A metal product containing two or more elements (1) as a solid solution, (2) as an intermetallic compound, or (3) as a mixture of metallic phases. Alloys are frequently described on the basis of their technical applications. They may also be categorized and described on the basis of compositional groups. For example, See also Beryllium alloys; Iron alloys.
Except for native copper and gold, the first metals of technological importance were alloys. Bronze, an alloy of copper and tin, is appreciably harder than copper. This quality made bronze so important an alloy that it left a permanent imprint on the civilization of several millennia ago now known as the Bronze Age. Today the tens of thousands of alloys involve almost every metallic element of the periodic table.
Alloys are used because they have specific properties or production characteristics that are more attractive than those of the pure, elemental metals. For example, some alloys possess high strength; others have low melting points; others are refractory with high melting temperatures; some are especially resistant to corrosion; and others have desirable magnetic, thermal, or electrical properties. These characteristics arise from both the internal and the electronic structure of the alloy. An alloy is usually harder than a pure metal and may have a much lower conductivity.
Bearing alloys are used for metals that encounter sliding contact under pressure with another surface; the steel of a rotating shaft is a common example. Most bearing alloys contain particles of a hard intermetallic compound that resist wear. These particles, however, are embedded in a matrix of softer material which adjusts to the hard particles so that the shaft is uniformly loaded over the total surface. The most familiar bearing alloy is babbitt. Bearings made by powder metallurgy techniques are widely used because they permit the combination of materials which are incompatible as liquids, for example, bronze and graphite, and also permit controlled porosity within the bearings so that they can be saturated with oil before being used, the so-called oilless bearings. See also Antifriction bearing; Wear.
Certain alloys resist corrosion because they are noble metals. Among these alloys are the precious-metal alloys. Other alloys resist corrosion because a protective film develops on the metal surface. This passive film is an oxide which separates the metal from the corrosive environment. Stainless steels and aluminum alloys exemplify metals with this type of protection. The bronzes, alloys of copper and tin, also may be considered to be corrosion-resisting. See also Corrosion; Stainless steel.
Dental alloys contain precious metals. Amalgams are predominantly silver-mercury alloys, but they may contain minor amounts of tin, copper, and zinc for hardening purposes. Liquid mercury is added to a powder of a precursor alloy of the other metals. After being compacted, the mercury diffuses into the silver-base metal to give a completely solid alloy. Gold-base dental alloys are preferred over pure gold because gold is relatively soft. The most common dental gold alloy contains gold, silver, and copper. For higher strengths and hardnesses, palladium and platinum are added, and the copper and silver are increased so that the gold content drops. Vitallium and other corrosion-resistant alloys are used for bridgework and special applications. See also Silver alloys.
Die-casting alloys have melting temperatures low enough so that in the liquid form they can be injected under pressure into steel dies. Such castings are used for automotive parts and for office and household appliances which have moderately complex shapes. Most die castings are made from zinc-base or aluminum-base alloys. Magnesium-base alloys also find some application when weight reduction is paramount. Low-melting alloys of lead and tin are not common because they lack the necessary strength for the above applications. See also Metal casting.
In certain alloy systems a liquid of a fixed composition freezes to form a mixture of two basically different solids or phases. An alloy that undergoes this type of solidification process is called a eutectic alloy. A homogeneous liquid of this composition on slow cooling freezes to form a mixture of particles of nearly pure copper embedded in a matrix (background) of nearly pure silver.
The advantageous mechanical properties inherent in composite materials have been known for many years. Attention is being given to eutectic alloys as they are basically natural composite materials. See also Eutectics; Metal matrix composite.
Fusible alloys generally have melting temperatures below that of tin (449°F or 232°C), and in some cases as low as 122°F (50°C). Using eutectic compositions of metals such as lead, cadmium, bismuth, tin, antimony, and indium achieves these low melting temperatures. These alloys are used for many purposes, for example, in fusible elements in automatic sprinklers, forming and stretching dies, filler for thin-walled tubing that is being bent, and anchoring dies, punches, and parts being machined.
High-temperature alloys have high strengths at high temperatures. In addition to having strength, these alloys must resist oxidation by fuel-air mixtures and by steam vapor. At temperatures up to about 1380°F (750°C), the austenitic stainless steels serve well. An additional 180°F (100°C) may be realized if the steels also contain 3% molybdenum. Both nickel-base and cobalt-base alloys, commonly categorized as superalloys, may serve useful functions up to 2000°F (1100°C). Nichrome, a nickel-base alloy containing chromium and iron, is a fairly simple superalloy. More sophisticated alloys invariably contain five, six, or more components; for example, an alloy called René-41 contains Cr, Al, Ti, Co, Mo, Fe, C, B, and Ni. Other alloys are equally complex. A group of materials called cermets, which are mixtures of metals and compounds such as oxides and carbides, have high strength at high temperatures, and although their ductility is low, they have been found to be usable. One of the better-known cermets consists of a mixture of titanium carbide and nickel, the nickel acting as a binder or cement for the carbide. See also Cermet.
Metals are bonded by three principal procedures: welding, brazing, and soldering. Welded joints melt the contact region of the adjacent metal; thus the filler material is chosen to approximate the composition of the parts being joined. Brazing and soldering alloys are chosen to provide filler metal with an appreciably lower melting point than that of the joined parts. Typically, brazing alloys melt above 750°F (400°C), whereas solders melt at lower temperatures. See also Brazing; Soldering.
