Fasestabilitet

Fasestabilitet, or phase stability, refers to the tendency of a particular phase to persist as the dominant form under specified conditions of temperature, pressure, and composition. It is a fundamental concept in chemistry, materials science, geology, and physical metallurgy, and it governs which phases are present in a system at equilibrium or over given timescales.

Thermodynamically, stability is determined by the Gibbs free energy. At a fixed temperature and pressure, the phase with the lowest Gibbs free energy is thermodynamically stable. When multiple phases can exist, phase diagrams illustrate which phases or combinations are stable under different conditions. In materials science, convex-hull constructions help identify stable phases and their mixtures. Metastable states occur when a phase is not the lowest-energy form but is kinetically trapped, so it can persist for practical times despite being less stable thermodynamically.

Kinetics also play a crucial role. The rate of transformations, diffusion, and nucleation can prevent a system from reaching thermodynamic equilibrium, so metastable or amorphous phases may be observed even though a more stable phase exists. This distinction between thermodynamic stability and kinetic accessibility is central to understanding real-world materials behavior.

Examples illustrate the concept. Water has solid, liquid, and gaseous phases, with phase stability depending on temperature and pressure. In alloys, the iron–carbon system exhibits distinct stable phases such as ferrite, cementite, and austenite, while different carbon allotropes (diamond vs graphite) reflect polymorphic stability that depends on pressure. Fasestabilitet thus underpins material design, processing, and interpretation of phase diagrams, enabling prediction of phase formation and transformation.