liquidus

Liquidus is a term used in phase diagrams and materials science to describe the boundary that separates the all-liquid region from the solid–liquid mixture region for a given composition. In a binary alloy diagram, the liquidus is the upper boundary: above it the material is completely liquid; below it there can be a mixture of solid and liquid until the temperature reaches the solidus, below which the material is entirely solid.

For pure elements, the solidus and the liquidus coincide at the melting point, resulting in a single melting temperature. In alloys, there is a melting range between the solidus and the liquidus, reflecting partial melting as the material is heated and partial solidification as it cools. The liquidus temperature at a given composition is the temperature at which the last solid has melted on heating; the solidus marks the temperature at which melting begins on heating.

The liquidus curve is an essential feature of phase diagrams and is used to predict solidification behavior, microstructure, and processing windows. It influences casting and welding performance, alloy design, and heat-treatment strategies because it helps determine how much of the material will be liquid at a given temperature and composition. The liquidus is often determined experimentally by calorimetry (such as DSC or DTA), thermal analysis, or in situ observation, and is complemented by equilibrium data that define the overall phase relationships, including eutectic and peritectic reactions. In geology, the term similarly describes temperatures above which rocks or minerals become fully molten.