Basics of Phase diagrams

      Phase diagrams are one of the most important sources of information concerning the behavior of elements, compounds and solutions. They provide us with the knowledge of phase composition and phase stability as a function of temperature (T), pressure (P) and composition (C). Furthermore, they permit us to study and control important processes such as phase separation, solidification, sintering, purification, growth and doping of single crystals for technological and other applications. Although phase diagrams provide information about systems at equilibrium, they can also assist in predicting phase relations, compositional changes and structures in systems not at equilibrium.

     The phase rule, also known as the Gibbs phase rule, relates the number of components and the number of degrees of freedom in a system at equilibrium by the formula
                                       F = C – P + 2

where F equals the number of degrees of freedom or the number of independent variables, C equals the number of components in a system in equilibrium and P equals the number of phases. The digit 2 stands for the two variables, temperature and pressure.

    The number of degrees of freedom (F) of a system is the number of variables that may be changed independently without causing the appearance of a new phase or disappearance of an existing phase.

    Please note that the value of F cannot be less than 0. So the maximum number of phases can be found out using the Gibbs Formula with taking thermodynamics into consideration.

    The point at which F = 0 , is called invariant point. The point at which the three phases can co-exist is called triple point. 

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Some of the info on this post has been taken from the url: http://web.mit.edu/3.091/www/archives/Notes_10.pdf. Please do refer to the same for more info.