Alloys: Composition and Everyday Uses
An alloy is a mixture of a metal with one or more other elements, usually other metals, that has properties different from and often better than those of the pure metal. In brief: steel is iron with a small amount of carbon, brass is copper with zinc and bronze is copper with tin; alloying a soft metal with other atoms makes it harder and stronger.
Most metal objects we use every day are alloys, not pure metals. Pure metals are often too soft or too reactive for practical use. This article explains what alloys are and why they are stronger, lists the composition and uses of common alloys, shows how to calculate the metal content of an alloy, works through the carat system for gold and gives the history of the Bronze and Iron Ages.
What is an alloy?
An alloy is made by melting the metals together, mixing them and letting the mixture solidify. Because the components are not chemically combined in a fixed ratio, an alloy is a mixture, although the atoms are mixed very evenly, so most alloys behave as a solid solution. The proportions can vary, which allows engineers to adjust the properties.
The word alloy covers combinations such as a metal with a small amount of a nonmetal. Steel contains carbon, which is a nonmetal.
Why are alloys harder than pure metals?
In a pure metal, the atoms are all the same size and are arranged in regular layers. The layers can slide over each other easily when a force is applied, so the metal bends and can be shaped. In an alloy, atoms of other elements have different sizes. They disturb the regular layers and make it harder for the layers to slide. The alloy is therefore harder and stronger, though it is often less malleable and a poorer conductor than the pure metal.
Common alloys, compositions and uses
| Alloy | Main components | Typical composition | Everyday uses |
|---|---|---|---|
| Brass | Copper, zinc | about 60 to 70% Cu, 30 to 40% Zn | Musical instruments, door handles, plumbing fittings |
| Bronze | Copper, tin | about 88% Cu, 12% Sn | Statues, bells, bearings |
| Steel (carbon steel) | Iron, carbon | about 0.1 to 1.5% C, rest Fe | Buildings, cars, tools |
| Stainless steel | Iron, chromium (and nickel) | at least 10.5% Cr | Cutlery, sinks, medical tools |
| Solder (lead-free) | Tin with silver or copper | mostly Sn | Joining electronics |
| Sterling silver | Silver, copper | 92.5% Ag | Jewelry, tableware |
| Duralumin (aluminum alloy) | Aluminum, copper | about 4% Cu, rest Al | Aircraft parts |
| Cupronickel | Copper, nickel | about 75% Cu, 25% Ni | Coins |
Compositions vary between manufacturers and standards, so the values are typical, not exact.
Why is steel so important?
Pure iron is soft. Adding a small amount of carbon, typically between 0.1% and 1.5%, makes it much harder and stronger. More carbon gives harder but more brittle steel, and less carbon gives softer and easier-to-shape steel. Other elements such as chromium, nickel, manganese and vanadium create special steels. Stainless steel contains at least 10.5% chromium, which forms a thin, protective oxide layer on the surface and prevents rusting.
Worked example: calculating alloy composition
- Brass. A brass sample has a mass of 250 g and contains 35% zinc. The mass of zinc is 250 × 0.35 = 87.5 g, and the mass of copper is 250 − 87.5 = 162.5 g.
- Making bronze. To make 500 g of bronze with 12% tin, the tin required is 500 × 0.12 = 60 g, and the copper is 440 g.
- Density of an alloy (approximate). Copper has a density of 8.96 g/cm3 and zinc 7.14 g/cm3. For the brass above, the volume of copper is 162.5 ÷ 8.96 = 18.1 cm3 and the volume of zinc is 87.5 ÷ 7.14 = 12.3 cm3. The total volume is 30.4 cm3 and the density is 250 ÷ 30.4 = 8.2 g/cm3. Real brass has a density of about 8.4 to 8.7 g/cm3, so this estimate, which assumes that volumes add, is not exact.
- Percent of iron in steel. A steel with 0.8% carbon has 99.2% iron and other elements. A 2,000 kg steel beam contains about 2,000 × 0.008 = 16 kg of carbon.
What does carat mean for gold?
The carat (karat in the United States) system describes the proportion of pure gold in an alloy in parts out of 24. Pure gold is 24-carat. The number of carats divided by 24 gives the fraction of gold.
| Carat | Calculation | Gold content (%) |
|---|---|---|
| 24 | 24 ÷ 24 | 100 |
| 22 | 22 ÷ 24 | 91.7 |
| 18 | 18 ÷ 24 | 75.0 |
| 14 | 14 ÷ 24 | 58.3 |
| 9 | 9 ÷ 24 | 37.5 |
A ring of 18 carats with a mass of 8.0 g contains 8.0 × 0.75 = 6.0 g of gold. Pure gold is too soft for jewelry that is worn daily, so it is alloyed with silver, copper or other metals, which also change the color.
How did alloys shape history?
Bronze was one of the first alloys used on a large scale, from about 3000 BCE in some regions, and it gave its name to the Bronze Age. Bronze was harder than copper and could be cast into tools, weapons and ornaments. Iron smelting came later, and the use of steel expanded greatly in the 19th century after Henry Bessemer introduced a process for the mass production of steel in 1856. Stainless steel was developed in the early 20th century. Modern alloys such as titanium alloys and nickel superalloys are used in jet engines and spacecraft.
Which mistakes are common?
Mistake 1: Saying that an alloy is a compound
Alloys are mixtures with variable composition, not compounds with a fixed formula.
Mistake 2: Thinking that alloys are always made only from metals
Steel contains carbon, a nonmetal.
Mistake 3: Assuming that an alloy has the average properties of its parts
Alloys often have properties that are quite different, such as much greater hardness.
Mistake 4: Believing that stainless steel can never rust
It resists rust well but can corrode under harsh conditions, such as long exposure to salt water or chlorides.