Analytic And Concepts

Since the early foundations of phase diagrams scientists and engineers have found them extremely helpful indescribing the various forms a certain material can take. By examining a phase diagram one can gain some understanding into the materails mechanical properties.

Microstrusture

Interpretation of phase digrams

When a binary system deviates from the behavior of ideal solutions, the resulting phase diagram looks like the one shown above. First note that it has all the same regions as a two-phase diagram, namely the alpha, beta, and liquid regions. In addition, there are the alpha+L, beta+L, and alpha+beta regions, resulting from the limited solubility of the two components. The most important feature of this kind of system is the invariant point, where all three phases are in equilibrium. That this is indeed a point can be verified with the Gibbs phase rule. The composition at which the invariant point appears is called the eutectic composition. The corresponding temperature is refered to as the eutectic temperature. Reactions at this composition are called eutectic reactions (eutectic means easily melted).

Several different types of microstructures arise in a eutectic system. These depend on the composition in question. The simplest case is when the maximum solubility of a compoment at room temperature has not been exceeded. In this case, upon cooling, the resulting microstructure consists simply of grains of the single phase.

The second case is when the composition exceeds the solubility limit but does not reach the solubility limit at the eutectic temperature. In this case, upon cooling, the same microstructure as in the previous casse results. However, when the temperature reaches the solvus line, the solubility of the alpha phase is exceeded and small grains of the beta phase will start to appear.

So far, we have considered diagrams with only two solid phases. Such cases are termed terminal solid solutions. Howver, other alloy systems often involve intermediate phases, and such systems are named intermediate solid solutions. One example is the ---Cu-Zn--- system, which contains five peritectic points.

For some sytems, instead of an intermediate phase, an intermetallic compound of specific composition forms. One example is the Mg2Pb compound that occurs at 19%wt Mg and 81%wt Pb. These compound have to be represented on the phase diagram as a vertical line, since the composition is a specific value.

Another interesting reaction that occurs for some systems is the peritectic reaction. This involves one solid phase transforming into another phase plus a liquid phase upon heating.

Conclusion

Terms