Solidification Thermodynamics of Binary Alloys

The stable state of a binary system under constant temperature and pressure is the one with the lowest Gibbs free energy. Phase stability can be determined by understanding the fluctuations of Gibbs free energy in different possible phases with composition and temperature. A liquid solution cools to a liquidus temperature, causing a solid phase to separate from it. This solid phase can be an almost pure component, a solid solution with similar or different composition, or a chemical complex formed by the reaction of multiple components. The composition of the solid phase in equilibrium with the liquid solution minimizes the system's Gibbs free energy. If liquid solutions are stable throughout the composition range, the Gibbs free energy of the liquid states is less than any solid-state phase. Conversely, the Gibbs free energy of solid states is always lower than liquid-state phases.