Electrical properties
The electrical conductivity of a metal (or its reciprocal, electrical resistivity) is determined by the ease of movement of electrons past the atoms under the influence of an electric field. This movement is particularly easy in copper, silver, gold, and aluminum—all of which are well-known conductors of electricity. The conductivity of a given metal is decreased by phenomena that deflect, or scatter, the moving electrons. These can be anything that destroys the local perfection of the atomic arrangement—for example, impurity atoms, grain boundaries, or the random oscillation of atoms induced by thermal energy. This last example explains why the conductivity of a metal increases substantially with falling temperature: in a pure metal at room temperature, most resistance to the motion of free electrons comes from the thermal vibration of the atoms; if the temperature is reduced to almost absolute zero, where thermal motion essentially stops, conductivity can increase several thousandfold.
Magnetic properties
When an electric current is passed through a coil of metal wire, a magnetic field is developed around the coil. When a piece of copper is placed inside the coil, this field increases by less than 1 percent, but, when a piece of iron, cobalt, or nickel is placed inside the coil, the external field can increase 10,000 times. This strong magnetic property is known as ferromagnetism, and the three metals listed above are the most prominent ferromagnetic metals. When the piece of ferromagnetic metal is removed from the coil, it retains some of this magnetism (that is, it is magnetized). If the metal is hard, as in a hardened piece of steel, the loss, or reversal, of magnetization will be slow, and the sample will be useful as a permanent magnet. If the metal is soft, it will quickly lose its magnetism; this will make it useful in electrical transformers, where rapid reversal of magnetization is essential.
In many types of solids, the atoms possess a permanent magnetic moment (they act like small bar magnets). In most solids, the direction of these moments is arranged at random. What is exceptional about ferromagnetic solids is that the interatomic forces cause the moments of neighbouring atoms spontaneously to align in the same direction. If the moments of all of the atoms in a single sample lined up in the same direction, the sample would be an exceptionally strong magnet with exceptionally high energy. That energy would be reduced if the sample broke up into domains, with all atomic moments in each domain being aligned but the direction of magnetization in adjacent domains being in opposite directions and thus tending to cancel one another. This is what happens when a ferromagnetic metal is magnetized: all domains do not take on the same orientation, but domains of one orientation grow at the expense of others. The alignment of atomic magnetic moments within a domain is weakened by thermally induced oscillations, and ferromagnetism is finally lost above the Curie point, which is 770° C (1,420° F) for iron and 358° C (676° F) for nickel.
Chemical properties
Almost any metal will oxidize in air, the only exception being gold. At room temperature a clean metal surface will oxidize very little, since a thin oxide film forms and protects the metal from further oxidation. At elevated temperatures, though, oxidation is faster, and the film is less protective. Many chemicals accelerate this corrosion process (that is, the conversion of a metal to an oxide in air or to a hydroxide in the presence of water).
A special property of metal surfaces is their ability to catalyze chemical reactions. For example, in the exhaust system of most automobiles, combustion gases pass over a dispersion of very fine platinum particles. The surfaces of these particles greatly accelerate the oxidation of carbon monoxide and hydrocarbons to carbon dioxide and water, thus reducing the toxicity of the exhaust gases.
Alloying
Almost all metals are used as alloys—that is, mixtures of several elements—because these have properties superior to pure metals. Alloying is done for many reasons, typically to increase strength, increase corrosion resistance, or reduce costs.
Processes
In most cases, alloys are mixed from commercially pure elements. Mixing is relatively easy in the liquid state but slow and difficult in the solid state, so that most alloys are made by melting the base metal—for instance, iron, aluminum, or copper—and then adding the alloying agents. Care must be taken to avoid contamination, and in fact purification is often carried out at the same time, since this is also done more easily in the liquid state. Examples can be found in steelmaking, including the desulfurizing of liquid blast-furnace iron in a ladle, the decarburization of the iron during its conversion to steel, the removal of oxygen from the liquid steel in a vacuum degasser, and finally the addition of tiny amounts of alloying agents to bring the steel to the desired composition.
