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Imperfections in Solids
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MATERIALS SCIENCE &
BITSPilaniPilani Campus
MATERIALS SCIENCE & ENGINEERING
Dr. Subrata Bandhu Ghosh
Department of Mechanical Engg.
Imperfections in SolidsImperfections in Solids
In our pervious Lecture when
discussing Crystals we
ASSUMED PERFECT ORDER
In real materials we find: Crystalline Defects or lattice irregularity
Most real materials have one or more “errors in Most real materials have one or more “errors in perfection”
with dimensions on the order of an atomic diameter to many lattice sites
Defects can be classification:
1. according to geometry
(point, line or plane)
2. dimensions of the defect
Imperfections in Solids
• The properties of some materials are
profoundly influenced by the
presence of imperfections.
• It is important to have knowledge • It is important to have knowledge
about the types of imperfections that
exist and the roles they play in
affecting the behavior of materials.
Atom Purity and Crystal Perfection
• If we assume a perfect crystal structure
containing pure elements, then
anything that deviated from this
concept or intruded in this uniform
homogeneity would be an homogeneity would be an
imperfection.
1. There are no perfect crystals.
2. Many material properties are improved
by the presence of imperfections and
deliberately modified (alloying and
doping).
Polycrystalline Materials
Grain Boundaries
• regions between
crystals
• transition from lattice of
one region to that of the one region to that of the
other
• ‘slightly’ disordered
• low density in grain
boundaries
– high mobility
– high diffusivity
– high chemical reactivity Adapted from Fig. 4.7, Callister 7e.
• Vacancy atoms
• Interstitial atoms
• Substitutional atomsPoint defects
1-2 atoms
Types of Imperfections
• Dislocations Line defects1-dimensional
• Grain Boundaries Area defects2-dimensional
• Vacancies:-vacant atomic sites in a structure.
Point Defects
Vacancy
distortion
of planes
• Self-Interstitials:-"extra" atoms positioned between atomic sites.
self-interstitial
distortion
of planes
Self Interstitials
• In metals, a self interstitial introduces
relatively large distortions (strain) in
the surrounding lattice since the atom
is substantially larger than the
interstitial site.
POINT DEFECTS
• The simplest of the point defect is a vacancy, or vacant lattice site.
• All crystalline solids contain vacancies.
• Principles of thermodynamics is used explain the necessity of the
existence of vacancies in crystalline solids.
• The presence of vacancies increases the entropy (randomness) of
the crystal.
• The equilibrium number of vacancies for a given quantity of
material depends on and increases with temperature as
follows:
Nv= N exp(-Qv/kT)
Equilibrium no. of vacancies
Total no. of atomic sites Energy required to form vacancy
T = absolute temperature in °Kelvin
k = gas or Boltzmann’s constant
Nv = exp − Qv
No. of defects
No. of potential
Activation energy –energy required for formation of vacancy
• Equilibrium concentration varies with temperature.
Equilibrium Concentration:
Point Defects
Boltzmann's constant
(1.38 x 10-23
J/atom-K)
(8.62 x 10-5eV/atom-K)
N= exp
kT
No. of potential
defect sitesTemperature
Each lattice site is a potential vacancy site
• We can get Qv from
an experiment. Nv
N= exp
−Qv
kT
Measuring Activation Energy
• Measure this...
Nv
• Replot it...
Nv slope
note:
A El
El
NN
A
ρ=
Nv
N
T
exponential dependence!
defect concentration1/T
N
Nvln
-Qv /k
slope
Example Problem 4.1
Calculate the equilibrium number of vacancies per cubic meter for
copper at 1000°C. The energy for vacancy formation is 0.9 eV/atom; the
atomic weight and density (at 1000 ° C) for copper are 63.5 g/mol and
8.4 g/cm3, respectively.
Solution.
