Transcript Chapter 4

AREA DEFECTS: GRAIN BOUNDARIES
Grain boundaries:
•
•
•
•
Burgers vector
are boundaries between crystals.
are produced by the solidification process, for example.
have a change in crystal orientation across them.
impede dislocation motion.
Schematic
grain
boundaries
Low-angle or tilt
grain boundary
Adapted from Fig. 4.7, Callister 6e.
Chapter 4- 15
OTHER DEFECTS:
• External surfaces of a material (where bonds are not
complete for
atoms).
• Stacking faults in FCC materials (i.e. a loss or interruption
in the
ABCABC… sequence)
• Bulk or Volume Defects: like cracks, voids or pores, foreign
relativelylarge inclusions, other material phases (to be studied more
later).
• Atomic Vibrations: a function of temperature T (actually
define it).
Typically 1013 vibrations/second.
Chapter 4- 15
OPTICAL MICROSCOPY (1)
• Useful up to 2000X magnification.
• Polishing removes surface features (e.g., scratches)
• Etching changes reflectance, depending on crystal
orientation.
close-packed planes
Adapted from Fig. 4.11(b) and (c),
Callister 6e. (Fig. 4.11(c) is courtesy
of J.E. Burke, General Electric Co.
micrograph of
Brass (Cu and Zn)
0.75mm
Chapter 4- 16
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].)
Or use line method: measure grain numbers intersecting
several (5-10) lines drawn on a photomicrograph.
Divide line length by average grain number by
magnification.
Chapter 4- 17
ELECTRON MICROSCOPY
• magnification > 2000X.
• Examples: Transmission Electron Microscope (TEM)
Scanning Electron Microscope (SEM)
• TEM and SEM use electron beams instead of light beams
• SEM pictures a top view of a sample (needs to be electrically
conductive but no need for polishing and etching)
• TEM “sees” through a thin foil of a specimen. Magnification up
to 1,000,000X. Used frequently to study dislocations.
• SEM has a great depth of field. Magnification from 10X50,000X.
Real dislocations
using TEM
Chapter 4- 16
SCANNING PROBE MICROSCOPY
(SPM)
9
• resolution in the nanometer range (mags. up to 10 X)
• Examples: Scanning Tunneling Microscope (STM), and Atomic
Force
Microscope (AFM)
• SPMs give three-dimensional images with surface topography
An AFM image of (111) atoms in gold
information.
Chapter 4- 16
SUMMARY
• Point, Line, and Area defects arise in solids.
• The number and type of defects can be varied
and controlled (e.g., T controls vacancy conc.)
• Defects affect material properties (e.g., grain
boundaries control crystal slip).
• Defects may be desirable or undesirable
(e.g., dislocations may be good or bad, depending
on whether plastic deformation is desirable or not.)
Chapter 4- 18