Aluminum and magnesium, with densities of 2.7 and 1.75 g/cm3, respectively, are the bases for most of the light-metal alloys. Titanium (4.5 g/cm3) may also be regarded as a light-metal alloy if comparisons are made with metals such as steel and copper. Aluminum and magnesium must be hardened to receive extensive application. Age-hardening processes are used for this purpose. See also Aluminum; Magnesium.
Low-expansion alloys include Invar, the dimensions of which do not vary over the atmospheric temperature range, and Kovar, which is widely used because its expansion is low enough to match that of glass.
Soft and hard magnetic materials involve two distinct categories of alloys. The former consists of materials used for magnetic cores of transformers and motors, and must be magnetized and demagnetized easily. For alternating-current applications, silicon-ferrite is commonly used. This is an alloy of iron containing as much as 5% silicon. Permalloy and some comparable cobalt-base alloys are used in the communications industry. Ceramic ferrites, although not strictly alloys, are widely used in high-frequency applications because of their low electrical conductivity and negligible induced-energy losses in the magnetic field. Permanent or hard magnets may be made from steels which are mechanically hardened, either by deformation or by quenching. The Alnicos are also widely used for magnets. Since these alloys cannot be forged, they must be produced in the form of castings. The newest hard magnets are being produced from alloys of cobalt and the rare-earth type of metals. See also Magnetic materials.
In addition to their use in coins and jewelry, precious metals such as silver, gold, and the heavier platinum metals are used extensively in electrical devices in which contact resistances must remain low, in catalytic applications to aid chemical reactions, and in temperature-measuring devices such as resistance thermometers and thermocouples. The unit of alloy impurity is commonly expressed in karats, where each karat is a 1/24 part. The most common precious-metal alloy is sterling silver (92.5% Ag, with the remainder being unspecified, but usually copper). The copper is very beneficial in that it makes the alloy harder and stronger than pure silver.
Metallic implants demand extreme corrosion resistance because body fluids contain nearly 1% NaCl, along with minor amounts of other salts, with which the metal will be in contact for indefinitely long periods of time. Type 316 stainless steels resist pitting corrosion but are subject to crevice corrosion. Vitallium and other cobalt-base alloys have orthopedic applications. Titanium alloys gained wide usage in Europe during the early 1970s for pacemakers and for retaining devices in artificial heart valves. While excellent for corrosion resistance, this alloy is subject to mechanical wear; therefore, it is not satisfactory in hip-joint prostheses and applications with similar frictional contacts. See also Prosthesis.
Shape memory alloys have a very interesting and desirable property. In a typical case, a metallic object of a given shape is cooled from a given temperature T1 to a lower temperature T2 where it is deformed so as to change its shape. Upon reheating from T2 to T1 the shape change accomplished at T2 is recovered so that the object returns to its original configuration. This thermoelastic property of the shape memory alloys is associated with the fact that they undergo a martensitic phase transformation (that is, a reversible change in crystal structure that does not involve diffusion) when they are cooled or heated between T1 and T2. Shape memory alloys are capable of being employed in a number of useful applications. One example is for thermostats; another is for couplings on hydraulic lines or electrical circuits.
Superconducting alloys, with zero resistivity, are of great interest in the design of certain fusion reactors which require very large magnetic fields to contain the plasma in a closed system. The advantage of the use of a material with a resistivity approaching zero is obvious. However, two significant problems are involved in the use of superconducting alloys in large electromagnetics: the critical temperature, and the fact that above a certain critical current density the superconducting materials tend to become normal conductors with a finite resistance. Serious materials problems still have to be solved before these materials can be used successfully. See also Superconductivity.
Thermocouple alloys include Chromel and Alumel. These two alloys together form the widely used Chromel-Alumel thermocouple, which can measure temperatures up to 2200°F (1204°C). Another common thermocouple alloy, constantan, is used to form iron-constantan and copper-constantan couples, employed at lower temperatures. See also Steel.
As discussed here, prosthetic alloys are alloys used in internal prostheses, that is, surgical implants such as artificial hips and knees. External prostheses are devices that are worn by patients outside the body; alloy selection criteria are different from those for internal prostheses. Alloy selection criteria for surgical implants can be stringent primarily because of biomechanical and chemical aspects of the service environment. The most widely used prosthetic alloys therefore include high-strength, corrosion-resistant ferrous, cobalt-based, or titanium-based alloys: for example, cold-worked stainless steel; cast Vitallium; a wrought alloy of cobalt, nickel, chromium, molybdenum, and titanium; titanium alloyed with aluminium and vanadium; and commercial-purity titanium. See also Prosthesis.
An alloy of niobium and titanium (NbTi) has a great number of applications in superconductivity; it becomes superconducting at 9.5 K (critical superconducting temperature, Tc). This alloy is preferred because of its ductility and its ability to carry large amounts of current at high magnetic fields, represented by Jc(H) [where Jc is the critical current and H is a given magnetic field], and still retain its superconducting properties. Novel high-temperature superconducting materials may have revolutionary impact on superconductivity and its applications. These materials are ceramic, copper oxide-based materials that contain at least four and as many as six elements. Typical examples are yttrium-barium-copper-oxygen (Tc 93 K); bismuth-strontium-calcium-copper-oxygen (Tc 110 K); and thallium-barium-calcium-copper-oxygen (Tc 125 K). These materials become superconducting at such high temperatures that refrigeration is simpler, more dependable, and less expensive. See also Ceramics; Superconductivity.
