Empire, in the Western tradition, was a unitary and universal thing. There was and could be only one empire at any one time, and it was, in principle at least, a world empire. In this case, all roads led to and from Rome. Herodotus had introduced the idea, if not the term, of translatio imperii, the transfer of empire from one ruler to another. In his account the succession was from the Assyrians to the Medes, to the Persians. Later writers saw the Macedonians, in the person of Alexander the Great, as successor to the Persians, and later still it was relatively easy to see the Romans, with their admiration for Greek culture, as heirs to the Hellenistic empire of Alexander.

The English have found it difficult for most of their history to separate themselves from a British identity. That is in good part because they created two empires: the empire of the United Kingdom and the more familiar overseas British Empire. As an imperial people, they have merged their identities in their creations. But with the loss of the overseas empire and the possible break-up of the United Kingdom itself, they have been forced to consider the question of who they are and what is a specifically English as opposed to a British identity. Brexit has made that task even more urgent. The problem is the relative lack of a tradition of reflection on English national identity. There is no lack of cultural resources to draw on, but the work remains to be done.

Introduction

In Chapter 4, we dealt with point and line defects. There is another class of defects called interfacial, or planar, defects. These imperfections, as the name signifies, occupy an area or surface and so are two-dimensional, as well as being of great importance. Examples of such defects are free surfaces of a material, grain boundaries, twin boundaries, domain boundaries, and antiphase boundaries. Of all these, grain boundaries are the most important from the point of view of the mechanical properties of the material. In what follows, we consider in detail the structure of grain and twin boundaries and their importance in various deformation processes, and, very briefly, the structure of other interfacial defects. Details regarding the strengthening of a material by grain boundaries are given in Section 5.3. Volumetric defects, such as voids, also play a major role in the mechanical properties of materials, affecting the strength and elastic properties of the material significantly. Volumetric defects are briefly described in Section 5.7. In Section 5.8, we present the defects occurring in polymers.

Grain Boundaries

Crystalline solids generally consist of a large number of grains separated by boundaries. Most industrial metals and ceramics are polycrystalline aggregates, and the mechanical properties of these polycrystals can be radically different from those of the monocrystals that form the individual grains. Figure 5.1 illustrates a polycrystalline aggregate, in which each grain has a distinct crystallographic orientation.

Introduction

We can define a composite material as a material consisting of two or more physically and/or chemically distinct phases, suitably arranged or distributed. A composite material usually has characteristics that are not depicted by any of its components in isolation. Generally, the continuous phase is referred to as the matrix, while the distributed phase is called the reinforcement. Three items determine the characteristics of a composite: the reinforcement, the matrix, and the interface between them. In this chapter, we provide a brief survey of different types of composite materials, highlight some of their important features, and indicate their various applications.

Types of Composites

We may classify composites on the basis of the type of matrix employed in them – for example, polymer matrix composites (PMCs), metal matrix composites (MMCs), and ceramic matrix composites (CMCs). We may also classify composites on the basis of the type of reinforcement they employ (see Figure 15.1):

1. Particle reinforced composites.

2. Short fiber, or whisker reinforced, composites.

3. Continuous fiber, or sheet reinforced, MMCs.

4. Laminate composite.

Figure 15.2 shows typical microstructures of some composites: boron fiber/Al (Figure 15.2(a)), short alumina fiber/Al (Figure 15.2(b)), and NbC/Ni–Cr, an in situ (eutectic) composite (Figure 15.2(c)). Examples of microstructure of a silicon carbide particle (three different volume fractions) reinforced aluminium matrix are given in Figure 15.3. These were made by hot pressing of powders followed by hot extrusion. Note the preferential alignment of SiC particles in the extrusion direction.

Introduction

An intermetallic is a compound phase of two or more normal metals (ordered or disordered). Interest in intermetallics waned in the 1960s and 1970s. However, the demand for materials that are strong, stiff, and ductile at high temperatures has led to a resurgence of interest in intermetallics, especially silicides and ordered intermetallics such as aluminides. A testimony to this resurgence was the appearance in 1994 on the subject of a two-volume set by J. H. Westbrook and R. L. Fleischer, Intermetallic Compounds: Principles and Practice (New York: John Wiley). Intermetallic aluminides and silicides can be very oxidation and corrosion resistant, because they form strongly adherent surface oxide films. Also, intermetallics span a wide range of unusual properties. An important example outside the field of high-temperature materials involves the exploitation of martensitic transformations, exotic colors, and the phenomenon of shape memory in gold-based intermetallics in jewelry making. In what follows, we first describe the silicides, then the ordered intermetallics, and finally the basic structure and properties of foams.

Silicides

About 300 intermetallic compounds melt at temperatures above 1,500 °C. A survey of some silicide intermetallics for high-temperature applications showed that, based on criteria such as availability, phase changes in the temperature range of interest, and oxidation resistance, Ti5S3 and MoSi2 seem to be the most promising materials: Ti5Si3 has the lowest density of all intermetallics, and MoSi2 has a superior oxidation resistance.

Introduction

A solution can be defined as a homogeneous mixture of two or more substances. Generally, one thinks of a solution as liquid, but gaseous or solid forms are possible as well. Indeed, we can have solutions of gases in a gas, gases in a liquid, liquids in a liquid, solids in a liquid, and solids in a solid. A solution can have one or more solutes dissolved in a solvent. The solute is the substance that is dissolved; the solvent is the substance in which the solute is dissolved. In a solution, there is always less solute than solvent. There are two kinds of solid solutions: substitutional and interstitial. Figure 10.1 shows examples of each in a schematic manner. Figure 10.1(a) is of brass, which is a substitutional solid solution of zinc (the solute) in copper (the solvent). We call such an alloy substitutional because the solute atoms merely substitute for the solvent atoms in their normal positions. In a substitutional solution, the atomic sizes of the solute and solvent atoms are fairly close. The maximum size difference is approximately 15%. When the atomic sizes of the solute and solvent are very different, as in the case of carbon or nitrogen in iron, we get an interstitial solid solution. Figure 10.1(b) shows such a solid solution of carbon in iron. We call these solutions interstitial solid solutions because the solute atoms occupy interstitial positions in the solvent lattice.

Introduction

Fracture of any material (be it a recently acquired child's toy or a nuclear pressure vessel) is generally an undesirable happening, resulting in economic loss, an interruption in the availability of a desired service, and, possibly, damage to human beings. Besides, one has good, technical reasons to do fracture testing: to compare and select the toughest (and most economical material) for given service conditions; to compare a particular material's fracture characteristics against a specified standard; to predict the effects of service conditions (e.g., corrosion, fatigue, stress corrosion) on the material toughness; and to study the effects of microstructural changes on material toughness. One or more of these reasons for fracture testing may apply during the design, selection, construction, and/or operation of material structures. There are two broad categories of fracture tests; qualitative and quantitative. The Charpy impact test exemplifies the former, and the plane-strain fracture toughness (KIc) test illustrates the latter. We describe briefly important tests in both of these categories.

Impact Testing

We saw in Chapter 7 that stress concentrations, like cracks and notches, are sites where failure of a material starts. It has been long appreciated that the failure of a given material in the presence of a notch is controlled by the material's fracture toughness. Many tests have been developed and standardized to measure this “notch toughness” of a material. Almost all are qualitative and comparative in nature.