Alloy is a material made up of a metal and at least one other element. Most alloys contain a large amount of the main metal, or base metal, and smaller amounts of other components. These components can be metals, such as iron; metalloids, such as silicon; or nonmetals, such as carbon. Many pure metals are too soft, rust too easily, or have other disadvantages. But often these disadvantages can be overcome if the metals are combined with other elements. Three, four, or more different substances may be present in a single alloy.
People usually make alloys by melting a base metal and adding other components. This liquid alloy then cools and solidifies. Many alloys need to be worked into a final shape after cooling. Other alloys can be made without melting the base metal. For example, manufacturers can blend metal powders together and heat them under pressure. The solid powder particles then bond to form the alloy.
Characteristics of alloys
Alloys consist of tiny crystals called grains. In every grain, the atoms are packed together in a particular geometric arrangement. Each grain is tilted differently from those next to it. Manufacturers can control grain size by the way they heat, form, and cool the material. Grain size determines particular characteristics in alloys. For example, smaller grains make stronger alloys. Boundaries between the grains also help determine alloy characteristics. Boundaries can become sources of weakness when an alloy has impurities, or when people use the alloy at high temperatures or with damaging chemicals.
There are two types of alloys: (1) single phase alloys and (2) polyphase alloys. Single phase alloys consist of grains that all have the same composition. In these alloys, one metal dissolves into another in the same way that salt dissolves into water. A mixture of copper and nickel in any proportion forms a single phase alloy.
Polyphase alloys have different types of grains mixed together. In these alloys, the different atoms bond together in various ways to form strong, stable compounds. The proportion of metals in polyphase alloys is the same in all grains of a particular type. But this proportion can vary significantly from one grain type to another. Steels are polyphase alloys that include some grains of iron containing small amounts of carbon and other grains of a chemical compound called iron carbide. Iron carbide has one atom of carbon to every three atoms of iron.
Most alloys are stronger and harder than the pure metals from which they are made. Alloys also usually have lower melting temperatures than pure metals. Most alloys are less ductile than pure metals—that is, they are more difficult to hammer into shape, roll into sheets, or draw into wires. However, some special superplastic alloys are extremely ductile. Few alloys can conduct electricity as well as pure metals. But there are some superconducting alloys that are excellent conductors.
Kinds of alloys
The first alloys.
People first discovered alloys in nature during prehistoric times. Such alloys included meteorites of iron and nickel and mixtures of gold and silver in river beds. The first alloy made by people was bronze. The oldest bronzes consisted of copper and arsenic. Prehistoric copper smelters first produced them accidentally in about 3500 B.C. Over the next few hundred years, people discovered that mixing tin with copper produced a more useful bronze. They began to make tools, ornaments, and weapons out of this bronze. Bronze is much harder than pure copper, and it is easier to melt and cast into useful shapes.
Alloys of iron.
Iron is the most important industrial metal. Manufacturers have almost always used it as an alloy rather than as a pure metal. Iron-based alloys are called ferrous alloys.
The most widely used ferrous alloys are the steels. Steels vary both in the way people make them and in their composition. All steels, however, contain small amounts of carbon and manganese and large amounts of iron.
Each variety of steel has certain advantages. Carbon steels rank as the most widely used steels. Most carbon steels contain less than 1 percent carbon. Their strength and durability make them popular materials for structural beams, automobile bodies, and food cans. Alloy steels contain nickel, chromium, and molybdenum. They are strong enough for such products as bicycle frames and aircraft landing gear. Stainless steels contain more than 12 percent chromium, and many varieties also use nickel. Stainless steels can resist corrosion (rust and other chemical damage) extremely well. They are common materials for kitchen utensils, pots and pans, and hospital equipment. Tool steels are ferrous alloys used to work and shape other materials. They contain such components as tungsten, chromium, and molybdenum. Machines used to shape metal are made with special tool steels that keep their hardness and sharp cutting edges, even when they become red-hot in use.
Alloys for strength and lightness.
Many alloys used in vehicles, and especially aircraft, must be strong and light. Aluminum is a common base metal for many of these alloys. Pure aluminum is too light and weak for construction purposes. But manufacturers can mix the metal with other components to make strong, durable, alloys. Some common aluminum alloys contain small amounts of copper, manganese, and magnesium. These alloys are only slightly heavier than pure aluminum and are as strong as some steels. Other aluminum alloys contain zinc, magnesium, and lithium and are even stronger materials. People make aluminum alloys into many different products, including beverage cans, bicycle rims, and house siding.
Magnesium is only about two-thirds as heavy as aluminum. It is not strong enough by itself for most structural purposes, but it serves as the base metal in many useful alloys. Products using magnesium alloys include aircraft and automobile parts, as well as various tools and equipment. Titanium is another base metal for many strong, light alloys. Industries use titanium alloys to make jet engines, aircraft parts, and corrosion-resistant equipment in chemical plants.
Costly and ornamental alloys.
People have long used gold and silver as alloys rather than as pure metals. Manufacturers usually add cheaper metals to the gold or silver. This reduces the cost of the alloy while keeping the appearance of the precious metal. Alloying also hardens the gold or silver and keeps it from wearing away rapidly through use. Yellow gold is a gold alloy containing copper and silver. It is common in jewelry and other ornamental items. Dentists use similar gold alloys as fillings for teeth. Jewelry and tableware are often made of alloys that contain silver and copper.
Several cheaper alloys make attractive but inexpensive household articles. German silver is a blue-white, copper-based alloy containing nickel and zinc. People use German silver for such objects as candlesticks and hardware. Pewter, a silver-colored, tin-based alloy, is easy to work and can be highly polished. Plates, mugs, and vases are often made of pewter. Coins are commonly made of copper-nickel alloys.
Other alloys.
Among the most common alloys are the brasses. These copper-based alloys contain up to about 40 percent zinc and small amounts of tin, lead, or other elements. Plumbing materials, locks, fasteners, and other hardware are among the many products containing brass. Monel is a nickel-copper alloy. People often use this alloy for materials that must resist corrosion in sea water, such as pump fittings and boat propellers.
Many alloys have specialized applications. Stellite, an extremely hard alloy, consists chiefly of cobalt, chromium, and tungsten. People use stellite as a surface layer on steel to improve its resistance to wear. Solder, which has a low melting point, is used to join metal surfaces. Wood’s metal, another alloy with a low melting point, is employed in fuses for automatic fire alarms and sprinkler systems. Invar, an iron-nickel alloy, barely expands or contracts when its temperature changes. It is used in products that must remain at a constant size, such as measuring devices and pendulum rods.
Several alloys make excellent magnets. One example is Alnico, a group of alloys containing aluminum, nickel, cobalt, iron, and copper. These alloys can lift up to 60 times their own weight. However, alloys containing large amounts of metallic elements called lanthanide elements, such as samarium, produce magnets hundreds of times more powerful than Alnico.
Scientists are developing many alloys that provide greater strength and durability than older alloys. For example, superalloys can resist extremely high temperatures and severely corrosive conditions. They contain the base metals nickel or cobalt alloyed with chromium and many other elements. These superalloys are important components of jet engines and spacecraft.
