The components of an alloy are the entities that are combined to make it. For the copper/nickel system the components are the atoms Cu and Ni. For the ceramic mullite, the components are the compounds alumina (Al₂ O₃ ) and silica (Si O₂).

When components are combined to form an alloy, the material system that results includes all possible alloy compositions.

Some alloy systems may contain several phases. A phase is a distinct region of a material that has a particular chemical composition and physical condition and is separated from other phases by a definite boundary.

Binary alloys, systems with only two components, will be used to explore the different ways that alloys can form in thermodynamic equilibrium.