The largest tonnages of alloys are melted in air, with the slag being used to protect the metal from oxidation. However, a large and increasing amount is melted and poured entirely in a vacuum chamber. This allows close control of the composition and minimizes oxidation. Most of the alloying elements needed are placed in the initial charge, and melting is done with electricity, either by induction heating or by arc melting. Induction melting is conducted in a crucible, while in arc melting the melted droplets drip from the arc onto a water-cooled pedestal and are immediately solidified.
Sometimes an inhomogeneous, composite structure is desired, as in cemented tungsten carbide cutting tools. In such cases, the alloy is not melted but is made by powder metallurgical techniques (see below).
Metallurgy
Increasing strength
The most common reason for alloying is to increase the strength of a metal. This requires that barriers to slip be distributed uniformly throughout the crystalline grains. On the finest scale, this is done by dissolving alloying agents in the metal matrix (a procedure known as solid solution hardening). The atoms of the alloying metals may substitute for matrix atoms on regular sites (in which case they are known as substitutional elements), or, if they are appreciably smaller than the matrix atoms, they may take up places between regular sites (where they are called interstitial elements).
The next coarser type of barrier to slip is a fine, solute-rich precipitate with dimensions of only tens or hundreds of atomic diameters. These particles are formed by heat treatment. The metal is heated to a temperature at which the solute-rich phase dissolves (e.g., 5 percent copper in aluminum at 540° C [1,000° F]), and then it is rapidly cooled to avoid precipitation. The next step is to form a fine precipitate throughout the sample by aging at an elevated temperature that is well below the temperature used for the initial dissolution.
In metals that undergo transformations from one crystal structure to another on heating (e.g., iron or titanium), the difference in solute solubility between the high- and low-temperature phases is often utilized. For example, in the low-alloy steels used for tools and gears, carbon forms the hardening precipitate. Carbon is much more soluble in the high-temperature fcc phase (gamma iron, also called austenite) than in the low-temperature bcc phase (alpha iron, or ferrite). The other alloying elements added (e.g., chromium, nickel, and molybdenum) retard the transformation of austenite on cooling, so that the fcc-to-bcc transformation occurs at a low temperature by a sudden, shear transformation; this allows no time for carbon precipitation and makes the steel harder. A final reheating tends to coarsen the precipitate and thereby increase ductility; this is commonly called tempering.
An array of barriers on the same scale as precipitation hardening can be created by plastically deforming the metal at room temperature. This is often done in a cold-working operation such as rolling, forging, or drawing. The deformation occurs through the generation and motion of line defects, called dislocations, on slip planes spaced only a few hundred atom diameters apart. When slip occurs on different planes, the intersecting dislocations form tangles that inhibit further slip on those planes. Such strain hardening can double or triple the yield stress of a metal.
Increasing corrosion resistance
Alloys can have much better high-temperature oxidation resistance than pure metals. The alloying elements most commonly used for this purpose are chromium and aluminum, both of which form an adherent film of stable oxide on the surface that protects the metal from further oxidation. Eleven percent or more chromium is added to iron to create a stainless steel, while 10 to 15 percent chromium and 3 to 5 percent aluminum are commonly added to the nickel- or cobalt-based superalloys used in the highest-temperature components of jet engines.
Inhibiting the corrosion of alloys in water is more varied and complex than inhibiting high-temperature oxidation. Nevertheless, one of the most common techniques is to add alloying elements that inhibit the corrosion.
Reducing costs
Gold and silver used in jewelry and coins are alloyed with other metals to increase strength and reduce cost. Sterling silver contains 7.5 percent base metal, commonly copper. The fraction of gold in gold jewelry is designated in karats, with 24-karat being pure gold and 18-karat being 75 percent gold by weight. In coins, alloys with the look and density of silver are commonly substituted for silver; for instance, all U.S. coins that appear to be made of silver actually have a surface layer of 75 percent copper and 25 percent nickel.
Lowering melting points
Alloying can also be done to lower the melting point of a metal. For example, adding lead to tin lowers the melting point of the tin-rich alloy, and adding tin to lead lowers the melting point of the lead-rich alloy. A 62-percent-tin 38-percent-lead alloy, which is called the eutectic composition, has the lowest melting point of all, much lower than that of either metal. Eutectic lead-tin alloys are used for soldering.
