Metal materials with metal characteristics formed by alloying process (melting, mechanical alloying, sintering, vapor deposition, etc.) that often contain two or more kinds of metal elements or add other nonmetallic elements on metal base are called alloys. But alloys may contain only one metallic element, such as steel. (Steel, is a general term for iron alloys with carbon content between 0.02% and 2.00% by mass percentage)

Here we need to note that the alloy is not a mixture in the general concept, and can even be pure, such as a single-phase metal intercompound alloy, the added alloying elements can form solid solutions, compounds, and produce endothermic or exothermic reactions, thereby changing the properties of the metal matrix.

The formation of alloys often improves the properties of elemental elements; for example, steel is stronger than its main component, iron. The physical properties of the alloy, such as density, reactivity, Young's modulus, electrical and thermal conductivity, may be similar to the constituent elements of the alloy, but the tensile and shear strength of the alloy is usually very different from the properties of the constituent elements. This is due to the fact that the arrangement of atoms in the alloy is very different from that in the elemental.

A small amount of an element can have a large effect on the properties of an alloy. For example, impurities in ferromagnetic alloys can cause the properties of the alloy to change.

Unlike pure metals, most alloys do not have a fixed melting point, and when the temperature is in the melting temperature range, the mixture is solid-liquid coexistence. It can therefore be said that the melting point of the alloy is lower than that of the component metal. See eutectic mixtures. Among the common alloys, brass is an alloy of copper and zinc; Bronze is an alloy of tin and copper used in statues, decorations and church bells. Some countries use alloys (such as nickel) in their currencies.