Alloy
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Alloy, substance composed of two or more metals. Alloys, like pure metals, possess metallic luster and conduct heat and electricity well, although not generally as well as do the pure metals of which they are formed. Compounds that contain both a metal or metals and certain nonmetals, particularly those containing carbon, are also called alloys. The most important of these is steel. Simple carbon steels consist of about 0.5 percent manganese and up to 0.8 percent carbon, with the remaining material being iron.
An alloy may consist of an intermetallic compound, a solid solution, an intimate mixture of minute crystals of the constituent metallic elements, or any combination of solutions or mixtures of the foregoing. Intermetallic compounds, such as NaAu2, CuSn, and CuAl2, do not follow the ordinary rules of valency. They are generally hard and brittle; although they have not been important in the past where strength is required, many new developments have made such compounds increasingly important. Alloys consisting of solutions or mixtures of two metals generally have lower melting points than do the pure constituents. A mixture with a melting point lower than that of any other mixture of the same constituents is called a eutectic. The eutectoid, the solid-phase analog of the eutectic, frequently has better physical characteristics than do alloys of different proportions.
The properties of alloys are frequently far different from those of their constituent elements, and such properties as strength and corrosion resistance may be considerably greater for an alloy than for any of the separate metals. For this reason, alloys are more generally used than pure metals. Steel is stronger and harder than wrought iron, which is approximately pure iron, and is used in far greater quantities. The alloy steels, mixtures of steel with such metals as chromium, manganese, molybdenum, nickel, tungsten, and vanadium, are stronger and harder than steel itself, and many of them are also more corrosion-resistant than iron or steel. An alloy can often be made to match a predetermined set of characteristics. An important case in which particular characteristics are necessary is the design of rockets, spacecraft, and supersonic aircraft. The materials used in these vehicles and their engines must be light in weight, very strong, and able to sustain very high temperatures. To withstand these high temperatures and reduce the overall weight, lightweight, high-strength alloys of aluminum, beryllium, and titanium have been developed. To resist the heat generated during reentry into the atmosphere of the earth, alloys containing heat-resistant metals such as tantalum, niobium, tungsten, cobalt, and nickel are being used in space vehicles.
A wide variety of special alloys containing metals such as beryllium, boron, niobium, hafnium, and zirconium, which have particular nuclear absorption characteristics, are used in nuclear reactors. Niobium-tin alloys are used as superconductors at extremely low temperatures. Special copper, nickel, and titanium alloys, designed to resist the corrosive effects of boiling salt water, are used in desalination plants.
Historically, most alloys have been prepared by mixing the molten materials. More recently, powder metallurgy has become important in the preparation of alloys with special characteristics. In this process, the alloys are prepared by mixing dry powders of the materials, squeezing them together under high pressure, and then heating them to temperatures just below their melting points. The result is a solid, homogeneous alloy. Mass-produced products may be prepared by this technique at great savings in cost. Among the alloys made possible by powder metallurgy are the cermets. These alloys of metal and carbon (carbides), boron (borides), oxygen (oxides), silicon (silicides), and nitrogen (nitrides) combine the advantages of the high-temperature strength, stability, and oxidation resistance of the ceramic compound with the ductility and shock resistance of the metal. Another alloying technique is ion implantation, which has been adapted from the processes used to produce computer chips; beams of ions of carbon, nitrogen, and other elements are fired into selected metals in a vacuum chamber to produce a strong, thin layer of alloy on the metal surface. Bombarding titanium with nitrogen, for example, can produce a superior alloy for prosthetic implants.
Sterling silver, 14-karat gold, white gold, and plantinum-iridium are precious metal alloys. Babbit metal, brass, bronze, Dow-metal, German silver, gunmetal, Monel metal, pewter, and solder are alloys of less precious metals. Commercial aluminum is, because of impurities, actually an alloy. Alloys of mercury with other metals are called amalgams.
An alloy may consist of an intermetallic compound, a solid solution, an intimate mixture of minute crystals of the constituent metallic elements, or any combination of solutions or mixtures of the foregoing. Intermetallic compounds, such as NaAu2, CuSn, and CuAl2, do not follow the ordinary rules of valency. They are generally hard and brittle; although they have not been important in the past where strength is required, many new developments have made such compounds increasingly important. Alloys consisting of solutions or mixtures of two metals generally have lower melting points than do the pure constituents. A mixture with a melting point lower than that of any other mixture of the same constituents is called a eutectic. The eutectoid, the solid-phase analog of the eutectic, frequently has better physical characteristics than do alloys of different proportions.
The properties of alloys are frequently far different from those of their constituent elements, and such properties as strength and corrosion resistance may be considerably greater for an alloy than for any of the separate metals. For this reason, alloys are more generally used than pure metals. Steel is stronger and harder than wrought iron, which is approximately pure iron, and is used in far greater quantities. The alloy steels, mixtures of steel with such metals as chromium, manganese, molybdenum, nickel, tungsten, and vanadium, are stronger and harder than steel itself, and many of them are also more corrosion-resistant than iron or steel. An alloy can often be made to match a predetermined set of characteristics. An important case in which particular characteristics are necessary is the design of rockets, spacecraft, and supersonic aircraft. The materials used in these vehicles and their engines must be light in weight, very strong, and able to sustain very high temperatures. To withstand these high temperatures and reduce the overall weight, lightweight, high-strength alloys of aluminum, beryllium, and titanium have been developed. To resist the heat generated during reentry into the atmosphere of the earth, alloys containing heat-resistant metals such as tantalum, niobium, tungsten, cobalt, and nickel are being used in space vehicles.
A wide variety of special alloys containing metals such as beryllium, boron, niobium, hafnium, and zirconium, which have particular nuclear absorption characteristics, are used in nuclear reactors. Niobium-tin alloys are used as superconductors at extremely low temperatures. Special copper, nickel, and titanium alloys, designed to resist the corrosive effects of boiling salt water, are used in desalination plants.
Historically, most alloys have been prepared by mixing the molten materials. More recently, powder metallurgy has become important in the preparation of alloys with special characteristics. In this process, the alloys are prepared by mixing dry powders of the materials, squeezing them together under high pressure, and then heating them to temperatures just below their melting points. The result is a solid, homogeneous alloy. Mass-produced products may be prepared by this technique at great savings in cost. Among the alloys made possible by powder metallurgy are the cermets. These alloys of metal and carbon (carbides), boron (borides), oxygen (oxides), silicon (silicides), and nitrogen (nitrides) combine the advantages of the high-temperature strength, stability, and oxidation resistance of the ceramic compound with the ductility and shock resistance of the metal. Another alloying technique is ion implantation, which has been adapted from the processes used to produce computer chips; beams of ions of carbon, nitrogen, and other elements are fired into selected metals in a vacuum chamber to produce a strong, thin layer of alloy on the metal surface. Bombarding titanium with nitrogen, for example, can produce a superior alloy for prosthetic implants.
Sterling silver, 14-karat gold, white gold, and plantinum-iridium are precious metal alloys. Babbit metal, brass, bronze, Dow-metal, German silver, gunmetal, Monel metal, pewter, and solder are alloys of less precious metals. Commercial aluminum is, because of impurities, actually an alloy. Alloys of mercury with other metals are called amalgams.
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