110 200 211 z x y a b c Diffraction angle 2 Diffraction pattern for

110 200 211 z x y a b c diffraction angle 2

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(110) (200) (211) z x y a b c Diffraction angle 2 Diffraction pattern for polycrystalline -iron (BCC) Intensity (relative) z x y a b c z x y a b c
1/17/2013 23 Chapter 3 - 67 Atoms may assemble into crystalline or amorphous structures. We can predict the density of a material, provided we know the atomic weight , atomic radius , and crystal geometry (e.g., FCC, BCC, HCP). SUMMARY Common metallic crystal structures are FCC , BCC , and HCP . Coordination number and atomic packing factor are the same for both FCC and HCP crystal structures. Crystallographic points , directions and planes are specified in terms of indexing schemes. Crystallographic directions and planes are related to atomic linear densities and planar densities . Ceramic crystal structures are based on: -- maintaining charge neutrality -- cation-anion radii ratios. Interatomic bonding in ceramics is ionic and/or covalent. Chapter 3 - 68 Some materials can have more than one crystal structure. This is referred to as polymorphism (or allotropy ). SUMMARY Materials can be single crystals or polycrystalline . Material properties generally vary with single crystal orientation (i.e., they are anisotropic ), but are generally non-directional (i.e., they are isotropic ) in polycrystals with randomly oriented grains. X-ray diffraction is used for crystal structure and interplanar spacing determinations.

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