Phase Diagrams 101

Interactive tutorial on reading and interpreting binary phase diagrams. Essential for materials selection and processing.

What a Phase Diagram Shows

A phase diagram maps which phases are stable at different temperatures, compositions, and sometimes pressures. For materials engineers, it is a roadmap for processing and microstructure.

The Axes

In a binary temperature-composition diagram, the x-axis usually shows composition and the y-axis shows temperature. A point on the diagram represents one alloy composition at one temperature.

Key Regions

Single-phase regions show where one phase is stable. Two-phase regions show where two phases coexist. Lines between regions mark phase transformations. The liquidus marks where solidification begins during cooling. The solidus marks where solidification is complete.

Reading a Two-Phase Field

If an alloy lies inside a two-phase region, a horizontal tie line can estimate the composition of each phase. The lever rule can estimate how much of each phase is present.

Why Processing Matters

Phase diagrams describe equilibrium. Real processing may involve cooling rates, diffusion limits, segregation, metastable phases, and non-equilibrium transformations. The diagram is a starting point, not the entire story.

Example Question

If an alloy is cooled from the liquid region into a liquid-plus-solid region, which phase appears first? The answer comes from the boundary crossed first and the composition at that temperature.

Study Checklist

• Identify the alloy composition.
• Locate the temperature.
• Name the phase field.
• If two phases are present, draw a tie line.
• Use the lever rule only when equilibrium assumptions are reasonable.
• Connect the phase field to likely microstructure and properties.