CH 6 CVIL 223 Structural Imperfections and Atom Movements.pdf

Structural engineering

1. Structural Imperfections
(Defects)
In Crystalline Solids

2. Real Crystalline solids are almost never
perfect. These imperfections (defects) may
be classified into four categories depending
on their dimension:

1.Point defects (0-Dimension)
2.Line defects (1-D)
3.Interfacial defects (2-D)
4.Bulk defects (3-D)

3. Why study imperfections in
solids?
•Mechanical properties
•Chemical properties (corrosion)
•Electrical properties
•Optical properties

The consideration of impurities,
imperfections, and atom movements enable
us to anticipate properties more accurately.

4. 1. POINT DEFECTS
These are defects of atomic dimensions that
usually result from:

1.The presence of an impurity atom
Substitutional →larger atoms
Interstitial → smaller atoms
2.The absence of a matrix atom (vacancy)
3.The presence of a matrix atom in a wrong
place (self-interstitial)

5. Point Defects:
Presence of an impurity atom:
-"extra" atoms positioned between atomic sites.
Substitutional
Interstitial

6. Vacancies: -vacant atomic sites in a structure.
Self-Interstitials: -"extra" atoms positioned between atomic sites.
Point Defects
Vacancy
distortion
of planes
self-
interstitial
distortion
of planes

7. Point Defects in Ionic Structure: The point defects
discussed so far occur in metallic structures. Those in ionic
structures differ because of the charge neutrally requirement.

Schottky defect: absence of pair of ions of opposite charges.
Frenkel defect: An ion is replaced from the lattice point to an
instertitial site.

8. The point defects discussed so far occur in metallic
structures. Those in ionic structures differ because of
the charge neutrally requirement.
An anion
and a cation
is missing
An anion or a
cation is at an
insterstital site

9. • are line defects,
• slip between crystal planes result when dislocations move,
• produce permanent (plastic) deformation.
Dislocations:
Schematic of Zinc (HCP):
• before deformation • after tensile elongation
slip steps
2. Line Defects (Dislocations)

10. Linear Defects (Dislocations)
–Are one-dimensional defects around which
atoms are misaligned
•Edge dislocation:
–extra half-plane of atoms inserted in a
crystal structure
–b  to dislocation line
•Screw dislocation:
–spiral planar ramp resulting from shear
deformation
–b  to dislocation line
Burger’s vector, b: measure of lattice distortion

11. First a closed circuit is drawn around the
dislocation by jumping from one atom to
another.
The same number of jumps will be made in a
perfect system.
The vector needed to complete the circuit is
called BURGERS VECTOR.
Burger’s vector b describes the direction and
magnitude of the dislocation.

12. Edge Dislocations
Edge dislocations can be described as an edge of
an extra plane of atoms within a crystal system.
Burger’s vector
is perpendicular to
dislocation in edge
dislocations.

13. If atoms are  to
dislocation line,
then such a
dislocation is
known as Screw
dislocation.
Screw Dislocations

15. Dislocation is a defect in which atoms slide or slip
on each other.
Presence of dislocation effects the mechanical
behaviour of materials.
When dislocation moves all through the material, the
material permanently deforms.
Dislocations are responsible for the DUCTİLİTY.
If a material free from dislocations could be obtained
it would be very brittle & useless as an engineering
material.

16. 3.INTERFACIAL DEFECTS
(BOUNDARIES)
Boundaries could be summarized into three:

1.Free surfaces: Interfaces between liquids and
gases.
2.Grain boundaries: Interfaces between crystal
systems having different orientation.
In each crystal system the atoms are
arranged orderly. However, at the boundary
there is a transition zone which is not alinged
with either of the crystal systems.

17. 3.Interphase boundaries: similar to grain
boundaries both in shape and behavior.
However, in these systems there are two
or more materials having different
crystal structures. Multiphase materials
having a change in physical and/or
chemical characteristics will also have
interphase boundaries. (Ex: ice-water)

18. Free Surfaces
Surface atoms have unsatisfied atomic
bonds, and higher energies than the bulk
atoms. Since they have atoms on only one
side.

Surface areas tend to minimize their
energies. e.g. liquid drop takes a spherical
shape to minimize the surface area &
therefore the surface energy.

19. Grain Boundaries
A one phase material may contain crystals of different
orientations. Within any particular grain, all of the atoms
are arranged with one orientation & one pattern,
characterized by the unit cell. However, at the grain
boundaries there are transition zones which are not aligned
with either of the grains.

grain boundary
The atoms along the
boundary have higher
energy than those within the
grains.
The lower atomic packing
at the boundary favors
atomic diffusion.

