Upload
chuong
View
218
Download
0
Embed Size (px)
Citation preview
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
1/17
Lecture 7. Imperfections in Solids (2)
Learning Objectives After this lecture, you should be able to do the following:
1. What type of defects exist in solids?
2. Can the number and type of defects be varied and controlled?
3. What are the solidification mechanisms?
4. How do defects affect material properties?
5. Are defects undesirable?
Reading
• Chapter 4: Imperfection in Solids (4.5–4.10)Multimedia
• Virtual Materials Science & Engineering (VMSE):
http://www.wiley.com/college/callister/CL_EWSTU01031_S/vmse/
Lecture 7 - 1MSE 3300 / 5300 UTA SPRING 2015
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
2/17
Line Defects
• 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
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 2
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
3/17
Imperfections in Solids
Linear Defects (Dislocations)• Are one-dimensional defects around which atoms are
misaligned (due to presence of an extra portion of plane ofatoms or half plane)
• Edge dislocation: • extra half-plane of atoms inserted in a crystal structure
• b perpendicular (⊥) to dislocation line
• Screw dislocation:• spiral planar ramp resulting from shear deformation
• b parallel (||) to dislocation line
Burger ’s vector, b: measure of lattice distortion
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 3
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
4/17
Imperfections in SolidsEdge Dislocation
Fig. 4.4, Callister & Rethwisch 9e. (Adapted from A. G. Guy, Essentials of Materials Science, McGraw-Hill
Book Company, New York, NY, 1976, p. 153.)
Copyright © 2014 John Wiley & Sons, Inc. All rights reserved
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 4
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
5/17
Imperfections in Solids
Screw Dislocation
Adapted from Fig. 4.5, Callister & Rethwisch 9e.[Figure (b) from W. T. Read, Jr.,Dislocations in Crystals,
McGraw-Hill Book Company, New York, NY, 1953.]
Burgers vector b
Dislocation
line
b
(a)
(b)
Screw Dislocation
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 5
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
6/17
Edge, Screw, and Mixed Dislocations
Adapted from Fig. 4.6, Callister & Rethwisch 9e.[Figure (b) from W. T. Read, Jr., Dislocations in Crystals,
McGraw-Hill Book Company, New York, NY, 1953.]
Edge
Screw
Mixed
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 6
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
7/17
Imperfections in Solids
Dislocations are visible in electron micrographs
Fig. 4.7, Callister & Rethwisch 9e.(Courtesy of M. R. Plichta, Michigan
Technological University.)
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 7
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
8/17
Dislocations & Crystal Structures
• Structure: close-packed
planes & directionsare preferred.
view onto two
close-packedplanes.
close-packed plane (bottom) close-packed plane (top)
close-packed directions
• 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)
Al (FCC)tensile direction
Brittle fracture
Ductile fracture
BCC
FCC
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 8
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
9/17
Catalysts and Surface Defects
• A catalyst increases therate of a chemicalreaction without beingconsumed
• Active sites on catalystsare normally surfacedefects
Fig. 4.11, Callister & Rethwisch 9e.
Fig. 4.12, Callister & Rethwisch 9e.[From W. J. Stark, L. Mädler, M. Maciejewski, S. E.
Pratsinis, and A. Baiker, “Flame Synthesis of
Nanocrystalline Ceria/Zirconia: Effect of Carrier
Liquid,” Chem. Comm., 588–589 (2003). Reproduced
by permission of The Royal Society of Chemistry.]
Single crystals of
(Ce0.5Zr 0.5)O2
used in an automotivecatalytic converter
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 9
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
10/17
Microscopic Examination
• Crystallites (grains) and grain boundaries.Vary considerably in size. Can be quite large.
• ex: Large single crystal of quartz or diamond or Si
• ex: Aluminum light post or garbage can - see the
individual grains• Crystallites (grains) can be quite small (mm or
less) – necessary to observe with amicroscope.
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 10
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
11/17
Optical Microscopy
• Useful up to 2000X magnification.
• Polishing removes surface features (e.g., scratches)• Etching changes reflectance, depending on crystal
orientation.
Micrograph of
brass (a Cu-Zn alloy)
0.75 mm
Fig. 4.14(b) & (c), Callister &
Rethwisch 9e.
crystallographic planes
C o ur t e s y of
J .E .B ur k e , G en er al E l e c t r i c C o.
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 11
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
12/17
Optical MicroscopyGrain boundaries...
• are imperfections,• are more susceptible
to etching,
• may be revealed as
dark lines,
• change in crystal
orientation across
boundary.Fig. 4.15(a) & (b), Callister &
Rethwisch 9e. [Fig. 4.15(b) is courtesy of L.C.
Smith and C. Brady, the National
Bureau of Standards, Washington,DC (now the National Institute of
Standards and Technology,
Gaithersburg, MD).]
Fe-Cr alloy(b)
grain boundary
surface groove
polished surface
(a)
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 12
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
13/17
Electron Microscopy
Optical resolution ca. 10
-7
m = 0.1μ
m = 100 nmFor 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.
e.g. - Transmission electron microscope and
scanning electron microscope.
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 13
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
14/17
Scanning Tunneling Microscope
(STM)
• Atoms can be arranged and imaged!
Carbon monoxide
molecules arranged
on a platinum (111)surface.
Photos produced from the
work of C.P. Lutz,
Zeppenfeld, and D.M. Eigler.
Reprinted with permission
from International Business
Machines Corporation,
copyright 1995.
Iron atoms arranged
on a copper (111)
surface. These Kanjicharacters represent
the word “atom”.
• Employs a tiny probe with a sharp tip.• Raster scans the surface
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 14
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
15/17
Summary
• Defects may be desirable or undesirable (e.g., dislocations
may be good or bad, depending on whether plastic
deformation is desirable or not).
• Microscopic Examination of Defects – Optical and
Electron Microscopy
• Line defects – Screw and Edge Dislocations.
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 15
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
16/17
Homework 3
1. Describe both vacancy and self-interstitial crystalline defects.
2. Calculate fraction of atom sites that are vacant for Cu at its melting temperature
of 1084 deg C (1357 K). Assume energy of vacancy formation of 0.90 eV/atom.
Repeat this calculation at room temperature (298 K).
3. Is it possible for three or more elements to form a solid solution. Explain your
answer.
4. Home – Ruthery Law: Using Table in Lecture 6.Which of these elements would
you expect to form the following with copper:
(a) A substitutional solid solution having complete solubility
(b) A substitutional solid solution of incomplete solubility
(c) An interstitial solid solution
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 16
8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids
17/17
5. For each of edge, screw, and mixed dislocations:
(a) describe and make a drawing of the dislocation;
(b) note the location of the dislocation line; and
(c) indicate the direction along which the dislocation line extends.
6. For an FCC crystal would you expect the surface energy for a (100) to be
greater or less than that of (111) plane . Explain why.
7. Explain what preparations are necessary for observation of the grain structure of
a polycrystalline material with an optical microscope.
8. Name and briefly describe the operation of each of the two types of electron
microscopes. How are they different from Scanning Probe Microscope.
MSE 3300 / 5300 UTA SPRING 2015 Lecture 7 - 17
Homework 3