Lec04 - Imperfections in Solids
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Transcript of Lec04 - Imperfections in Solids
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Chapter 4- 2
• Vacancy atoms • Interstitial atoms • Substitutional atoms
• Dislocations
• Grain Boundaries
Point defects
Line defects
Area defects
TYPES OF IMPERFECTIONS
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Chapter 4- 3
• Vacancies :
-vacant atomic sites in a structure.
Vacancydistortionof planes
• Self-Interstitials :-"extra" atoms positioned between atomic sites.
self-interstitialdistortion
of planes
POINT DEFECTS
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Chapter 4-
Boltzmann's constant
(1.38 x 10-23
J/atom K)(8.62 x 10 -5 eV/at om K)
ND N
exp Q Dk T
No. of defects
No. of potentialdefect sites.
Activation energy
Temperature
Each lattice siteis a potentialvacancy site
4
• Equilibrium concentration varies with temperature!
EQUIL. CONCENTRATION:POINT DEFECTS
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Chapter 4- 6
• Find the equil. # of vacancies in 1m of Cu at 1000C. • Given:
3
8.62 x 10 -5 eV/atom-K
0.9eV/atom
1273K
ND N
expQ Dk T
For 1m 3 , N = NAACu
x x 1m 3 = 8.0 x 10 28 sites
= 2.7 · 10 -4
• Answer:
MEASURING ACTIVATION ENERGY
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Chapter 4- 7
• Low energy electron
microscope view ofa (110) surface of NiAl.• Increasing T causes
surface island ofatoms to grow.
• Why? The equil. vacancyconc. increases via atommotion from the crystalto the surface, where
they join the island.Reprinted with permission from Nature (K.F. McCarty,J.A. Nobel, and N.C. Bartelt, "Vacancies in
Solids and the Stability of Surface Morphology",Nature, Vol. 412, pp. 622-625 (2001). Image is5.75 mm by 5.75 mm.) Copyright (2001) MacmillanPublishers, Ltd.
OBSERVING EQUIL. VACANCY CONC.
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Chapter 4- 8
Two outcomes of impurity (B) added to host (A):• Solid solution of B in A (i.e., random dist. of point defects)
• Solid solution of B in A plus particles of a newphase (usually for a larger amount of B)
OR
Substitutional alloy(e.g., Cu in Ni)
Interstitial alloy(e.g., C in Fe )
Second phase particle--different composition --often different structure.
POINT DEFECTS IN ALLOYS
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Chapter 4- 9
• Low energy electron microscope view ofa (111) surface of Cu.
• Sn islands move along the surface and "alloy"the Cu with Sn atoms,to make "bronze".
• The islands continually move into "unalloyed"
regions and leave tinybronze particles intheir wake.
• Eventually, the islands disappear.
Reprinted with permission from: A.K. Schmid, N.C.Bartelt, and R.Q. Hwang, "Alloying at Surfaces bythe Migration of Reactive Two-Dimensional Islands",Science, Vol. 290, No. 5496, pp. 1561-64 (2000).Field of view is 1.5 mm and the temperature is290K.
ALLOYING A SURFACE
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Chapter 4- 10
Definition: Amount of impurity (B) and host (A)
in the system .
• Weight %
Two descriptions:• Atom %
• Conversion between wt % and at% in an A -B alloy:
C B =C' BAB
C' AAA + C' BABx 100
• Basis for conversion:
COMPOSITION
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Chapter 4- 11
• are line defects, • cause slip between crystal plane when they move, • produce permanent (plastic) deformation.
Dislocations :
Schematic of a Zinc (HCP):• before deformation • after tensile elongation
slip steps
LINE DEFECTS
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Chapter 4- 12
• Dislocations slip planes incrementally ...
• The dislocation line (the moving red dot)... ...separates slipped material on the left
from unslipped material on the right.
Simulation of dislocationmotion from left to right
as a crystal is sheared.
(Courtesy P.M. Anderson)
INCREMENTAL SLIP
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Chapter 4- 13
• 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 edgedislocation motion fromleft to right as a crystalis sheared.
(Courtesy P.M. Anderson)
BOND BREAKING AND REMAKING
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Chapter 4- 15
Grain boundaries :
• are boundaries between crystals. • are produced by the solidification process. • for example, a change in crystal orientation across them. • impede dislocation motion.
grain boundaries
Schematic
Adapted from Fig. 4.7, Callister 6e. Adapted from Fig. 4.10, Callister 6e. (Fig.
4.10 is from Metals Handbook , Vol. 9, 9th edition,Metallography and Microstructures , Am. Society for Metals,Metals Park, OH, 1985.)
~ 8cmMetal Ingot
AREA DEFECTS: GRAIN BOUNDARIES
An ingot is a material,usually metal , that is cast into ashape suitable for further
processing. Non-metallic andsemiconductor materials prepared in
bulk form may also be referred to asingots, particularly when cast bymold based methods.
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Chapter 4- 16
• Useful up to 2000X magnification.
• Polishing removes surface features (e.g., scratches) • Etching changes reflectance, depending on crystal
orientation.
close-packed planes
micrograph ofBrass (Cu and Zn)
Adapted from Fig. 4.11(b) and (c), Callister6e. (Fig. 4.11(c) is courtesyof J.E. Burke, General Electric Co.
0.75mm
OPTICAL MICROSCOPY (1)
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Chapter 4- 17
Grain boundaries... • are imperfections, • are more susceptible
to etching,• may be revealed as
dark lines,• change direction in a
polycrystal. Adapted from Fig. 4.12(a)and (b), Callister 6e. (Fig. 4.12(b) is courtesyof L.C. Smith and C. Brady,the National Bureau ofStandards, Washington, DC[now the National Institute ofStandards and Technology,Gaithersburg, MD].)
OPTICAL MICROSCOPY (2)
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Chapter 4- 18
• Point , Line , and Area defects arise 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.)
SUMMARY