpendicular to
the dislocation line, whereas in
the cases of the screw type it is
parallel. In metallic materials, b
is aligned with close-packed
crytallographic directions and its
magnitude is equivalent to one
interatomic spacing.
Dislocations can move if the
atoms from one of the surrounding
planes break their bonds and
rebond with the atoms at the
terminating edge.
It is the presence of
dislocations and their ability to
readily move (and interact) under
the influence of stresses induced
by external loads that leads to
the characteristic
malleability of metallic
materials.
Dislocations can be observed
using
transmission electron microscopy,
field ion microscopy and
atom probe techniques.
Disinclinations are line
defects corresponding to "adding"
or "subtracting" an angle around a
line. Basically, this means that
if you track the crystal
orientation around the line
defect, you get a rotation.
Planar defects
- Grain boundaries occur where
the crystallographic direction
of the lattice abruptly changes.
This commonly occurs when two
crystals begin growing
separately and then meet.
- Anti phase boundaries occur
in ordered alloys: in this case,
the crystallographic direction
remains the same, each side of
the boundary has an opposite
phase: For example if the
ordering is usually ABABABAB, an
anti phase boundary takes the
form of ABABBABA.
- Stacking faults occur in a
number of crystal structures,
but the common example is in
close packed structures. face
centered cubic (fcc) structures
differ from hexagonal close
packed (hcp) structures only in
stacking order: both structures
have close packed atomic planes
with six fold symmetry -- the
atoms form equilateral
triangles. When stacking one of
these layers on top of another,
the atoms are not directly on
top of one another -- the first
two layers are identical for hcp
and fcc, and labelled AB. If the
third layer is placed so that
its atoms are directly above
those of the first layer, the
stacking will be ABA -- this is
the hcp structure, and it
continues ABABABAB. However
there is another location for
the third layer, such that its
atoms are not above the first
layer. Instead, the fourth layer
is placed so that its atoms are
directly above the first layer.
This produces the stacking
ABCABCABC, and is actually a
cubic arrangement of the atoms.
A stacking fault is a one or two
layer interruption in the
stacking sequence, for example
if the sequence ABCABABCAB were
found in an fcc structure.
Bulk defects
- Voids are small regions
where there are no atoms, and
can be thought of as clusters of
vacancies.
- Impurities can cluster
together to form small regions
of a different phase. These are
often called precipitates.
