A graph showing the temperature and composition ranges within which each of the phases of a particular alloy exist under equilibrium conditions.
More-accurately known as an equilibrium phase diagram or constitution diagram. Steel is normally shown as the simple iron–carbon equilibrium diagram since the low metal alloy content of up to 1.5%, found in the most common steels used in engineering, has little effect on the diagram. High alloy contents can have a significant effect and require very complicated diagrams to explain their phases.
Where the diagram involves the base metal with one alloy element, such as iron-carbon, it is known as a binary phase diagram. If an additional alloy element is added, it is called a ternary phase diagram – for three constituents, such as iron–carbon–nitrogen.