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IMPERFECTIONS IN SOLIDS. ISSUES TO ADDRESS. • 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? How do point defects in ceramics differ from those in metals?

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imperfections in solids
IMPERFECTIONS IN SOLIDS

ISSUES TO ADDRESS...

  • • 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?
  • How do point defects in ceramics differ from those
  • in metals?
  • In ceramics, how are impurities accommodated
  • in the lattice and how do they affect properties?
types of imperfections
TYPES OF IMPERFECTIONS

• Vacancy atoms

• Interstitial atoms

• Substitutional atoms

Point defects

Line defects

Area defects

• Dislocations

• Grain Boundaries

point defects
POINT DEFECTS

• Vacancies:

-vacant atomic sites in a structure.

• Self-Interstitials:

-"extra" atoms positioned between atomic sites.

observing equil vacancy conc
OBSERVING EQUIL. 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.

Click on image to animate

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 is

5.75 mm by 5.75 mm.) Copyright (2001) Macmillan Publishers, Ltd.

line defects
LINE DEFECTS

Dislocations:

• are line defects,

• cause slip between crystal plane when they move,

• produce permanent (plastic) deformation.

Schematic of a Zinc (HCP):

• before deformation

• after tensile elongation

slip steps

incremental slip
INCREMENTAL SLIP

• 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 dislocation

motion from left to right

as a crystal is sheared.

Click on image to animate

(Courtesy P.M. Anderson)

bond breaking and remaking
BOND BREAKING AND REMAKING

• 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.

(Courtesy P.M. Anderson)

Click on image to animate

dislocations during cold work
DISLOCATIONS DURING COLD WORK

• Ti alloy after cold working:

• Dislocations entangle

with one another

during cold work.

• Dislocation motion

becomes more difficult.

Adapted from Fig. 4.6, Callister 6e.

(Fig. 4.6 is courtesy of M.R. Plichta, Michigan Technological University.)

dislocation dislocation trapping
DISLOCATION-DISLOCATION TRAPPING

• Dislocation generate stress.

• This traps other dislocations.

area defects grain boundaries
AREA DEFECTS: GRAIN BOUNDARIES

Grain boundaries:

• are boundaries between crystals.

• are produced by the solidification process, for example.

• have a change in crystal orientation across them.

• impede dislocation motion.

Metal Ingot

Schematic

~ 8cm

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.)

optical microscopy 2
OPTICAL MICROSCOPY (2)

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 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].)

dislocations materials classes
DISLOCATIONS & MATERIALS CLASSES

• Metals: Disl. motion easier.

-non-directional bonding

-close-packed directions

for slip.

electron cloud

ion cores

• Covalent Ceramics

(Si, diamond): Motion hard.

-directional (angular) bonding

• Ionic Ceramics (NaCl):

Motion hard.

-need to avoid ++ and --

neighbors.

dislocation motion
DISLOCATION MOTION

Plastically

stretched

zinc

single

crystal.

• Produces plastic deformation,

• Depends on incrementally breaking

bonds.

Adapted from Fig. 7.9, Callister 6e. (Fig. 7.9 is from C.F. Elam, The Distortion of Metal Crystals, Oxford University Press, London, 1935.)

Adapted from Fig. 7.1, Callister 6e. (Fig. 7.1 is adapted from A.G. Guy, Essentials of Materials Science, McGraw-Hill Book Company,

New York, 1976. p. 153.)

• If dislocations don't move,

deformation doesn't happen!

Adapted from Fig. 7.8, Callister 6e.

incremental slip18
INCREMENTAL SLIP

• 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 dislocation

motion from left to right

as a crystal is sheared.

(Courtesy P.M. Anderson)

bond breaking and remaking19
BOND BREAKING AND REMAKING

• 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.

(Courtesy P.M. Anderson)

dislocations crystal structure
DISLOCATIONS & CRYSTAL STRUCTURE

• Structure: close-packed

planes & directions

are preferred.

view onto two

close-packed

planes.

• Comparison among crystal structures:

FCC: many close-packed planes/directions;

HCP: only one plane, 3 directions;

BCC: none

• Results of tensile

testing.

Mg (HCP)

tensile direction

Al (FCC)

stress and dislocation motion
STRESS AND DISLOCATION MOTION

• Crystals slip due to a resolved shear stress, tR.

• Applied tension can produce such a stress.

slip plane

normal, ns

slip

direction

slip

direction

slip

direction

critical resolved shear stress
CRITICAL RESOLVED SHEAR STRESS

• Condition for dislocation motion:

• Crystal orientation can make

it easy or hard to move disl.

disl motion in polycrystals
DISL. MOTION IN POLYCRYSTALS

• Slip planes & directions

(l, f) change from one

crystal to another.

• tRwill vary from one

crystal to another.

• The crystal with the

largest tR yields first.

• Other (less favorably

oriented) crystals

yield later.

Adapted from Fig. 7.10, Callister 6e.

(Fig. 7.10 is courtesy of C. Brady, National Bureau of Standards [now the National Institute of Standards and Technology, Gaithersburg, MD].)

300 mm