Understanding Defects in Solids: Vacancies, Dislocations, and Grain Boundaries, Slides of Material Science and Technology

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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?

IMPERFECTIONS IN SOLIDS

• Vacancy atoms
• Interstitial atoms
• Substitutional atoms
  • Dislocations
  • Grain Boundaries

Point defects

Line defects

Area defects

TYPES OF IMPERFECTIONS

• 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 in Solids and the Stability of Surface Morphology", Nature, Vol. 412, pp. 622-625 (2001). Image is 5.75 m by 5.75 m.) Copyright (2001) Macmillan Publishers, Ltd.

OBSERVING EQUIL. VACANCY CONC.

Click on image to animate

• 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

• 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) INCREMENTAL SLIP Click on image to animate

• 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) BOND BREAKING AND REMAKING Click on image to animate

• 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

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.) ~ 8cm Metal Ingot AREA DEFECTS: GRAIN BOUNDARIES

• +^ +^ +^ +^ +

• 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

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• Produces plastic deformation,
• Depends on incrementally breaking bonds. Plastically stretched zinc single crystal.
• 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 edge dislocation motion from left to right as a crystal is 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 two close-packed planes. DISLOCATIONS & CRYSTAL STRUCTURE