Eutectic Point

Entry by Emily Redston

 www.tulane.edu/.../geol212/2compphasdiag.html Figure 1 shows a common and relatively simple binary phase diagram known as a eutectic phase diagram. A eutectic diagram can be thought of as the intersection of two solid solution diagrams. At the intersection of the two liquidus lines, the melt is in equilibrium with the two solid phases. In other words, a single liquid phase is transformed into two solid phases upon cooling, and the opposite occurs upon heating. This is called a eutectic reaction, and can be written as $L \rightleftharpoons A + B$ According to Gibbs Phase Rule, at this phase transition, we know that the number of degrees of freedom is $F=\left( C+2-P \right)=2+2-3=1$. There is only one degree of freedom, so if we assume constant pressure (a typical choice), then all three phases can only be in equilibrium at an invariant point --- the eutectic point! This means that once we choose the pressure, the eutectic temperature and composition of each phase is fixed, which we can see in Figure 1 at point E.

References

[1] Spaepen, Frans. Applied Physics 282: Solids: Structure and Defects. Harvard University

[2] Haasen, Peter. Physical Metallurgy. Cambridge: Cambridge UP, 1996.

[3] Callister, William D. Materials Science and Engineering: an Introduction. New York: John Wiley & Sons, 2007.