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lecture%205 - ENGR 2220 ENGR 2220 – Structure &...

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Unformatted text preview: ENGR 2220 ENGR 2220 – Structure & Properties of Materials Structure & Properties of Materials Lecture 5 Lecture 5 Structure of Structure of Crystalline Solids Crystalline Solids hapter 3 hapter 3 Chapter 3 Chapter 3 CHAPTER 3: CRYSTAL TRUCTURES & PROPERTIES STRUCTURES & PROPERTIES ISSUES TO ADDRESS... How do atoms assemble into solid structures? • How do atoms assemble into solid structures? (for now, focus on metals) • How does the density of a material depend on its structure? • When do material properties vary with the sample (i.e., part) orientation? Fundamental Concepts • Crystalline material epeating or periodic arrays Repeating or periodic arrays – Large atomic distances • One neighbor bonded to the next one g – All metals are crystalline any ceramics Many ceramics – Some polymers ome properties depend upon cry Some properties depend upon cry. structure Fundamental Concepts • Atoms are solid hard spheres • eighboring spheres touch each other Neighboring spheres touch each other • Lattice – Array of points coinciding with atom centers Unit Cells Small repeat entity Chosen to represent symmetry of crystal structure Building block Usually corners coincide with atom centers ENERGY AND PACKING F • Non dense, random packing Energy typical neighbor ond length r r bond length typical neighbor bond energy • Dense, regular packing Energy typical neighbor bond length ense regular acked structures tend to have r typical neighbor bond energy Dense, regular-packed structures tend to have lower energy. MATERIALS AND PACKING • atoms pack in periodic, 3D arrays Crystalline materials... • typical of:-metals-many ceramics- ome polymers rystalline SiO some polymers Si Oxygen crystalline SiO 2 Adapted from Fig. 3.18(a), Callister 6e. • atoms have no periodic packing • occurs for: Noncrystalline materials...-complex structures-rapid cooling noncrystalline SiO 2 " Amorphous " = Noncrystalline Adapted from Fig. 3.18(b), Callister 6e. METALLIC CRYSTALS • tend to be densely packed. have several reasons for dense packing: • have several 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. • have the simplest crystal structures. e will look at three such structures We will look at three such structures... SIMPLE CUBIC STRUCTURE (SC) • Rare due to poor packing (only Po has this structure) • Close-packed directions are cube edges. • Coordination # = 6 (# nearest neighbors) (Courtesy P.M. Anderson) ATOMIC PACKING FACTOR APF = Volume of atoms in unit cell* Volume of unit cell *assume hard spheres APF for a simple cubic structure = 0 52 • APF for a simple cubic structure = 0.52 atoms tom volume APF = 4 3 π (0.5a) 3 1 unit cell atom a R=0.5a a 3 unit cell volume close-packed directions ontains 8 x 1/8 = contains 8 x 1/8 1 atom/unit cell Adapted from Fig. 3.19, Callister 6e. BODY CENTERED CUBIC...
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This note was uploaded on 10/29/2008 for the course ENGR 2220 taught by Professor Rizvi during the Fall '08 term at UOIT.

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lecture%205 - ENGR 2220 ENGR 2220 – Structure &...

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