Use equation 4.1. Find the value of N, number of atomic sites per cubic
meter for copper, from its atomic weight Acu, its density, and meter for copper, from its atomic weight Acu, its density, and
Avogadro’s number NA.
toequal ie )1273( 1000at vacanciesofnumber theThus,
/8.0x10
/5.63
)/10)(/4.8)(/10023.6(
3 28
336323
KC
matoms
molg
mcmcmgmolatomsx
A
NN
Cu
A
o
=
==ρ
5
328
)1273)(/1062.8(
9.0(exp)/(8.0x10
exp
=
−=
− KKeVx
eVmatoms
kT
QNN v
v
Continuing:
325
5
/m vacancies2.2x10
)1273)(/1062.8(
=
− KKeVx
And Note: for MOST MATERIALS just
below Tm ���� Nv/N = 10-4
Here: 0.0022/8 = .000275 = 2.75*10-4
Two outcomes if impurity (B) added to host (A):• Solid solution of B in A (i.e., random dist. of point defects)
OR
Substitutional solid soln. Interstitial solid soln.
Point Defects in Alloys
• Solid solution of B in A plus particles of a new
phase (usually for a larger amount of B)
Substitutional solid soln.
(e.g., Cu in Ni)
Interstitial solid soln.
(e.g., C in Fe)
Second phase particle
--different composition
--often different structure.
Imperfections in Solids
Conditions for substitutional solid solution (S.S.)
• Hume – Rothery rules
– 1. ∆r (atomic radius) < 15%
– 2. Proximity in periodic table
• i.e., similar electronegativities• i.e., similar electronegativities
– 3. Same crystal structure for pure metals
– 4. Valency equality
• All else being equal, a metal will have a greater tendency
to dissolve a metal of higher valency than one of lower
valency (it provides more electrons to the “cloud”)
Imperfections in SolidsApplication of Hume–Rothery rules – Solid
Solutions
1. Would you predict
more Al or Ag
Element Atomic Crystal Electro- Valence
Radius Structure nega-
(nm) tivity
Cu 0.1278 FCC 1.9 +2
C 0.071
H 0.046more Al or Ag
to dissolve in Zn?
2. More Zn or Al
in Cu?
Table on p. 106, Callister 7e.
H 0.046
O 0.060
Ag 0.1445 FCC 1.9 +1
Al 0.1431 FCC 1.5 +3
Co 0.1253 HCP 1.8 +2
Cr 0.1249 BCC 1.6 +3
Fe 0.1241 BCC 1.8 +2
Ni 0.1246 FCC 1.8 +2
Pd 0.1376 FCC 2.2 +2
Zn 0.1332 HCP 1.6 +2
More Al because size is closer and val. Is
higher – but not too much – FCC in HCP
Surely Zn since size is closer thus
causing lower distortion (4% vs 12%)
Imperfections in Solids
• Specification of composition
– weight percent100x
21
11
mm
mC
+=
m1 = mass of component 1
100x 21
1'
1
mm
m
nn
nC
+=
nm1 = number of moles of component 1
– atom percent
Wt. % and At. % -- An example
Typically we work with a basis of 100g or 1000g
given: by weight -- 60% Cu, 40% Ni alloy
6009.44
63.55 /Cu
gn m
g m= =
'
'
4006.82
58.69 /
9.44.581 or 58.1%
9.44 6.82
6.82.419 or 41.9%
9.44 6.82
Ni
Cu
Ni
gn m
g m
C
C
= =
= ≈+
= ≈+
Converting Between: (Wt% and At%)
' 1 21
1 2 2 1
' 2 12
100
100
C AC
C A C A
C AC
C A C A
∗= ×
∗ + ∗
∗= ×
∗ + ∗
Converts
from wt% to
At% (Ai is 2
1 2 2 1
'
1 11 ' '
1 1 2 2
'
2 22 ' '
1 1 2 2
100
100
C A C A
C AC
C A C A
C AC
C A C A
∗ + ∗
∗= ×
∗ + ∗
∗= ×
∗ + ∗
At% (Ai is
atomic
weight)
Converts
from at% to
wt% (Ai is
atomic
weight)
Determining Mass of a Species per
Volume
" 311
1 2
10C
CC C
ρ ρ
= × +
• ρi is density of pure
element in g/cc
• Computed this way,
gives “concentration”
1 2
" 322
1 2
1 2
10C
CC C
ρ ρ
ρ ρ
+
= × +
gives “concentration”
of speciesi in kg/m3 of
the bulk mixture
(alloy)