an equilibrium phase diagram is a graphic mapping of the natural tendencies of a material or a material tendencies of a material or a material system. there are three primary variables to be considered: temperature, pressure, and composition. The simplest example of phase diagram is a pressure-temperature diagram for a fixed-composition material. However, most engineering processes are conducted at atmospheric pressure, and variations are more likely to occur in temperature and composition. Thus, the most useful mapping is usually a temperature-composition phase diagram at 1 atmospheric pressure.