mse 3300-lecture note 07-chapter 05 imperfections in solids

8/18/2019 MSE 3300-Lecture Note 07-Chapter 05 Imperfections in Solids

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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


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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


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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



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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



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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



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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



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Dislocations are visible in electron micrographs

Fig. 4.7, Callister & Rethwisch 9e.(Courtesy of M. R. Plichta, Michigan

Technological University.)



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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



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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



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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.



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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.



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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)



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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.



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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



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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.



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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



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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.


Homework 3

mse 3300-lecture note 07-chapter 05 imperfections in solids

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