What is described by the solidus line in a thermal equilibrium diagram?

The solidus line in a thermal equilibrium diagram, also known as the phase diagram, represents the boundary that separates solid and liquid + solid phases. This line indicates the temperature and composition conditions below which a material exists solely in a solid state, and above which it begins to melt and exist as a mixture of solid and liquid phases.

At temperatures below the solidus line, the material is completely solid, and no liquid is present. As the temperature increases and crosses this line, the material begins to transition into the liquid phase. Understanding the solidus line is crucial in processes where heat treatment or material processing is involved, as it helps indicate the conditions under which a solid can start to melt and form a liquid phase, which is essential for applications like welding and casting.

The other options relate to different concepts in the phase diagrams. The boundary between liquid and vapor phases, for example, pertains to the vaporization process, while the area where only liquid phase is present differs from the solidus line which deals specifically with solid and liquid interactions. The zone where both solid and liquid phases exist is typically indicated by a range rather than a singular line, underscoring the unique role of the solidus line in defining specific phase boundaries.