ssp_7y - 2 Continued Structure of metallic glasses Dense...

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2 Continued
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2-17 Structure of metallic glasses Dense random packing of (soft) spheres FT S(q) g(r) structure factor pair correlation function (see Chpt. 3) Fig. 2.21. Pair correlation functions of Ni in the liquid and glassy states. Note the structure in the second maximum for the glassy state, which is explained in terms of dense random packing of spheres (bottom). Refined models: e.g., icosahedral packing with defects.
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2-18 Icosahedral packing with defects Fig. 2.22. Close packing of atoms in two dimensions leads to triangles which form hexagons. They can be arranged in a space-filling manner in two dimension (left, a) In three dimensions the tetrahedron is the closest packing of atoms. 20 tetrahedra arranged with slight distortions, form an icosahedron (left, b; right, a-c). Here 12 atoms are grouped around a central atom. Space-filling is not possible with icosahedra, which often form upon rapid quenching of metallic melts. Metallic glasses have, therefore, been described in terms of local icosahedral packing with defects between the icosahedra. Icosahedra have 5-fold symmetry and have also been observed to form one component of the tiling of quasicrystals with 5-fold symmetry. Quasicrystals have no long-range periodicity, although atoms are arranged in a regular way (cf., Fig. 2.5).
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2-19 Local and global close packing Close packing of atoms in a layer fcc: ABC/ABC/. .., hcp: AB/AB/. .. fcc, hcp: global close packing (space filling!), regular packing of tetrahedra and octahedra tetrahedra: local close packing Fig. 2.23. The fcc structure is made up of a regular arrangement of tetrahedra and octahedra * .
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ssp_7y - 2 Continued Structure of metallic glasses Dense...

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