Manufacturing and Mechanical Properties of Engineering Materials
Structure of Matter:
The properties of a material are intimately connected to its basic molecular structure. Some knowledge of this structure is therefore essential for understanding the various macroscopic properties exhibited by the material. A general characteristic of all solids is their capability to retain definite shapes, and so we start from the mechanics of bonding between the molecules forming a solid.
Bonding of Solids:
When two atoms are sufficiently close to each other, the outer electrons are shared by both the nuclei. This results in an attractive force between the two atoms. This force increases with the decrease in distance between the two atoms, as shown in Fig. 1.1. However, the two atoms do not collapse as a repulsive force is generated when the two nuclei come very close.
This repulsive force increases rapidly with decreasing interatomic distance. The equilibrium interatomic distance de is that distance when the attractive and the repulsive forces are equal in magnitude (Fig. 1.1). The slope of the repulsive force curve is always more than that of the attractive force curve at the point of intersection A of the curves. Therefore, the equilibrium is of stable nature.
The mechanism is one of the various possible interactions resulting in bonding between atoms, and is known as covalent bonding. In a given solid, one or more bonding mechanisms can be simultaneously operative.
The nature of a bonding mechanism depends on the electronic structure of the atoms involved. The bonding mechanisms predominant in solids include metallic bonding (in metals) and the van der Waals bonding (in molecular crystals). In a metal, a large number of free electrons are present, resulting in the formation of a common electron cloud, the rest of the system consists of positively charged ions which are held together by the cloud (Fig. 1.2a).
The mechanism of bonding in alloys is similar. Since the inert atoms do not possess free electrons, the metallic bonding mechanism cannot be operative. In such instances, however, a very weak short range attraction is generated due to the van der Waals force. The origin of this force is attributed to a rapidly-fluctuating dipole moment.
Figure 1.2b shows two molecules at a distance d, each of which has a symmetric charge distribution. All the three different overall configurations of the charge distributions, shown in the figure, lead to the development of an attractive force though individually the molecules are neutral. This of bonding is very weak and is active in weak and low melting point materials such as paraffin and plastics. It is obvious that the strength of the bond controls the properties, e.g. melting point, of a material.
Crystal Structure:
The properties of a material depend not only on the bond strength but also on the arrangement of the atoms. In all metals and in many nonmetallic solids, the atoms are arranged in a well-ordered pattern. Such solids are commonly called crystalline solids. Of course, in a large number of situations, the whole solid is seldom composed of one single crystal. Instead, a very large number of small, randomly-oriented crystalline grains form the whole solid. Such materials are termed poly crystalline. Figures 1.3a and 1.3b show a single crystal and a polycrystalline solid, respectively.
In a crystal, we can identify the unit cell the repetition of which forms the whole crystal. The structure of a crystal is identified and described by this unit cell. The three commonly-observed crystal structures in metals are shown in Fig. 1.4.
Of these three basic structures, the fcc and the cph crystals have the most dense packing. The interatomic distance in such crystals is of the order of 10-7 mm. The crystal structures of some common metals are given in Table 1.1.
When a liquid metal solidifies by cooling, the atoms arrange themselves in regular space lattices, forming a crystal. The crystallization starts simultaneously at various places within the liquid mass. Figure 1.5 shows the growth of the crystal grains and the ultimate formation of the polycrystalline metal.
Most metals have only one crystal structure. A few metals, however, can have more than one type of crystal structure. Such metals are called allotropic. Table 1.1 indicates that iron is an allotropic metal.
A number of material properties, in general, can be associated with the type of crystal structure. For example- the bcc structures are usually harder, whereas the fee structures are more ductile. In cph structures, the ductility is low.
Crystal Imperfections:
Some properties of a crystalline solid depend on the basic crystal structure of the solid. However, in almost all instances, the crystals are not perfect, i.e., the lattices are not without imperfections. These imperfections govern most of the mechanical properties of crystalline solids (see Table 1.2).
The study of the crystal imperfections and their effects on the properties of a material is a subject by itself. In our discussion therefore, we shall give only those concepts that will be required for an understanding of the various phenomena associated with different processes, e.g., plastic deformation and heat treatment.
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