Solvus

A solvus is a boundary on a phase diagram that marks the limit of solubility of one component in another within a solid solution. It is the locus of temperature and composition at which a single solid phase becomes unstable and separates into two distinct solid phases. In binary systems, the solvus separates the single-phase region from the two-phase region and represents the maximum amount of solute that can dissolve in the solvent at a given temperature.

The solvus line typically curves downward as temperature decreases, reflecting the general tendency for solid solubility to decrease with cooling in many alloy systems. Above the solvus, the solid solution exists as a homogeneous phase; below the solvus, the system comprises two solid phases with different compositions in equilibrium, connected by a tie line.

In metallurgy and materials science, the solvus informs heat-treatment strategies and precipitation behavior. For example, in aluminum–silicon alloys, silicon solubility in aluminum is higher at elevated temperatures and decreases as temperature drops, leading to precipitation of silicon-rich phases upon cooling. In mineralogy, solvus boundaries describe exsolution processes where a fully soluble solid solution becomes separated into distinct minerals or phases as conditions change.

Overall, the solvus is a fundamental concept for understanding solid-state phase equilibria, complementing the liquidus and solidus boundaries on phase diagrams and guiding the prediction of microstructures and properties.