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ch03 - Chapter 3 Structures of Metals Ceramics ISSUES TO...

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Chapter 3 - 1 ISSUES TO ADDRESS... How do atoms assemble into solid structures? How does the density of a material depend on its structure? When do material properties vary with the sample (i.e., part) orientation? Chapter 3: Structures of Metals & Ceramics How do the crystal structures of ceramic materials differ from those for metals?
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Chapter 3 - 2 Non dense, random packing Dense, ordered packing Dense, ordered packed structures tend to have lower energies. Energy and Packing Energy r typical neighbor bond length typical neighbor bond energy Energy r typical neighbor bond length typical neighbor bond energy
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Chapter 3 - 3 atoms pack in periodic, 3D arrays Crystalline materials... -metals -many ceramics -some polymers atoms have no periodic packing Noncrystalline materials... -complex structures -rapid cooling crystalline SiO 2 noncrystalline SiO 2 " Amorphous " = Noncrystalline Adapted from Fig. 3.40(b), Callister & Rethwisch 3e. Adapted from Fig. 3.40(a), Callister & Rethwisch 3e. Materials and Packing Si Oxygen typical of: occurs for:
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Chapter 3 - 4 Metallic Crystal Structures How can we stack metal atoms to minimize empty space? 2-dimensions vs. Now stack these 2-D layers to make 3-D structures
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Chapter 3 - 5 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 order to lower bond energy. - Electron cloud shields cores from each other Have the simplest crystal structures. We will examine three such structures... Metallic Crystal Structures
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Chapter 3 - 6 Rare due to low packing density (only Po has this structure) Close-packed directions are cube edges. Coordination # = 6 (# nearest neighbors) (Courtesy P.M. Anderson) Simple Cubic Structure (SC)
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Chapter 3 - 7 APF for a simple cubic structure = 0.52 APF = a 3 4 3 π (0.5 a ) 3 1 atoms unit cell atom volume unit cell volume Atomic Packing Factor (APF) APF = Volume of atoms in unit cell* Volume of unit cell *assume hard spheres 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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Chapter 3 - 8 Coordination # = 8 Adapted from Fig. 3.2, Callister & Rethwisch 3e. (Courtesy P.M. Anderson) 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 QuickTime and a Cinepak decompressor are needed to see this picture. 2 atoms/unit cell: 1 center + 8 corners x 1/8
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Chapter 3 - 9 Atomic Packing Factor: BCC a APF = 4 3 π ( 3 a /4) 3 2 atoms unit cell atom volume a 3 unit cell volume length = 4 R = Close-packed directions: 3 a APF for a body-centered cubic structure = 0.68 a R Adapted from Fig. 3.2(a), Callister & Rethwisch 3e.
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