An alloy is a substance that is formed by combining a metal with other metals, or nonmetals. For example, brass is an alloy of the metals copper and zinc, and steel is alloy of the metal iron with the nonmetal carbon. The special characteristics of metals, such as hardness, strength, flexibility, and weight are called its properties. By the process of alloying it is possible to create materials with the exact combinations of properties for particular use.

In the aircraft industry, there is a need for metals that are both strong and light. Steel is strong but too heavy, whereas aluminum is light but not strong. By mixing aluminum with copper and other metals, an alloy is created that is strong to withstand the stresses of flight, but light enough to reduce the cost of fuel to lift the aircraft.

There is an important difference between the alloys created by us and the combination of metals that occur accidentally as impure metals.. Both are mixtures, but alloys are mixtures that have been deliberately combined in specific proportions for definite purpose.