===منحنى اطوار الحديد والكربون ===
Other much more complex types of phase diagrams can be constructed, particularly when more than one pure component is present. In that case, [[concentration]] becomes an important variable. Phase diagrams with more than two dimensions can be constructed that show the effect of more than two variables on the phase of a substance. Phase diagrams can use other variables in addition to or in place of temperature, pressure and composition, for example the strength of an applied electrical or magnetic field, and they can also involve substances that take on more than just three states of matter.
[[Image:Brosen ironcarbon.svg|thumb|250px|left|The iron–iron carbide (Fe–Fe<sub>3</sub>C) phase diagram. The percentage of carbon present and the temperature define the phase of the iron carbon alloy and therefore its physical characteristics and mechanical properties. The percentage of carbon determines the type of the ferrous alloy: iron, steel or cast iron]]