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SOLID MATERIALS 32.4
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slip more difficult to initiate. The following sections will explain the manner in which the various thermal and mechanical processes affect the structure of a material, which in turn determines the mechanical properties. The next section presents a brief review of atomic structure.
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32.2 ATOMIC BONDING FORCES
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The smallest particles that must be considered in the preceding context are atoms. The manner in which atoms are arranged in a solid material determines the material s crystal structure. The crystal structure and the type of interatomic bonding forces determine the strength and ductility of the material. The simple model of an atom is a dense nucleus, consisting of protons and neutrons, surrounded by discrete numbers of planetary electrons orbiting in shells at specific distances from the nucleus. Each proton has a positive electric charge of unity (1+). The number of protons in the nucleus determines the nuclear charge of the atom and is called the atomic number. The neutrons have no charge, but they do have mass. The atomic weight of an atom is the sum of the number of protons and neutrons.The electrons have negligible mass and a negative charge of unity (1 ).The number of electrons in a given type of atom is also equal to the atomic number of that element. The maximum number of electrons in any shell is 2n2, where n is the quantum number of the shell. Thus the maximum number of electrons that can be present in the first (innermost) shell is 2, and 8 is the maximum in the second shell. However, no more than 8 electrons are ever present in the outermost shell of an atom. The valence of an element is either the number of electrons in its outermost shell or the number of electrons necessary to fill that shell, whichever number is lower. The interatomic bonding forces are determined by the valence, or outer-shell, electrons. There are four types of atomic bonding forces that hold the atoms of a solid material in their relatively fixed positions. The three strongest (ionic, covalent, and metallic) types of bond are referred to as primary; the fourth (molecular) is referred to as a secondary type of bond.
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32.2.1 Ionic Bonds From the preceding brief description of atomic structure, it is evident that the uncombined atom is electrically neutral the number of protons (+ charges) in the nucleus exactly equals the number of electrons ( charges). When atoms combine, only the valence electrons are involved and not the nuclei. When a metal combines with a nonmetal, each metal atom loses its valence electrons and thus acquires a positive charge that is equal to the number of electrons so lost. Likewise each nonmetallic atom gains a number of electrons equal to its valence and acquires an equal negative charge. While in this state, the positively charged metallic atom and the negatively charged nonmetallic atom are called ions. Like-charged particles repel each other and oppositely charged particles attract each other with an electric force called the Coulomb force. When a material is maintained in the solid state by the mutual attraction of positively and negatively charged ions, the interatomic bonding force is called ionic. The Coulomb forces attracting oppositely charged ions are very large. Therefore, ionic-bonded solids exhibit very high strength and relatively low melting tempera-
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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SOLID MATERIALS 32.5
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tures. However, they exhibit very low ductility under normal conditions because the interatomic bonds must be broken in order for the atoms to slide past each other. This is one of the most important distinctions between ionic (or covalent) bonding and metallic bonding and is discussed later.
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32.2.2 Covalent Bonds Covalent bonds are those in which the atoms reach a stable configuration (filled outer shell) by sharing valence electrons. Unlike ionic bonds, which are nondirectional, covalent bonds act between specific pairs of atoms and thus form molecules. Covalent bonds are most prevalent in gas molecules. Covalent bonding also results in the formation of very large molecules which are present as solids rather than as liquids and gases. Diamond, silicon, and silicon carbide are examples of such covalent-bonded solids. They are characterized by high strength and melting temperature and low ductility. The atoms in the diamond structure are arranged on two interpenetrating face-centered cubic lattices. The entire crystal is composed of only one molecule, and in order to fracture the crystal, the strong covalent interatomic bonds must be broken.
32.2.3 Metallic Bonds Of the three primary bonding forces, the metallic bond is by far the most important for an understanding of the mechanical properties of the materials with which the practicing engineer is concerned.The metallic bond is a special type of covalent bond wherein the positively charged nuclei of the metal atoms are attracted by electrostatic forces to the valence electrons that surround them. Unlike the common covalent bond, which is directional, i.e., between a pair of atoms, the metallic bond is nondirectional, and each nucleus attracts as many valence electrons as possible. This leads to a dense packing of the atoms, and thus the most common crystal structures of the metals are the close-packed ones: face- and body-centered cubic and hexagonal close-packed structures. The reason that metal atoms have their own unique type of bonding force is the looseness with which their valence electrons are held in the outer shell. This is evident from the fact that the ionization potential of metal atoms is one-half to twothirds that of nonmetal atoms. The mean radius of the valence electrons in a free (isolated) metal atom is larger than the interatomic distance of that metal in the solid crystalline state. This means that the valence electrons are closer to a nucleus in the solid metal than they are in a free atom, and thus their potential energy is lower in the solid. Since the valence electrons are not localized between a pair of positive ions, they are free to move through the solid. Thus the structure of the solid metal is a closepacked arrangement of positive ion cores (the nucleus plus the nonvalence electrons) that is permeated by an electron gas or cloud. This ability of the valence electrons to move freely through the solid explains the high thermal and electrical conductivities of metals. Also, the fact that the valence electrons are nondirectional (not shared by only two atoms) explains the relatively low strength and high ductility of elemental metals, since the positive ions can move relative to one another without breaking any primary bonds. This mechanism is referred to as slip and is discussed in more detail in a following section on crystal structures.
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