AREA DEFECTS GRAIN BOUNDARIES Grain boundaries Burgers vector

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 Low-angle or tilt grain boundary Adapted from Fig. 4. 7, Callister 6 e. 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 2000 X 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 6 e. (Fig. 4. 11(c) is courtesy of J. E. Burke, General Electric Co. micrograph of Brass (Cu and Zn) 0. 75 mm 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 6 e. (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 > 2000 X. • 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 X. Used frequently to study dislocations. • SEM has a great depth of field. Magnification from 10 X-50, 000 X. 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