Structure of Metals and Alloys

Since the electrons in a metallic lattice are in a gas, we must use the core electrons and nuclei to determine the structure in metals. This will be true of most solids we will describe, regardless of the type of bonding, since the electrons occupy such a small volume compared to the nucleus. For ease of visualization, we consider the atomic cores to be hard spheres. Because the electrons are delocalized, there is little in the way of electronic hindrance to restrict the number of neighbors a metallic atom may have. As a result, the atoms tend to pack in a close-packed arrangement, or one in which the maximum number of nearest neighbors (atoms directly in contact) is satisfied. 

Remember that both HCP and FCC are close-packed structures and that each has a coordination number of 12, but that their respective unit cells contain 6 and 4 total atoms. We will now see how these two special close-packed structures fit into a larger assembly of crystal systems. 

Our description of atomic packing leads naturally into crystal structures. While some of the simpler structures are used by metals, these structures can be employed by heteronuclear structures, as well. We have already discussed FCC and HCP, but there are 12 other types of crystal structures, for a total of 14 space lattices or Bravais lattices. These 14 space lattices belong to more general classifications called crystal systems, of which there are seven. 

1 Crystal Systems. The cubic crystal system is composed of three space lattices, or unit cells, one of which we have already studied simple cubic (SC), body-centered cubic (BCC), anA face-centered cubic (FCC). The conditions for a crystal to be considered part of the cubic system are that the lattice parameters be the same (so there is really only one lattice parameter, a) and that the interaxial angles all be 90°. 

Body-centered cubic (BCC) is the unit cell of many metals and, like SC, is not a close-packed structure. The number of atoms in the BCC unit cell are calculated as follows  

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W. B. Pearson, Handbook oJEattice Spacings and Structures of Metals and Alloys, International Series on MetalPhysics and Physical Metallurgy, Pergamon Press, New York, 1958, p. 130. 

Hume-Rothery (1936) The Structure of Metals and Alloys (The Institute of Metals, London). 

In recent years it has become clear that the structure of metals and alloys may be described in terms of covalent bonds that resonate among the alternative interatomic positions in the metals, and that this resonance is of greater importance for metals than for substances of any Other class, including the aromatic hydrocarbons. Moreover, the phenomenon of metallic resonance of the valency bonds must be given explicit consideration in the discussion of metallic valency it is necessary in deducing the metallic valency from the number of available electrons and bond orbitals to assign to one orbital a special r le in the metallic resonance. 

The indication from interatomic distances that less than 4 bonding electrons per atom are operating in white tin has been recognized by W. Hume-Rothery, The Structure of Metals and Alloys/ The Institute of Metals Monograph and Report Series No. 1, p. 26... 

Pearson, W.B. (1967) A Handbook of Lattice Spacings and Structures of Metals and Alloy, (Pergamon Press, Oxford). 

Considerable progress has been made in accumulating information on the electronic structure of metals and alloys, on some aspects of the structure of hydrocarbon adsorption complexes, etc. Also, information on the relative importance of the electronic structure effects of alloying—as contrasted to the geometric, ensemble size effects—has grown appreciably. 

Source W. Hume-Rothery, R.E. Smallman, and C.W. Haworth, The Structure of Metals and Alloys, Institute of Metals and the Institution of Metallurgists, London, 1969,175.)... 

Ordering lowers the electrical resistivity of copper-gold alloys. Adapted from C. S. Barrett, Structure of Metals and Alloys (New York McGraw-Hill, 1943), p. 244, figure 14. 

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