Part I : Preliminaries (Thermodynamics and Kinetics)
Module 3: G versus xB curves and phase diagrams
 
4.4 An eutetic phase diagrm
Consider a phase diagram which has an eutectic point as shown in Fig. 15. Eutectic point is a point of co-existence of three phasses. Hence, the number of degrees of freedom is zero; that is both the compositions of the phases that make up the mechanical mixture (that forms from the given liquid) and temperature of the eutectic point is fixed. Hence, the line indicating the eutectic reaction in the phase diagram is known as invariant line. In Fig. 16, we show the $x_B$ versus $G_{1}$ lines at the four temperatures through marked on the phase diagram.
Figure 15: An eutectic phase diagram.
Figure 16: Free energy versus composition plots for a system with an eutectic phase diagram. Note that we have assumed that the phases that make up the eutectic mixture have the same structure.