Alloy

A mixture of two or more metals is called an alloy. Alloys are distinguished from composite metals in that alloys are thoroughly mixed, creating, in effect, a synthetic metal. In metal composites, the introduced metal retains its identity within the matrix in the form of fibers, beads, or other shapes.

Alloys are 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 can be created by mixing the metals while in a molten state or by bonding metal powders. Various alloys have different desired properties such as strength, visual attractiveness, or malleability. The number of possible alloy combinations is almost endless, since any metal can be alloyed in pairs or in multiples.

An entire period of human prehistory is named for the earliest known alloy—bronze. During the Bronze Age (c. 3500–1000 BC) humans first fashioned tools and weapons from something other than basic materials found in nature. Humans combined copper and tin to form a strong metal that was still easily malleable. Modern bronze contains a 25:75 ratio of tin to copper. The use of bronze in early times was greatest in areas where tin deposits were most plentiful, like Asia Minor, and among countries that traded with tin-mining nations.

Brass is an alloy of copper and zinc. It is valued for its light weight and rigid strength. It has a ratio of about one-third zinc to two-thirds copper. The exact ratio of metals determines the qualities of the alloy. For example, brass, having less than 63% copper, must be heated to be worked. Brass is noted for its beauty when polished. Brass was perhaps first produced in Palestine between 1400 to 1200 BC The Romans later used it for coins. Many references to brass in the Bible and other ancient documents are incorrect translations of bronze.

Pewter is an alloy of copper, tin, and antimony. It is a very soft mixture that can be worked when cold and beaten repeatedly without becoming brittle. It was used in Roman times, but its greatest period of popularity began in England in the fourteenth century and continued into the eighteenth. Colonial American metalworkers produced some notable pewter work. As a cheaper version of silver, it was used in plates, cups, pitchers, and candelabras.

The various types of steel and iron are all alloys classifiable by their content of other materials. For instance, wrought iron has a very small carbon content, while cast iron has at least 2% carbon.

Steels contain varying amounts of carbon and metals such as tungsten, molybdenum, vanadium, and cobalt. These materials give them the strength, durability, and anti-corrosion capabilities required by their different uses. Stainless steel, which has 18% chromium and 8% nickel alloyed to it, is valued for its anti-corrosive qualities.

Duraluminum contains one-third steel and two-thirds aluminum. It was developed during World War I for the superstructures of the Zeppelin airships built in Germany.

Many alloys add function to physical beauty. For example, sterling silver is made with 8% copper to add strength so that it can be made into chalices and silverware. American coins are made from copper alloy, sometimes sandwiched between layers of silver.

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 steelmaking industry was changed drastically by the Bessemer process, which burned out impurities in iron with the use of blast furnaces. In addition, two developments in the latter half of the nineteenth century advanced the study of alloys. 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 technique 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 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, 000 different types of alloys that have been developed.

Alloys greatly enhance the versatility of metals. Without them, there would be total dependency on pure metals, which would affect their cost and availability. Alloys are a very important part of humankind’s past and future.

See also Metallurgy.

Alloy

An alloy is a mixture of two or more metals. Some familiar examples of alloys include brass, bronze, pewter, cast and wrought iron, steel, coin metals, and solder (pronounced SOD-der; a substance used to join other metallic surfaces together). Alloys are usually synthetic materials, developed by scientists for special purposes. They generally have specially desirable properties quite different from the metals from which they are made. As an example, Wood's metal is a mixture of about 50 percent bismuth, 10 percent cadmium, 13 percent tin, and 27 percent lead that melts at 70°C (160°F). This low melting point makes Wood's metal useful as a plug in automatic sprinkler systems. Soon after a fire breaks out, the heat from the flames melts the Wood's metal plug, releasing water from the sprinkler system.

Important Alloys, Their Composition, and Typical Uses

Alloy

A mixture of two or more metals is called an alloy. Alloys are distinguished from composite metals in that alloys are thoroughly mixed, creating, in effect, a synthetic metal . In metal composites, the introduced metal retains its identity within the matrix in the form of fibers, beads, or other shapes.

Alloys can be created by mixing the metals while in a molten state or by bonding metal powders. Various alloys have different desired properties such as strength, visual attractiveness, or malleability. The number of possible alloy combinations is almost endless since any metal can be alloyed in pairs or in multiples.

An entire period of human prehistory is named for the earliest known alloy—bronze. During the Bronze age (c.3500-1000 b.c.) humans first fashioned tools and weapons from something other than basic materials found in nature. Humans combined copper and tin to form a strong metal that was still easily malleable. Modern bronze contains a 25:75 ratio of tin to copper. The use of bronze in early times was greatest in nations where tin deposits were most plentiful, like Asia Minor, and among countries that traded with tin-mining nations.

