Lecture 10 - Ch 4 pt 3

Lecture 10 - Ch 4 pt 3 - Dislocations & Crystal...

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Unformatted text preview: Dislocations & Crystal Structures • Structure: close-packed planes & directions preferred: Slip system = a slip direction and a slip plane close-packed plane (bottom) view onto two close-packed planes close-packed directions close-packed plane (top) • Comparison between crystal structures: FCC: many close-packed planes/directions HCP: only one plane, 3 directions BCC: none - (111) is not a close-packed plane Mg (HCP) Specimens that were tensile tested tensile direction Al (FCC) Chapter 4 - 36 Other Planar Defects in Solids • One case is a twin boundary (plane): Specific mirror lattice symmetry…essentially a reflection of atom positions across the twin plane Caused by atomic displacements under shear Adapted from Fig. 4.9, Callister 7e. • Another case is Stacking Faults: For FCC metals, an error in the ABCABC packing sequence e.g.: ABCABABC Chapter 4 - 37 Microscopic Examination • Crystallites (grains) and grain boundaries: – Vary considerably in size – Can be quite large (up to several cm) • e.g.: Large single crystal of quartz, diamond or Si • e.g.: Aluminum light pole or galvanized garbage can see the individual grains • Crystallites (grains) can also be quite small (mm, m or less) - necessary to observe using a microscope… Chapter 4 - 38 Optical Microscopy • Useful up to 1000-2000X magnification • Polishing removes surface features (e.g., scratches) • Etching changes reflectance, depending on crystal orientation Crystallographic planes of different orientations Micrograph of Polycrystalline brass (a Cu-Zn alloy) Adapted from Fig. 4.13 (b) and (c), Callister 7e. (Fig. 4.13(c) is courtesy of J. E. Burke, General Electric Co. 0.75 mm Chapter 4 - 39 Optical Microscopy Grain Boundaries: are imperfections are more susceptible to etching may be revealed as dark lines changes in crystal orientation across boundary polished & etched surface surface groove grain boundary (a) ASTM Grain Size number N = 2n-1 Fe-Cr alloy Adapted from Fig. 4.14(a) and (b), Callister 7e. (Fig. 4.14(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].) (b) grains/in2 Average number of at 100x magnification Chapter 4 - 40 Optical Microscopy • Polarized light: – metallographic microscopes often use polarized light to increase contrast – Also used for transparent samples such as polymers Chapter 4 - 41 Microscopy Optical resolution ca. 10-7 m = 0.1 μm = 100 nm For higher resolution need higher frequency – X-Rays? Difficult to focus! – Electrons: • Wavelengths ca. 3 pm (0.003 nm) – (Magnification - 1,000,000 X) • Atomic resolution possible • Electron beam focused by magnetic lenses Chapter 4 - 42 Scanning Tunneling Microscopy (STM) • Atoms can be arranged and imaged! Photos produced from the work of C.P. Lutz, Zeppenfeld, and D.M. Eigler. Reprinted with permission from International Business Machines Corporation, copyright 1995. Carbon monoxide molecules arranged on a Platinum (111) surface. Iron atoms arranged on a Copper (111) surface. These Kanji characters represent the word “atom”. Chapter 4 - 43 Summary Point, Line and Area defects exist 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 - 44 ...
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