An alloy is a homogenous mixture of two or more metals, or a metal and a non-metal. Alloys are combinations of metals or metals and other elements. They are formed by adding certain other metals/elements to the metals in specific ratios to form alloys. The purpose of creating alloys is to impart certain properties to metals or to strengthen some of their existing properties. For example, pure aluminum is a relatively soft metal, and pure copper is also quite soft. However, when aluminum is alloyed with copper, the strength of the resulting alloy is far greater than that of its parent metals.

Alloys can be classified into two categories based on the arrangements of atoms in their respective lattices: substitutional alloys and interstitial alloys. Substitutional alloys are formed when the atoms of the alloying element replace the atoms of the base metal in the crystal lattice. In interstitial alloys, the atoms of the alloying element occupy the interstitial sites in the crystal lattice of the base metal.

Some examples of alloys are:

• Brass: an alloy of copper and zinc.
• Steel: an alloy of iron and carbon.
• Bronze: an alloy of copper and tin.
• Duralumin: an alloy of aluminum, copper, and magnesium.
• Nichrome: an alloy of nickel, chromium, and iron.
• Solder: an alloy of lead and tin.

Alloys are used in a wide variety of industries because they combine different elements and contain atoms of different sizes, which gives them unique properties. The physical properties of an alloy, such as conductivity, reactivity, and density, may not differ significantly from its constituent elements. However, the engineering properties of an alloy, such as shear strength and tensile strength, may be substantially different.