Phase diagrams of condensed systems

The phase diagrams of two-component systems are represented in the two-dimensional space, where the composition is shown on the x axis (in molar or in mass fractions) in agreement with the lever rule, and the temperature is given on the y axis (in °C or in Kelvin). They are the so-called isobaric diagrams, since the constant pressure, mostly the atmospheric one, is assumed. The Gibbs phase law attains thus the form 

The individual curves or straight lines in the phase diagrams, which are denoted as boundary lines, represent the equilibrium between two phases. In a phase diagram of the binary system, the plane above both the boundary lines has the highest degree of freedom possible, i.e. v = 2. On the boundary lines, the degrees of freedom decreases to v = 1 and in the eutectic point, where the boundary lines meet, the degree of freedom is equal to zero. 

In the following text, the composition in all cases will be expressed in the coordinates of the molar fraction of the component B, thus x(B). 

Points Tfus, A and 7fus, b represent the melting points of components A and B. The lines starting from the individual melting points represent curves of primary crystallization, i.e. their liquidus curves. Both the liquidus curves meet at the eutectic temperature Te in the eutectic point with the composition Xg. The eutectic temperature is the lowest temperature at which the liquid phase is present in the system. 

At cooling of the melt with composition x(B), from the temperature representing to the figurative point X in the plane L, the composition of the melt does not change until the point pc, A is attained. In the plane L, the system has two degrees of freedom k = 2, /= 1, V = 2), thus we can arbitrarily change the temperature and composition of the melt without the appearance of a new phase. 

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The calculation of the phase diagrams of condensed systems using the coupled analysis of the thermodynamic and phase diagram data is based on the solution of a set of equations... 

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