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1/17/201323Chapter 3 -67• Atoms may assemble into crystallineor amorphousstructures. • We can predict the densityof 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 numberand atomic packing factorare the same for both FCC and HCP crystal structures.• Crystallographic points, directionsand planesare specified in terms of indexing schemes. Crystallographic directions and planes are related to atomic linear densitiesand 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 crystalsor 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 diffractionis used for crystal structure and interplanar spacingdeterminations.