There is no single simple definition of a metal. In general, however, any chemical element that displays "metallic properties", or metal properties, is classed as a metal. Typical metal properties include luster, good thermal and electrical conductivity, and the ability to be permanently shaped or deformed at room temperature. The wide variation among these properties of metals is precisely what makes them suitable for so many different uses in everyday life.
Chemical elements that lack these typical metal properties are classed as nonmetals. A small number of elements, known as metalloids, sometimes behave like a metal and at other times like a nonmetal. Examples given for this in-between group include carbon, phosphorus, silicon, and sulfur.
The different properties of metals can be combined by mixing two or more of them together, producing a substance called an alloy. Because pure elemental metals are often too soft to be of practical use, much of metallurgy focuses on formulating useful alloys. By creating an alloy with a specific composition, it is possible to achieve a particular metal property, or set of properties, for a predetermined application.
Steel, for example, is a mixture of iron with small amounts of carbon and other elements. Other alloys, such as brass (copper and zinc) and bronze (copper and tin), are easy to shape and attractive in appearance. Bronze is also used frequently in shipbuilding because it resists corrosion from seawater.
The properties of individual metals and alloys determine where each is used. Titanium is much lighter and less dense than steel yet equally strong. Compared with aluminum it is heavier, but it is twice as strong. Titanium is also highly resistant to corrosion, and together these metallic properties make it an excellent material. Titanium alloys are used in aircraft, ships, and spacecraft, as well as in paints, bicycles, and even laptop computers.
Copper is a good conductor of electricity and is ductile, which is why it is widely used for electrical cables. Gold and silver are very malleable, ductile, and largely non-reactive, making them ideal for intricate jewelry. Gold is especially suitable for this purpose because it does not tarnish, and it can also be used for oxidation-free electrical connections.
Iron and steel are both hard and strong, so they are used to construct bridges and buildings. One disadvantage of iron is that it tends to rust, and although most steels also rust, they can be formulated to be rust-free. Aluminum is a good conductor of heat and is malleable, which makes it useful for saucepans and foil, as well as for aircraft bodies, thanks to its very light weight.
Material selection becomes especially important in demanding environments. Stainless steel and galvanized steel are used where resistance to corrosion is essential, while aluminum alloys and magnesium alloys are chosen for applications that require both strength and light weight.
For particularly aggressive conditions, copper-nickel alloys such as Monel are used in highly corrosive environments and for non-magnetic applications. Nickel-based superalloys such as Inconel are used in high-temperature applications, including turbochargers, pressure vessels, and heat exchangers. Where temperatures are extremely high, single-crystal alloys are used to minimize creep.
Figure 1: Relationship Between Metal Properties, Performance Benefits, and Engineering Applications
Properties tell you how a metal behaves, but they make the most sense when you see where they come from and how they are put to use. The other guides in our Metal series join up the picture. Metal Composition explains how a material's chemistry sets its properties in the first place, and Metal Grades shows how those materials are sorted into the families you actually specify and buy. To see how a material's requirements are written down and formally agreed, read Metal Specifications and Metal Standards, which cover the defining documents and the systems behind them. Finally, Metal Equivalents and Cross-Referencing explains how a grade in one system is matched to its equivalent in another, something that relies directly on the properties described here.
Typical metal properties include luster, good thermal and electrical conductivity, and the ability to be permanently shaped or deformed at room temperature. The variation among these properties is what makes different metals suitable for different applications.
Metals display typical metallic properties, while nonmetals lack them. Metalloids are elements that sometimes behave like a metal and at other times like a nonmetal, occupying an intermediate position between the two.
Pure elemental metals are often too soft for practical use. Combining two or more metals creates an alloy whose composition can be tailored to deliver a specific property, or combination of properties, for a particular application. Steel, brass, and bronze are common examples.
Stainless steel and galvanized steel are widely used where corrosion resistance is important. Bronze resists seawater corrosion and is popular in shipbuilding, while copper-nickel alloys such as Monel perform well in highly corrosive environments.
Nickel-based superalloys such as Inconel are used in high-temperature applications like turbochargers, pressure vessels, and heat exchangers. For extremely high temperatures, single-crystal alloys are used to minimize creep.
Titanium combines low density with high strength and excellent corrosion resistance. It is lighter than steel but comparably strong, and about twice as strong as aluminum, which is why it is used in aircraft, ships, spacecraft, bicycles, and many other products.