About three quarters of the elements available can be classified as metals, and about half of these are of at least some industrial or commercial importance. Although the word metal, by strict definition, is limited to the pure metal elements, common usage gives it wider scope to include metal alloys. Although pure metallic elements have a broad range of properties, they are quite limited in commercial use. Metal alloys, which are combinations of two or more elements, are far more versatile and for this reason are the form in which most metals are used by industry.
Metallic materials are crystalline solids. Individual crystals are composed of unit cells repeated in a regular pattern to form a three-dimensional crystal-lattice structure. A piece of metal is an aggregate of many thousands of interlocking crystals (grains) immersed in a cloud of negative-valence electrons detached from the atoms of the crystals. These loose electrons serve to hold the crystal structures together because of their electrostatic attraction to the positively charged metal atoms (ions). The bonding forces, which are large because of the close-packed nature of metallic crystal structures, account for the generally good mechanical properties of metals. Also, the electron cloud makes most metals good conductors of heat and electricity.
Metals are often identified as to the method used to produce the forms in which they are used. When a metal has been formed or shaped in the solid, plastic state, it is referred to as a wrought metal. Metal shapes that have been produced by pouring liquid metal into a mold are referred to as cast metals.
There are two families of metallic materials — ferrous and nonferrous. The basic ingredient of all ferrous metals is the element iron. These metals range from cast irons and carbon steels, with over 90% iron, to specialty iron alloys, containing a variety of other elements that add up to nearly half the total composition.
Except for commercially pure iron, all ferrous materials, both irons and steels, are considered to be primarily iron-carbon alloy systems. Although the carbon content is small (less than 1% in steel and not more 4% in cast irons) and often less than other alloying elements, it nevertheless is the predominant factor in the development and control of most mechanical properties.
By definition, metallic materials that do not have iron as their major ingredient are considered to be nonferrous metals. There are roughly a dozen nonferrous metals in relatively wide industrial use. At the top of the list is aluminum, which next to steel is the most widely used structural metal today. It and magnesium, titanium, and beryllium are often characterized as light metals because their density is considerably below that of steel.
Copper alloys are the second nonferrous material in terms of consumption. There are two major groups of copper alloys: brass, which is basically a binary alloy system of copper and zinc, and bronze, which was originally a copper-tin alloy system. Today, the bronzes include other copper-alloy systems.
Zinc, tin, and lead, with melting points below 427°C, are often classified as low-melting alloys. Zinc, whose major structural use is in die castings, ranks third to aluminum and copper in total consumption.
Lead and tin are rather limited to applications where their low melting points and other special properties are required. Other low-melting alloys are bismuth, antimony, cadmium, and indium.
Another broad group of nonferrous alloys is referred to as refractory metals. Such metals as tungsten, molybdenum, and chromium, with melting points above 1649°C, are used in products that must resist unusually high temperatures. Although nickel and cobalt have melting points below 1649°C, they serve as the base metal or as alloying elements of many heat-resistant alloys.
Finally, the precious metals, or noble metals, have the common characteristic of high cost. In addition, they generally have high corrosion resistance, many useful physical properties, and generally high density.
