MSE 528

Crystal structures and Defects

Fall 2010

ISSUES TO ADDRESS...

• What types of defects arise in solids?

• Can the number and type of defects be variedand controlled?

• How do defects affect material properties?

• Are defects undesirable?

• How do point defects in ceramics differ from thosein metals?

• In ceramics, how are impurities accommodatedin the lattice and how do they affect properties?

IMPERFECTIONS IN SOLIDS

• Vacancy atoms• Interstitial atoms• Substitutional atoms

• Dislocations

• Grain Boundaries

Point defects

Line defects

Area defects

TYPES OF IMPERFECTIONS

• Vacancies:-vacant atomic sites in a structure.

Vacancydistortion of planes

• Self-Interstitials:-"extra" atoms positioned between atomic sites.

self-interstitialdistortion

of planes

POINT DEFECTS

• 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.

Island grows/shrinks to maintain equil. vancancy conc. in the bulk.

Reprinted with permission from Nature (K.F. McCarty, J.A. Nobel, and N.C. Bartelt, "Vacancies inSolids and the Stability of Surface Morphology",Nature, Vol. 412, pp. 622-625 (2001). Image is5.75 m by 5.75 m.) Copyright (2001) Macmillan Publishers, Ltd.

OBSERVING EQUIL. VACANCY CONC.

• 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

• Ti alloy after cold working:

• Dislocations entangle with one another during cold work.• Dislocation motion becomes more difficult.

0.9 m

Adapted from Fig. 4.6, Callister 6e. (Fig. 4.6 is courtesy of M.R. Plichta, Michigan Technological University.)

DISLOCATIONS DURING COLD WORK

• Dislocation generate stress.• This traps other dislocations.

DISLOCATION-DISLOCATION TRAPPING

Red dislocation generates shear at pts A and B that opposes motion of green disl. from left to right.

A

B

+ +

+ +

+ + + + + + + + + + + + +

+ + + + + + +

• 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.

DISLOCATIONS & MATERIALS CLASSES

+ + + +

+ + +

+ + + +

- - -

- - - -

- - -

• Produces plastic deformation,• Depends on incrementally breaking bonds.

Plasticallystretchedzincsinglecrystal.

• If dislocations don't move, deformation doesn't happen!

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.)

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. 7.8, Callister 6e.

DISLOCATION MOTION

• 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

• Structure: close-packed planes & directions are preferred.

• Comparison among crystal structures: FCC: many close-packed planes/directions; HCP: only one plane, 3 directions; BCC: none

close-packed plane (bottom)close-packed plane (top)

close-packed directions

Mg (HCP)

Al (FCC)tensile direction

• Results of tensile testing.

view onto twoclose-packedplanes.

DISLOCATIONS & CRYSTAL STRUCTURE

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.

grain boundaries

heat flow

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

Fe-Cr alloy

microscope

grain boundarysurface groove

polished surface

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 of Standards, Washington, DC [now the National Institute of Standards and Technology, Gaithersburg, MD].)

ASTM grain size number

N = 2n-1

no. grains/in2 at 100x magnification

OPTICAL MICROSCOPY (2)