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Lecture note 6 (09-22-2011)

Lecture note 6 (09-22-2011) - MAE 581 Advanced Materials...

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1 MAE 581 Advanced Materials Science Lecture note 6 Metallic Crystal Structures From the viewpoints of symmetry, higher symmetry crystals (trigonal, tetragonal, hexagonal, and cubic) are more abundant among metals, oxides, and halides with relatively simple chemical formulae since many of these structures are based on close-packing principles (because, in most cases, metallic bonding is very strong). Metal ions are relatively small, with diameters in the order of 0.25 nm. A millimeter cube of metal therefore contains about 10 20 atoms. The like ions in pure solid metal are packed together in a highly regular manner and, in the majority of metals, are packed so that ions collectively occupy the minimum volume. Metals are normally crystalline and for all of them, irrespective of whether the packing of ions is close or open, it is possible to define and express atomic arrangements in terms of structure cells. Furthermore, because of the non-directional nature of the metallic bond, it is also possible to simulate these arrangements by simple ‘hard-sphere’ modeling. There are two ways of packing spheres of equal size together so that they occupy the minimum volume. The structure cells of the resulting arrangements, face-centered cubic (fcc ) and close- packed hexagonal (cph) , are shown in Figures 1.15a and b. The other structure cell (Figure 1.15c) has a body-centered cubic (bcc)
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