Phase Diagrams

A phase is defined as a homogeneous portion of matter. This definition implies that different regions of a phase have the same chemical composition, atomic arrangement, and physical properties. For example, the three phases of water are vapor, liquid, and ice. A similar phenomenon occurs as two metals are mixed in an alloy. Phase diagrams are "equilibrium diagrams" since they show the phases present in a system for a given composition and temperature at equilibrium.

Quantities of bismuth (Bi) and tin (Sn) are measured into crucibles and heated to obtain a homogeneous liquid phase. The crucibles are removed from the furnaces and a thermocouple is inserted in each crucible. The temperature data are recorded and plotted versus time. When solid phases precipitate, the cooling rate changes and there is an inflection on the temperature-time graph. The temperature at the inflection point indicates that a phase boundary exists. Data from a series of tests with different portions of the two metals may be used to construct a phase diagram for the Bi-Sn system.