20. When the degree of misorientation of the
grain boundaries is less than 10° they are
called low-angled boundaries which may be
further divided into tilt & twist
boundaries.

The tilt boundary can result from a set of
edge dislocations where twist boundary
can result from a set of screw
dislocations.

.

Grain Boundaries

21. 4. BULK or VOLUME DEFECTS
They are either introduced during the
production of the material or during its
fabrication.

For example → inclusions (cracks,
notches, air bubbles & etc.) added
during production.

22. IMPORTANCE OF IMPERFECTIONS
Most of the properties of materials are affected
by imperfections:
Small amount of impurity atoms may increase
the electrical conductivity of semi-conductors.
Dislocations are responsible for ductility.
Strength of materials can be increased to a
large extent by the mechanism “strain-
hardening” which produces line defects that act
as a barrier to control the growth of other
imperfections.

23. IMPORTANCE OF IMPERFECTIONS
Most of the properties of materials are affected
by imperfections:
Presence of bulk defects such as cracks,
notches, holes causes brittle materials, which
break at very low stresses without showing
large deformations.

e.g. Adding 0,1% arsenic to silicon its conductivity will
increase 10,000 times.
e.g. İf line defects were not present in metals, they would
be very brittle.

24. Atom Movements - Atomic
Vibrations
Many changes in materials seen in practice
(hardening, sedimentation, recrystallization) and
processes like welding & galvanization are
dependent primarily on the motion of atoms.

Diffusion: İs a movement of atoms, ions or
molecules as the result of thermal agitation.
Diffusion is very rapid in gasses and liquids, but it is
a slow process in solids, where the movement of
atoms are greatly restricted.

25. Mechanisms of Atom Movements
The important points about diffusion of
atoms may be listed as follows:

1)Smaller atoms diffuse more easily.

2)Diffusion rate is higher in a medium with a
low melting point & therefore weaker bond.

3)Energy requirement is less for conditions in
which APF is low.

4)Diffusion is easier at locations like grain
boundaries where atom density is less when
compared with grains. e.g. Corrosion
starts at grain boundaries.

26. Impure Phases: Solid Solutions
Some metals used commercially for engineering
purposes are pure.
e.g. Copper  used for electrical wiring.
Zinc  used for coating on the galvanized steel.
But in some cases, foreign elements are intentionally
added to a material in order to enhance its properties.

e.g. Brass  Zinc (Zn) + Copper (Cu)

If such an addition becomes an integral part of the
solid phase, the resulting phase is called a solid
solution.

27. Solid Solutions
Solid solutions are made of a host (the solvent or
matrix) which dissolves the minor component
(solute). The ability to dissolve is called solubility.
Solid solutions can be divided into two groups
depending on the position of the impurity atoms in
the lattice system as;
• Substitutional solid solutions.
• Interstitial solid solutions.

28. Substitutional Solid Solutions (SSS)
Zn
When the solvent and solute atoms have similar sizes &
electron structures, these easily replace one another in the
lattice system to form a substitutional solid solutions.
When Zinc is added to copper it substitutes readily for the
copper within the FCC lattice, until a maximum of 40% of
the copper has been replaced.
r
Cu = 1,28 A°
r
Zn = 1,39 A°
Cu
29=1s
2
,2s
2
,2p
6
,3s
2
,3p
6
,3d
10
,4s
1
Zn
30=1s
2
,2s
2
,2p
6
,3s
2
,3p
6
,3d
10
,4s
2

29. Substitutional Solid Solutions (SSS)
If there is to be extensive replacement in a SSS, the atoms
must be nearly the same size.
e.g. Ni & Cu, r
Cu = 1,28 A° r
Ni = 1,25 A°; therefore for Ag
(German Silver) Cu + Ni 100% substitution is possible.


As the difference in size increases, less substitution can
occur. Extensive solid solubility rearly occurs if there is
more than 15% difference in radius between the two kinds
of atoms.

30. Interstitial Solid Solutions
Small atoms may be located in the interstices between larger atoms.
Above 910°C iron has FCC structure. Carbon being a small atom
(r
carbon=0,71 A°) can move into the ‘hole’ in the center of the unit cell to
produce a solid solution of iron and carbon.
At lower temperatures iron has BCC structure, so the instertices between
iron atom gets much smaller and the solubility is greatly reduced.
0,025% in BCC,
2% in FCC