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Objectives

Crystal Structures. Crystal:Lattice Constantsa,b,c lattice vectorsalpha, beta, gamma interaxial angles. Why Crystals?. Atoms arrange themselves into crystals in order to _______ their free energy. Structures that deviate from the crystalline ideal will result in _________ energies in the atoms..

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Objectives

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    1. Objectives Identify and describe the three primary metallic crystal structures. Explain the terms: Anisotropic, anisotropy Polycrystalline, polycrystallinity Polymorphic, polymorphism Allotropic, allotropy

    2. Crystal Structures Crystal: Lattice Constants a,b,c – lattice vectors alpha, beta, gamma – interaxial angles

    3. Why Crystals? Atoms arrange themselves into crystals in order to _______ their free energy. Structures that deviate from the crystalline ideal will result in _________ energies in the atoms.

    5. FCC: Face Centered Cubic

    8. General Characteristics FCC Metals Fairly soft Malleable (easily formed) and do not harden much as they deform Will deform a great deal before failure

    9. BCC: Body Centered Cubic

    12. General Characteristics BCC Metals Generally harder than FCC metals Will not deform as much before failure Tend to get harder as they deform

    13. Hexagonal Close Packed

    15. General Characteristics HCP Metals Tend to be______ Generally will harden rapidly as they deform Cannot usually deform a lot without failure

    16. Anisotropic- “Not Isotropic”; properties depend on ______

    17. Isotropy Isotropic Particle Board: Properties are ________ of ___________

    18. Polycrystalline – containing many crystals.

    19. Polymorphic – having different ________________. (eg Iron is BCC ? FCC ?BCC upon cooling from liquid to room temperature)

    20. Polymorphic – having different crystal structures. Allotropic – same as polymorphic but an elemental solid (many alloys are polymorphic, only a few elements are) Amorphous – no structure

    21. Self-Test What do BCC, FCC and HCP stand for? If a metal turns from BCC to HCP structure as it is heated, is it polymorphic or polycrystalline? If a metal turns from BCC to FCC, what happens to the volume of the metal?

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