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ENME382 S2010 0201 ch03_1

ENME382 S2010 0201 ch03_1 - Chapter 3 Structures of Metals...

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ISSUES TO ADDRESS... How do atoms assemble into solid structures? How does the density of a material depend on its structure? Chapter 3: Structures of Metals & Ceramics Chapter 3 - ENME 382 Spring 2010 Section 0201 1 When do material properties vary with the sample (i.e., part) orientation? How do the crystal structures of ceramic materials differ from those for metals?
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Non dense, random packing Energy and Packing Energy r typical neighbor bond length typical neighbor bond energy Chapter 3 - ENME 382 Spring 2010 Section 0201 2 Dense, ordered packing Dense, ordered packed structures tend to have lower energies. Energy r typical neighbor bond length typical neighbor bond energy
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atoms pack in periodic, 3D arrays Crystalline materials... -metals -many ceramics -some polymers crystalline SiO 2 Adapted from Fig. 3.40(a), Callister & Rethwisch 3e. Materials and Packing typical of: Chapter 3 - ENME 382 Spring 2010 Section 0201 3 atoms have no periodic packing Noncrystalline materials... -complex structures -rapid cooling noncrystalline SiO 2 " Amorphous " = Noncrystalline Adapted from Fig. 3.40(b), Callister & Rethwisch 3e. Si Oxygen occurs for:
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Metallic Crystal Structures How can we stack metal atoms to minimize empty space? 2-dimensions Chapter 3 - ENME 382 Spring 2010 Section 0201 4 vs. Now stack these 2-D layers to make 3-D structures
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Tend to be densely packed. Reasons for dense packing: - Typically, only one element is present, so all atomic radii are the same. - Metallic bonding is not directional. - Nearest neighbor distances tend to be small in Metallic Crystal Structures Chapter 3 - ENME 382 Spring 2010 Section 0201 5 order to lower bond energy. - Electron cloud shields cores from each other Have the simplest crystal structures. We will examine three such structures...
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Rare due to low packing density (only Po has this structure) Close-packed directions are cube edges. Coordination # = 6 (# nearest neighbors) Simple Cubic Structure (SC) Chapter 3 - ENME 382 Spring 2010 Section 0201 6 (Courtesy P.M. Anderson)
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APF for a simple cubic structure = 0.52 volume Atomic Packing Factor (APF) APF = Volume of atoms in unit cell* Volume of unit cell *assume hard spheres Chapter 3 - ENME 382 Spring 2010 Section 0201 7 APF = a 3 4 3 π (0.5 a ) 3 1 atoms unit cell atom unit cell volume Adapted from Fig. 3.42, Callister & Rethwisch 3e. close-packed directions a R =0.5 a contains 8 x 1/8 = 1 atom/unit cell
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Coordination # = 8 Atoms touch each other along cube diagonals. --Note: All atoms are identical; the center atom is shaded differently only for ease of viewing. Body Centered Cubic Structure (BCC) ex: Cr, W, Fe ( α ), Tantalum, Molybdenum Chapter 3 - ENME 382 Spring 2010 Section 0201 8 Adapted from Fig. 3.2, Callister & Rethwisch 3e. (Courtesy P.M. Anderson) QuickTime™ and a Cinepak decompressor are needed to see this picture.
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