Which zone in a thermal equilibrium diagram signifies the presence of only liquid?

The presence of only liquid in a thermal equilibrium diagram is indicated by the liquidus line. This line defines the boundary between the liquid phase and the solid + liquid phase. When the temperature is above the liquidus line, the material is completely in the liquid state.

As the temperature decreases and approaches the liquidus line, some solid may begin to form, marking the transition to the solid + liquid phase. Therefore, the area above the liquidus line is where only liquid exists, while below it, there will be a mixture of solid and liquid or just solid, depending on the specific point in the diagram. This understanding is crucial for analyzing phase changes in materials during heating and cooling processes, which plays a vital role in engineering practices involving materials development and processing.