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Section 9: Modern Materials—Alloys
As we learned in Section 5 of this unit, alloys are materials that contain more than one element and have the characteristic properties of metals: They are ductile, malleable, and good electrical conductors. Alloys are widely used in all kinds of modern applications, from jewelry to medical implants and aircraft parts. Many have been developed to combine strength with other attributes. For example, titanium alloys are widely used to make artificial hips, knees, and other body implants because they are strong yet lightweight, and do not cause reactions in human tissue.
Figure 13-16. Atomic Arrangements in Alloys
When a metal is mixed with another material, forming an alloy, atoms of the added component may either take the place of host metal atoms or fit between them.
© Wikimedia Commons, Public Domain.
Alloys that are homogeneous mixtures in which the components are distributed uniformly are known as "solution alloys." The components can blend in two ways. Atoms of the added component(s) can take up spaces that are normally occupied by atoms of the host metal, forming a substitutional alloy; or they can occupy spaces in between atoms of the host metal, forming an interstitial alloy. (Figure 13-16) Substitutional alloys are much more common than interstitial alloys.
Alloys can also be heterogeneous mixtures in which the component materials are not uniformly distributed. However the alloy is structured, combining a metal with another substance changes its properties. The first known manmade alloy to be produced was bronze (a substitutional alloy), which came into use more than 3,000 years BCE, giving the Bronze Age its name. Bronze is roughly 70 to 90 percent copper mixed with tin; the resulting alloy is harder and more durable than copper, and easier to melt and cast. It is also harder than iron and much more resistant to corrosion.
Other well-known alloys include brass (copper and zinc), pewter (tin, copper, and bismuth), solder (tin and lead), steel (iron, carbon, and other metals), and stainless steel (steel and chromium). Brass and pewter are substitutional alloys. Steel is interstitial: Small carbon atoms fill spaces between larger iron atoms. Stainless steel is both substitutional and interstitial: Carbon atoms fit between the iron atoms, but nickel and chromium atoms replace some iron atoms.
Figure 13-17. Common Steel Alloying Agents
Many metals can be alloyed with steel to alter its characteristics for different products and applications.
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To form a substitutional alloy, two components must have atomic radii of similar sizes (no more than about 15 percent difference) and similar chemical bonding characteristics. Metals from the d block of the periodic table have similar radii and form a wide range of alloys with each other. Common examples include gold-silver alloys, widely used for making jewelry, and brass, which is an alloy of copper and zinc.
As we learned in The Chemistry of Steelmaking Sidebar in Section 6, mass production of steel (an alloy of iron mixed with small amounts of interstitial elements, mainly carbon) developed in the 19th century. In the 20th century, manufacturers learned that alloying steel with other metals could further improve its properties. Nickel, chromium, molybdenum, vanadium, and copper are among the metals that modern steelmakers use to alter properties such as the hardness and corrosion resistance of their products. (Figure 13-17) For example, stainless steel is a steel alloy that contains about 10 percent chromium by mass. Stainless steel is not completely immune to corrosion, however. As one example, the Gateway Arch in St. Louis, which has a skin made of stainless steel, shows visible stains and corrosion, which National Park Service managers say are cosmetic.
To understand why alloys are usually stronger than pure metals, remember that metals are crystalline solids. When a metal is mixed with another material to make a substitutional alloy, the less abundant atoms of the solute element distort the crystal lattices of the more abundant element. This makes it more difficult for planes of the material to slide past each other, which makes the alloy hard and strong. In interstitial alloys, the nonmetal atoms fit between the metal atoms and provide extra bonding to neighboring atoms, which makes the alloy harder, stronger, and less ductile.
However, a few alloys are designed to be weaker than their constituent metals. The best-known example is solder, which is essentially metallic glue used to join metal parts together. Solder is a blend of lead and tin, sometimes with other metals added to make the alloy harder or more elastic. Another fusible alloy (one that melts at low temperatures) is Wood's metal, a blend of bismuth, lead, tin, and cadmium that melts at 70°C. Wood's metal is used as the triggering element in fire sprinkler systems: Heat from nearby flames melts a plug of Wood's metal, releasing water from sprinklers.