Interactive Gibbs Free Energy and Phase Diagram for the Lead-Tin System

Below is an interactive diagram that demonstrates the concept of Gibbs free energy minimization that is used to calculate the equilibrium state of a chemical system. This diagram describes the binary lead-tin (Pb-Sn) alloy system.

Hover your mouse pointer over the diagram and then turn the mouse wheel forward and backward. You will see how the Gibbs free energy vs. composition plane (the free energy diagram) moves to higher and lower temperatures. The white curve (the minimum G curve) on the free energy diagram indicates the minimum Gibbs free energy of the system as a function of composition at the selected temperature.

The black lines that connect a red dot on the free energy diagram with a red dot on the temperature vs. composition plane (the phase diagram) show how transition points on the minimum G curve map to zero phase fraction lines, or phase boundaries, on the phase diagram.

At low temperatures, the solid FCC and BCT phases have lower G values and are therefore stable, while at high temperatures the liquid phase is more stable.