Alloy

Alloy

An alloy is a substance with metallic properties that is composed of a mixture of two or more elements. Alloys can be classified as interstitial or substitutional. In an interstitial alloy, smaller elements fill holes that are in the main metallic structure. The smaller element may be a nonmetallic element, such as boron, carbon, nitrogen, or silicon. For example, steel is an interstitial alloy in which carbon atoms fill the holes between the crystal structure of iron. In substitutional alloys, some of the atoms of the main metal are substituted with atoms of another metal. If the two metal atoms are about the same size and have the same crystallographic structure, then the two metals may form a solid solution. Hume-Rothery rules predict which metals will form solid-solutions based on the relative sizes and electronic properties of the metal atoms. Brass, an alloy composed of copper and zinc, is an example of a substitutional alloy.

Alloys have played an important role in human history. Early Mesopotamian civilizations first used bronze, an alloy of copper and tin as early as 3000 B.C.. Bronze is more easily cast than pure copper because of its lower melting point. It is also stronger than copper after it solidifies. Because of the widespread use of weapons and other implements made of bronze, this period has become known as the Bronze Age. Around 1200 B.C. the raw materials to make bronze were becoming scarce in the Mesopotamian area, so local metal workers began to use the more readily available iron. Pure iron is soft, ductile, and malleable because it is made of small spherical atoms that can move easily past each other. It is also more difficult to work with than bronze and it rusts easily. Early metal workers improved the pure iron by a process called carburizing which added 0.5 - 1% carbon. This produced an interstitial alloy which is very similar to modern day steel. Adding carbon creates strong iron to carbon bonds which prevent atoms from sliding past each other as easily as they do in pure iron. The resulting alloy is stronger and harder than iron or bronze.

Metallurgy, or the study of metals and their alloys, remained relatively unchanged from antiquity until the end of the eighteenth century. The Industrial Revolution greatly increased the need for steel so practical inventors and scientists developed new techniques for making alloys. For example, in 1850 the steel making industry was drastically changed by the Bessemer process which burned out impurities in iron through the use of a blast furnace. The study of alloys was also advanced by two developments in the latter half of the nineteenth century. In 1863 Henry Clifton Sorby of Sheffield (1826-1908) developed a technique for polishing and etching metals so that they could be observed under a microscope. This enabled scientists to correlate crystalline structures with the strength, ductility, and other properties of alloys. In 1887 Hendrik Willem Bakhuis Roozeboom (1854-1907) applied Josiah Willard Gibb's (1839-1903) phase rule to alloys. The phase rule applied thermodynamic principles to chemical equilibria and allowed Roozeboom to develop a phase diagram of the iron carbon system. A phase diagram shows the phases that can be present in an alloy at different temperatures, pressures, and compositions at thermodynamic equilibrium. Roozeboom's phase diagram enabled him and others to improve the quality of steel. Later, other procedures, such as electron microscopy and x-ray techniques, also contributed greatly to the study of alloys.

Alloys have many applications. Alloys based on tin, cadmium, copper, or silver are used to make bearings which reduce friction between two sliding surfaces. Dental fillings can be made from alloys of silver and mercury or alloys of gold, silver, and copper. Stainless steels, iron alloys with more than 12% chromium, are examples of corrosion-resistant alloys. Other alloys, such as Nichrome, a nickel based alloy with 12-15% chromium and 25% iron, are very strong at high temperatures. They are used in power-generating plants, jet engines, and gas turbines. Light weight alloys, like the aluminum, zinc, and magnesium system, are used in aircraft. Some alloys, like solder, which is usually 60% tin and 40% lead, are used to make electrical circuits. Prosthetic devices, like artificial knees and hips, are made from high-strength, corrosion-resistant alloys that are usually based on iron, cobalt, or titanium. Alloys can also be superconductors, which are materials that have zero resistance to the flow of electrical current at low temperatures. One alloy of Niobium and Titanium becomes superconducting at -442.3°F (263.5°C). Alloys of precious metals, like gold, silver, and platinum, are used as coins, catalysts for chemical reactions, electrical devices, temperature sensing devices, and jewelry. Yellow gold contains gold, silver, and copper in a 2:1:1 ratio. Some iron-based alloys like Alnico-4, which is 55% iron, 28% nickel, 12% aluminum, and 5% cobalt, are used as magnets. Many other applications exist for the over 10 thousand different types of alloys that have been developed.