Alloys

Alloys are solid combinations of metals or of metals and nonmetallic elements that have technologically desirable properties. The discoveries of various alloys have marked significant turning points in human history.

Background

Alloys are mixtures of metal—such as iron, coal, copper, tin, and lead—with other metals or with nonmetallic elements developed to add desirable properties to those possessed by the metallic elements. These properties include strength, hardness, resistance to corrosion, and the ability to withstand high temperatures. The properties of alloys depend not only on their chemical composition but also on the way they have been prepared. Steel, a family of alloys based on the addition of carbon and other elements to iron, is perhaps the most familiar example in modern technology, but alloys based on aluminum, cobalt, gold, nickel, mercury, titanium, and many other elements are also of great practical importance. In many cases the role they play in alloy formation is the determining factor in the importance attached to these elements as natural resources. The metals used in alloys must be extracted from their ores, a process that often leaves environmentally troublesome by-products such as sulfur oxides. The manufacture of alloys generally requires sustained high temperatures, creating a demand for fossil fuels and raising concern about thermal pollution.

History

Archaeologists and historians have named the stages of early civilization after the principal materials used for tools in each of them. Thus at various times in different parts of the world, civilization progressed from the stone Age to a Bronze Age, and then to an Iron Age. Bronze, a mixture of copper and tin, was the first alloy to receive extensive use. Bronze artifacts dated as early as 3500 b.c.e. have been found in both Asia Minor and China. The Hittites are believed to have been the first peoples, in about 1500 b.c.e., to have discovered how to extract metallic iron from its ores. The superior strength of iron led to the replacement of bronze by iron in armor, weaponry, and knives. The iron used by early civilizations was undoubtedly an alloy, though it was not understood as such. Steel, formed by the addition of carbon to iron, was made in India by 1000 b.c.e. Brass, a mixture of copper and zinc, appears to have been known to the Romans.

Modern Alloys

Alloys are generally grouped into ferrous alloys, those containing iron, and nonferrous alloys. Bronze and brass remain among the most common nonferrous alloys. Bronze is used in numerous industrial applications and as a durable material for sculptures. Brass is readily machined and widely used in hardware, electrical fixtures, and decorations. Aluminum, extracted from bauxiteore by high-temperature electrolysis, is alloyed with manganese, magnesium, or other elements to produce a lightweight rigid material.

Ferrous alloys include steels and cast iron. Cast irons are alloys of iron with 2 to 4 percent carbon and up to 3 percent silicon. Steels are alloys of iron that contain a smaller amount of carbon as well as other elements. The manufacture of steel requires extremely high temperatures. Numerous forms of steel exist. Chromium steel has increased hardness and rust resistance. Stainless steel is a special form of chromium steel with admixtures of manganese, silicon, and nickel. Molybdenum, titanium, phosphorus, and selenium may also be added. Manganese is added to steel to increase strength and durability. Tungsten steels are stronger at high temperatures. Vanadium steel has greater elasticity and is suited to parts that must bend and regain their shape.

Alloys of gold and silver are important in coinage and for decorative purposes. Gold is alloyed with silver and copper for jewelry. Sterling silver is an alloy of silver with copper.

Certain alloys are employed in dentistry and medicine. Throughout most of the twentieth century dentists made liberal use of mercury amalgam, a moldable mixture of mercury, silver, and other elements, as a filling material for dental caries (cavities). Concern about mercury toxicity led to a reduction in use of this material. Orthopedic surgeons frequently use stainless steel screws, pins, and rods to hold fractured bones in place so that they can heal properly. Alloys also play a role in a variety of orthopedic implants used to replace badly worn or damaged joints.

Another important group of alloys is those used for permanent magnets. These include alnico, a combination of aluminum, nickel, and cobalt. Other magnetic materials include iron-nickel and iron-aluminum combinations. The rare earth elements also play a role in some magnetic materials.

Superalloys are materials based on nickel, cobalt, or an iron-nickel mixture and contain carefully controlled amounts of trace elements designed to exhibit high strength at temperatures above 1,000° Celsius. These materials are used in jet engines, in heat exchangers, and in chemical production plants.

Impact of Alloys on Natural Resources

The development and refinement of alloy technology have had a dual effect on natural resource utilization. By making a larger variety of consumer goods available, the development of new alloys has tended to accelerate the use of mineral ores and energy sources. However, the emergence of alloys that are lighter, more corrosion resistant, and amenable to recycling, as well as the replacement of some alloys by polymer-based materials and other alloys, slowed the rate of resource use somewhat after its peak in the 1970’s.

Bibliography

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