tvåfasregion

A tvåfasregion, in thermodynamics and materials science, is a region in a phase diagram where two distinct phases are in thermodynamic equilibrium. In this region, a system with a given temperature, pressure and composition would not remain a single homogeneous phase, but would separate into two coexisting phases with different compositions and properties. The overall composition is a weighted average of the two phase compositions.

In binary systems, phase diagrams plotted as temperature versus composition show two-phase regions bounded by phase boundaries (the binodals). Within these regions, any state point corresponds to two phases whose compositions lie at the ends of a tie line through the overall composition. The fractions of the two phases are given by the lever rule, based on how far the overall composition is from each phase composition.

Common examples include water–ice–vapor regions in honest phase diagrams of pure substances, such as solid–liquid coexistence below the melting point at a given pressure, or liquid–vapor coexistence along the vaporization curve. In alloy systems, two-phase regions occur between the solidus and liquidus lines, where mixtures of two solid phases or a solid and a liquid can be in equilibrium; eutectic and peritectic reactions are typical features in such diagrams.

The concept highlights that the properties of a material in a two-phase region depend on the properties and proportions of both phases, not solely on the overall composition. The two-phase region is bounded by phase boundaries, and crossing these boundaries changes the system to a single phase or to a different two-phase region.