Brass is an alloy of copper and zinc. It is valued for its light weight and rigid strength. It has a ratio of about one-third zinc to two-thirds copper. The exact ratio of metals determines the qualities of the alloy. For example, brass, having less than 63% copper, must be heated to be worked. Brass is noted for its beauty when polished. Brass was perhaps first produced in Palestine from 1400 to 1200 b.c. It was later used by the Romans for coins. Many references to brass in the Bible and other ancient documents are really mistranslations of mentions of bronze.

Pewter is an alloy of copper, tin, and antimony. It is a very soft mixture that can be worked when cold and beat repeatedly without becoming brittle. It was used in Roman times, but its greatest period of popularity began in England in the fourteenth century and continued into the eighteenth. Colonial American metalworks produced some notable pewter work. As a cheaper version of silver, it was used in plates, cups, pitchers, and candelabras.

The various types of steel and iron are all alloys classifiable by their content of other materials. For instance, wrought iron has a very small carbon content, while cast iron has at least 2% carbon.

Steels contain varying amounts of carbon and metals such as tungsten, molybdenum, vanadium, and cobalt, giving them the strength, durability, and anti-corrosion capabilities required by their different uses. Stainless steel, which has 18% chromium and 8% nickel alloyed to it, is valued for its anti-corrosive qualities.

Duraluminum contains one-third steel and twothirds aluminum . It was developed during World War I for the superstructures of the Zeppelin airships built in Germany.

Many alloys add function to physical beauty. For example, sterling silver is made with 8% copper to add strength so that it can be made into chalices and silverware.

All American coins are made from copper alloy, sometimes sandwiched between layers of silver.

Alloys greatly enhance the versatility of metals. Without them there would be total dependency on pure metals, which would affect their cost and availability. Alloys are a very important part of humankind's past and future.

See also Metallurgy.

Alloy

An alloy is a mixture of two or more elements, at least one of which is metallic, that itself has metallic properties (ductility, conductivity, etc.). Compounds that involve metals but do not have metallic properties are not alloys. Alloying occurs naturally; most raw gold, for example, is alloyed with silver, and natural nickel-iron alloys occur both in terrestrial rocks and as a common ingredient of meteorites. However, all alloys used for modern technological purposes are created industrially. This is necessary both because most raw metals exist as chemical compounds in rocks and because the balance of ingredients in a useful alloy must be precise.

In a given alloy, one metal is usually present in higher concentration than any other element; this is termed the parent metal or solvent of the alloy. Most alloys are solid at room temperature , and are assumed to be in the solid state when their properties are specified. Three common alloys are steel (parent metal iron , main additive carbon ), bronze (parent metal copper, main additive tin), and brass (parent metal copper, main additive zinc).

The nature of the mixing in an alloy depends on the chemical properties of its ingredients. The atoms of the different elements in an alloy can be roughly classed as indifferent to each other, as attracting each other, or as repelling each other. If all atoms in an alloy are indifferent to each other, they mix randomly and produce an alloy that is uniform at all levels above the atomic. Such an alloy is termed a random solid solution. If the atoms of unlike elements in an alloy attract each other, some orderly pattern develops when the alloy cools from its molten to its solid state. Such a solid is termed a superlattice or ordered solid solution. For example, a half-copper, half-aluminum alloy is an ordered solid solution in which planes of aluminum atoms alternate with planes of copper atoms. However, if the unlike atoms in a substance are attracted by strong electrical forces, the result is not an ordered solid solution with metallic properties but a true chemical compound. Salt, for example (sodium chloride, NaCl), is considered an ionic compound, not an alloy of sodium.

If the unlike atoms in an alloy attract each other less than the like atoms, the elements tend to segregate into distinct crystal domains upon solidification. The alloy is then a mass of pure, microscopic crystals of its component elements and is termed a phase mixture.

See also Crystals and crystallography; Industrial minerals; Metals; Precious metals; Phase state changes

al·loy • n. / ˈaˌloi/ a metal made by combining two or more metallic elements, esp. to give greater strength or resistance to corrosion: an alloy of nickel, bronze, and zinc. ∎  an inferior metal mixed with a precious one.• v. / ˈaˌloi; əˈloi/ [tr.] mix (metals) to make an alloy: alloying tin with copper to make bronze. ∎ fig. debase (something) by adding something inferior.