AREA DEFECTS: GRAIN BOUNDARIES
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Chapter 4-15 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 Schematic Adapted from Fig. 4.7, Callister 6e. AREA DEFECTS: GRAIN BOUNDARIES Low-angle or tilt grain boundary Burgers vector
description
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. Burgers vector. Schematic. - PowerPoint PPT Presentation
Transcript of AREA DEFECTS: GRAIN BOUNDARIES
Chapter 4-15
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
Schematic
Adapted from Fig. 4.7, Callister 6e.
AREA DEFECTS: GRAIN BOUNDARIES
Low-angle or tilt grain boundary
Burgers vector
Chapter 4-15
• 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 relatively- large inclusions, other material phases (to be studied more later).• Atomic Vibrations: a function of temperature T (actually define it). Typically 1013 vibrations/second.
OTHER DEFECTS:
Chapter 4-16
• Useful up to 2000X magnification.• Polishing removes surface features (e.g., scratches)• Etching changes reflectance, depending on crystal orientation.
microscope
close-packed planes
micrograph ofBrass (Cu and Zn)
Adapted from Fig. 4.11(b) and (c), Callister 6e. (Fig. 4.11(c) is courtesyof J.E. Burke, General Electric Co.
0.75mm
OPTICAL MICROSCOPY (1)
Chapter 4-
Fe-Cr alloy
microscope
grain boundarysurface groove
polished surface
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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)
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 4-16
• 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 10X-50,000X.
ELECTRON MICROSCOPY
Real dislocationsusing TEM
Chapter 4-16
• resolution in the nanometer range (mags. up to 109X)• Examples: Scanning Tunneling Microscope (STM), and Atomic Force Microscope (AFM)• SPMs give three-dimensional images with surface topography information.
SCANNING PROBE MICROSCOPY (SPM)
An AFM image of (111) atoms in gold
Chapter 4-18
• 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.)
SUMMARY
