5. Chpt 5: Imperfections in Solids
ISSUES TO ADDRESS...
• What types of defects arise in solids? Describe them.
• Can the number and type of defects be varied
and controlled?
• How do defects affect material properties?
• Are defects undesirable?
5
7. TYPES of DEFECTS
•
•
•
•
•
Vacancy atoms
Interstitial atoms
Frankel defect
Impurity atoms
Dislocations
Point defects
Line defects
Edges, Screws, Mixed
• Grain Boundaries
Area/Planar defects
• Stacking Faults
• Anti-Phase and Twin Boundaries
We need to describe them and understand their effects.
7
8. Length Scale of Imperfections
point, line, planar, and volumetric defects
Vacancies,
impurities
dislocations
Grain and twin
boundaries
Voids
Inclusions
precipitates
8
9. Point Defects
• The defects which are confined to a point are
known as point defects
• Types of point defects
– Vacancy
– Interstitialcy
– Frankel defect
– Impurity atoms
10. Vacancy
• When an atom is missing from its regular
lattice site, it is known as VACANCY.
13. Vacancy
Vacancy: a vacant lattice site
It is not possible to create a crystal free of vacancies.
About 1 out of 10,000 sites are vacant near melting.
13
14. Vacancy
Thermodynamics provides an expression for
Vacancy Concentration: CV =
Q
Nv
= exp − v
N
k BT
CV = Concentration of vacancies
Nv = Number of vacancies
N = Total number of atomic sites
Qv = vacancy formation energy
kB = 1.38 x 10–23 J/atom-K = 8.62 x 10–5 eV/atom-K
kB/mole = R = 1.987 cal/mol-K
Defects ALWAYS cost energy!
14
15. Equilibrium Concentration of Vacancy
• Equilibrium concentration varies with temperature!
No. of defects
Cd=
Activation energy
−Q
ND =
D
exp
kT
N
No. of potential
Temperature
defect sites.
Boltzmann's constant
(1.38 x 10-23 J/atom K)
(8.62 x 10 -5 eV/atom K)
Each lattice site
is a potential
vacancy site
15
16. Estimating Vacancy Concentration
• Find the equil. # of vacancies in 1m 3of Cu at 1000C.
• Given: ρ = 8.4 g/cm3
ACu = 63.5g/mol
QV = 0.9eV/atom NA = 6.02 x 1023 atoms/mole
0.9eV/atom
−Q
ND
D
= exp
= 2.7 · 10-4
CV=
kT
N
For 1m3, N =
1273K
8.62 x 10-5 eV/atom-K
NA
x 1m3 = 8.0 x 1028 sites
ρ x
ACu
• Solve: ND = 2.7 · 10 -4 · 8.0 x 10 28 sites = 2.2x 1025 vacancies
* What happens when temperature is slowly
reduced to 500 C, or is rapidly quenched to 500 C?
106 cm3 = 1 m3
16
17. Equilibrium concentration of vacancies as a
function of temperature for aluminium (after Bradshaw and
Pearson, 1957).
20. Interstitialcy
• When an atom is present at an interstitial
position in its lattice, it is known as
interstitialcy.
• In this case an atom is present at a place
where it should not have been.
22. Interstitialcy
Self-interstitial: atom crowded in ‘holes’
Self-interstitials are much less likely in metals, e.g.,,
as it is hard to get big atom into small hole - there is
large distortions in lattice required that costs energy.
22
23. Frenkel Defect
• It is a combination of vacancy and
interstitialcy defects.
• When an atom leaves its regular lattice site
and goes to an interstitial site, it is known as
Frenkel Defect.
24.
25. Impurity Atom
• When atom of another element is present as
an impurity in the lattice of an element, it is
known as impurity atom.
26. Impurity Atom
• Impurity atoms can occupy any of the
following position
– Interstitial position
– Substitutional position
27. Impurity Atom
• Types of Impurity Atom
– Interstitial Impurity Atom
• When impurity atom is present at an interstitial site, it
is known as interstitial impurity atom.
– Small atoms C, N, B and H can occupy interstitial sites
– Substitutional Impurity Atom
• When impurity atom substitutes a parent atom at its
regular lattice site, it is known as substitutional
impurity atom.
44. Burgers Vector
Burgers circuit round a dislocation A fails to
close when repeated in a perfect lattice unless completed by
a closure vector FS equal to the Burgers vector b.
47. The Edge Dislocations and Burger’s Vector
Looking along line direction of edge
Burger’s vector = extra step
• Edge looks like extra plane of atoms.
• Burger’s vector is perpendicular to line.
• Positive Edge (upper half plane)
Stress fields
at an Edge
dislocation
• Is there a Negative Edge?
• Where is it?
• What happens when edge gets to
surface of crystal?
• What are the stresses near edge?
47
60. Stacking Faults: Messed up stacking
FCC
HCP
or C
slip
or C
• All defects cost energy (J/m2 or erg/cm2).
• Stress, dislocation motion can create Stacking Faults.
• What is stacking of FCC and HCP in terms of A,B, and C positions in (111) planes?
…ABCABCABC…
hcp
...ABCACABABCABC...
…….fcc
fcc ………..
60
61. Grain Boundary
• It is the boundary between two grains having
different orientations.
63. Relative Energies of Grain Boundaries
Which GB is lower in energy, low-angle GB or high-angle GB?
Hint: compare number of reduced preferred bonds.
Why?
high-angle
• The grains affect properties
• mechanical,
• electrical, …
Grain I
• Recall they affect diffraction so you
know they’re there.
• What should happen to grains
as temperature increases?
Grain 2
Hint:
surfaces (interfaces) cost energy.
Grain 3
low-angle
63
70. Twin Boundaries: an atomic mirror plane
original atomic positions
before twinning
• There has to be another opposite twin nearby to get back to perfect crystal,
because all defects cost energy (J/m2 or erg/cm2) and to much defect costly.
• Stress twins can be created (e.g., Tin) in which case the atoms must move
at the speed of sound.
What happens when something moves at speed of sound?
70