1. CHAPTER 4: IMPERFECTIONS IN SOLIDS
ISSUES TO ADDRESS...
• What are the solidification mechanisms?
• What types of defects arise in solids?
• Can the number and type of defects be varied
and controlled?
• How do defects affect material properties?
• Are defects undesirable?

2. Imperfections in Solids
Solidification-
result of casting of molten material( metals and alloys)
the size and shape of the structure depends on the
cooling rate
2 steps
Nuclei form
Nuclei grow to form crystals – grain structure
Start with a molten material – all liquid
nuclei
crystals growing
grain structure
liquid
Crystals grow until they meet each other

3. Polycrystalline Materials
Grain Boundaries
regions between crystals
transition from lattice of one region to that of the other
slightly disordered
low density in grain boundaries
high mobility
high diffusivity
high chemical reactivity

4. Solidification
Grains can be - equiaxed (roughly same size in all directions)
- columnar (elongated grains)
~ 8 cm
heat
flow
Shell of equiaxed grains due to rapid cooling (greater T) near wall
Columnar in area with less undercooling
Grain Refiner - added to make smaller, more uniform, equiaxed grains.

5. Imperfections in Solids
There is no such thing as a perfect crystal.
What are these imperfections?
Why are they important?
Many of the important properties of materials are due to the presence of imperfections.

6. Types of Imperfections
Point defects
• Vacancy atoms
• Interstitial atoms
• Impurities defects ( for solid solution)
Line defects ( dislocation)
Edge dislocation
Screw dislocation
mixed dislocation

7. Planar defects (interfacial defects) external surfaces,
grain boundaries,
twin boundaries,
stacking fault
anti phase boundaries
Bulk defects ( volume 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.

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

9. Impurities defects: Substitutional or interstitial
- Substitutional : impurity atoms type A substitutional atoms
type B in structure
- Interstitial : atoms type A fill the voids or interstices among
the atoms type B
substit
interstitial

10. Equilibrium Concentration: Point Defects
• Equilibrium concentration varies with temperature!
Equil.No. of defects
Activation energy æ - ö N Q v ç v ÷ =
exp ç ÷ è ø
Total No. of atomic sites N k T
Temperature
Boltzmann's constant
-23
(1.38 x 10
J/atom-K) -5
(8.62 x 10
eV/atom-K)

11. Measuring Activation Energyç ÷ N v =
exp - Q k T æ è ö ø
• We can get Qv from
an experiment.
• Measure this... N v T
exponential
dependence!
defect concentration
• Replot it... 1/ T N v ln - Q /k
slope

12. Estimating Vacancy Concentration
• Find the equil. # of vacancies in 1 m3 of Cu at 1000C.
• Given: 3 r
= 8.4 g / cm A
= 63.5 g/mol Cu Q
= 0.9 eV/atom N
= 6.02 x 1023
atoms/mol A v
8.62 x 10-5
eV/atom-K
0.9 eV/atom
1273K ç ÷ N v =
exp - Q k T æ è ö ø
= 2.7 x 10-4
For 1 m3
, N = N A Cu r x
1 m3
= 8.0 x 1028 sites
• Answer: N v =
(2.7 x 10-4)(8.0 x 1028) sites = 2.2 x 1025 vacancies

13. Observing Equilibrium Vacancy Conc.
• Low energy electron
microscope view of
a (110) surface of NiAl.
• Increasing T causes
surface island of
atoms to grow.
• Why? The equil. vacancy
conc. increases via atom
motion from the crystal
to the surface, where
they join the island. I
sland grows/shrinks to maintain
equil. vancancy conc. in the bulk.

14. Point Defects in solid solution
SOLID SOLUTIONS: A solid solution forms when, as the solute atoms are added to the solvent atoms,
the crystal structure is maintained, and no new structures are formed.
Solvent (host atoms):represents the element or compound that is present in the greatest
Solute: is used to denote an element or compound present in a minor concentration. It is dissolved in the solvent.

15. There are several features of the solute and solvent atoms that determine the degree to which the former dissolves
in the latter; these are as follows(for perfect solid solution)
Atomic size factor: Δr less than 15% for solute and solvent
Crystal structure: must be the same for both
Electro-negativity: the more e.negativ.one element and the more e.positiv. Another for good solid solution
Valences: Other factors being equal, a metal will have more of a tendency to dissolve another metal of higher valency than one of a lower valency.

16. Point Defects in solid solution
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.
(e.g., Cu in Ni)
Interstitial solid soln.
(e.g., C in Fe)
• Solid solution of B in A plus particles of a new
phase (usually for a larger amount of B)
Second phase particle
--different composition
--often different structure.

17. Imperfections in Solids
Conditions for substitutional solid solution (S.S.)
W. Hume – Rothery rule
1. ( atomic size factor) r (atomic radius) < 15%
2. Proximity in periodic table
i.e., similar electronegativities
3. Same crystal structure for pure metals of both
atoms type
4. Valency
ex. of substitu. solid solution copper and nickel

18. Ex. of substitu. solid solution copper and nickel
r = .125 nm
both have FCC crystal structure
electronegat. = 1.9
valences +2 for both
Ex. Of interstit. Solid solution is carbon added to iron
r of C << r of iron
the maximum concent. Of C in iron is less than 10%

19. Imperfections in Solids
Application of Hume–Rothery rules – Solid Solutions
1. Would you predict more Al or Ag to dissolve in Zn?
2. More Zn or Al
in Cu?
Element Atomic Crystal Electro- Valence Radius Structure nega- (nm) tivity
Cu FCC C H O Ag FCC Al FCC Co HCP Cr BCC Fe BCC Ni FCC Pd FCC Zn HCP

