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. 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 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. close-packed planes micrograph of Brass (Cu and Zn) Adapted from Fig. 4.11(b) and (c), Callister 6e. (Fig. 4.11(c) is courtesy of J.E. Burke, General Electric Co. 0.75mm OPTICAL MICROSCOPY (1)

Chapter 4-17 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].) 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 dislocations using TEM

Chapter 4-16 resolution in the nanometer range (mags. up to 10 9 X) 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