23.6 Alloys

An alloy is a material that contains more than one element and has the characteristic properties of metals. The alloying of metals is of great importance because it is one of the primary ways of modifying the properties of pure metallic elements. For example, nearly all the common uses of iron involve alloy compositions. As another example, pure gold is too soft to be used in jewelry, whereas alloys of gold and copper are quite hard. Pure gold is termed 24 karat; the common alloy used in jewelry is 14 karat, meaning that it is 58 percent gold PM23025

A gold alloy of this composition has suitable hardness to be used in jewelry. The alloy can be either yellow or white, depending on the elements added. Some further examples of alloys are given in Table 23.4.

Alloys can be classified as solution alloys, heterogeneous alloys, and intermetallic compounds. Solution alloys are homogeneous mixtures in which the components are dispersed randomly and uniformly. Atoms of the solute can take positions normally occupied by a solvent atom, thereby forming a substitutional alloy, or they can occupy interstitial positions, thereby forming an interstitial alloy. These types are diagrammed in Figure 23.18.

Figure 23.18 (a) Substitutional and (b) interstitial alloys. The blue spheres represent host metal; the yellow spheres represent the other components of the alloy.

Substitutional alloys are formed when the two metallic components have similar atomic radii and chemical-bonding characteristics. For example, silver and gold form such an alloy over the entire range of possible compositions. When two metals differ in radii by more than about 15 percent, solubility is more limited.

For an interstitial alloy to form, the component present in the interstitial positions between the solvent atoms must have a much smaller covalent radius than the solvent atoms. Typically, an interstitial element is a nonmetal that participates in bonding to neighboring atoms. The presence of the extra bonds provided by the interstitial component causes the metal lattice to become harder, stronger, and less ductile. For example, steel is an alloy of iron that contains up to 3 percent carbon. Steel is much harder and stronger than pure iron. Mild steels contain less than 0.2 percent carbon; they are malleable and ductile and are used to make cables, nails, and chains. Medium steels contain 0.2 to 0.6 percent carbon; they are tougher than mild steels and are used to make girders and rails. High-carbon steel, used in cutlery, tools, and springs, contains 0.6 to 1.5 percent carbon. In all these cases other elements may be added to form alloy steels. Vanadium and chromium may be added to impart strength and to increase resistance to fatigue and corrosion. For example, a rail steel used in Sweden on lines bearing heavy ore carriers contains 0.7 percent carbon, 1 percent chromium, and 0.1 percent vanadium.

One of the most important iron alloys is stainless steel, which contains 0.4 percent carbon, 18 percent chromium, and 1 percent nickel. The chromium is obtained by carbon reduction of chromite, FeCr2O4, in an electric furnace. The product of the reduction is ferrochrome, FeCr2, which is then added in the appropriate amount to molten iron that comes from the converter to achieve the desired steel composition. The ratio of elements present in the steel may vary over a wide range, imparting a variety of specific physical and chemical properties to the materials.

In heterogeneous alloys the components are not dispersed uniformly. For example, in the form of steel known as pearlite, two distinct phases—essentially pure iron and the compound Fe3C, known as cementite—are present in alternating layers. In general, the properties of heterogeneous alloys depend not only on the composition but also on the manner in which the solid is formed from the molten mixture. Rapid cooling leads to distinctly different properties than are obtained by slow cooling.

Intermetallic Compounds

Intermetallic compounds are homogeneous alloys that have definite properties and compositions. For example, copper and aluminum form a compound, CuAl2, known as duraluminum. Intermetallic compounds play many important roles in modern society. The intermetallic compound Ni3Al is a major component of jet aircraft engines because of its strength and low density. Razor blades are often coated with Cr3Pt, which adds hardness, allowing the blade to stay sharp longer. The compound Co5Sm is used in the permanent magnets in lightweight headsets (Figure 23.19) because of its high magnetic strength per unit weight.

These examples illustrate some unusual ratios of combining elements. Nothing we have yet discussed in this text would lead us to predict such compositions. Among the many fundamental problems that remain unresolved in chemistry is the problem of developing a good theoretical model to predict the stoichiometries of intermetallic compounds.