20. Imperfections in Solids
Specification of composition( or concentration
of alloy in terms of its constituent elements)
weight percent
m1 = mass of component 1
nm1 = number of moles of component 1
atom percent

21. problem
What is the composition , in atom percent of an alloy that contains 33 g copper and 47 g zinc?
Solution: first becomes necessary to compute the number of moles of both Cu and Zn, Thus, the number of moles of Cu is just
Likewise, for Zn

22. now
also

23. problem2
What is the composition, in atom percent, of an alloy that consists of 5.5 wt% Pb and 94.5 wt% Sn(tin)?
Solution:

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

25. Imperfections in Solids
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

26. Imperfections in Solids
Edge Dislocation
Fig. 4.3, Callister 7e.

27. Motion of Edge Dislocation
• Dislocation motion requires the successive bumping
of a half plane of atoms (from left to right here).
• Bonds across the slipping planes are broken and
remade in succession.
Atomic view of edge
dislocation motion from
left to right as a crystal
is sheared.

28. Imperfections in Solids
Screw Dislocation
Screw Dislocation b
Dislocation
line
Burgers vector b
(b)
(a)

29. Edge, Screw, and Mixed Dislocations

30. Imperfections in Solids
Dislocations are visible in electron micrographs

31. Dislocations & Crystal Structures
• Structure: close-packed
planes & directions
are preferred.
view onto two
close-packed
planes.
close-packed directions
close-packed plane (bottom)
close-packed plane (top)
• Comparison among crystal structures:
FCC: many close-packed planes/directions;
HCP: only one plane, 3 directions;
BCC: none
• Specimens that
were tensile
tested.
Mg (HCP)
tensile direction
Al (FCC)

32. Surface energy

33. Planar (interfacial)Defects in Solids
External surfaces one of most obvious boundaries, along which the crystal structure terminates. Surface atoms are not bonded to the maximum number of nearest neighbors, and are therefore in a higher energy state than the atoms at interior positions
Grain boundaries occur where the crystallographic direction of the lattice abruptly changes. This usually occurs when two crystals begin growing separately and then meet.

34. Planar Defects in Solids
twin boundary (plane)
Essentially a reflection of atom positions across the twin plane.
there is a specific mirror lattice symmetry; that is, atoms on one side of the boundary are located in mirror-image positions of the atoms on the other side. The region of material between these boundaries is appropriately termed a twin

35. 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
For FCC metals an error in ABCABC packing sequence
Ex: ABCABABC
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

36. problem For each of the following stacking sequences found in
FCC metals, cite the type of planar defect that exists:
…..A B C A B C B A C B A
……A B C A B C B C A B C

37. Solution:
The interfacial defect that exists for this stacking sequence is a twin boundary, which occurs at the indicated position
The stacking sequence on one side of this position is mirrored on the other side
b) The interfacial defect that exists within this FCC stacking sequence is a stacking fault, which occurs between the two lines.
Within this region, the stacking sequence is HCP.

38. Bulk(volume) defects
These include pores, cracks, foreign inclusions, and other phases. They are normally introduced during processing and fabrication steps
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.

39. Microscopic Examination
Crystallites (grains) and grain boundaries. Vary considerably in size. Can be quite large
ex: Large single crystal of diamond
ex: Aluminum light garbage can see the individual grains
Crystallites (grains) can be quite small (mm or less) – necessary to observe with a microscope.

40. Microscopic Examination
Several important applications of micro-structural examinations are as follows:
To ensure that the association between the properties and structure ( and defects ) are properly understood
To predict the properties of materials once these relationship have been established
To design the alloys with new property combinations
To determine whether or not a material has been correctly heat treated
To ascertain the mode of mechanical fracture

41. Optical Microscopy • Useful up to 2000X magnification.
• Polishing removes surface features (e.g., scratches)
• Etching changes reflectance, depending on crystal
orientation.
0.75mm
the chemical reactivity of the grains of some single-phase materials depends on crystallographic orientation
crystallographic planes
Micrograph of
brass (a Cu-Zn alloy)

42. Optical Microscopy Grain boundaries... Fe-Cr alloy
• are imperfections,
• are more susceptible
to etching,
• may be revealed as
dark lines,
• change in crystal
orientation across
boundary.
Fe-Cr alloy
(b)
grain boundary
surface groove
polished surface
(a)

43. Optical Microscopy Polarized light
metallographic scopes often use polarized light to increase contrast
Also used for transparent samples such as polymers

44. Microscopy Optical resolution ca. 10-7 m = 0.1 m = 100 nm
For higher resolution need higher frequency
X-Rays? Difficult to focus.
Electrons
wavelengths ca. 3 pm (0.003 nm)
(Magnification - 1,000,000X)
Atomic resolution possible
Electron beam focused by magnetic lenses.
Optical microscopy good to ca. wavelength of light
Higher frequencies
X-rays – good idea but difficult to focus
Electrons - wavelength proportional to velocity with high voltage (high acceleration) get wavelengths ca. 3pm (0.003nm) (x1,000,000)

45. Scanning Tunneling Microscopy (STM)
• Atoms can be arranged and imaged!
Carbon monoxide molecules arranged on a platinum (111) surface.
Iron atoms arranged on a copper (111) surface. These Kanji characters represent the word “atom”.

46. Summary • Point, Line, and Area defects exist in solids.
• The number and type of defects can be varied
and controlled (e.g., T controls vacancy conc.)
• Defects affect material properties (e.g., grain
boundaries control crystal slip).
• Defects may be desirable or undesirable
(e.g., dislocations may be good or bad, depending
on whether plastic deformation is desirable or not.)