Phase Diagrams and Phase Stability
Use phase diagrams to identify stable phases, compositions and transformation limits.
A phase diagram is an equilibrium map
It identifies stable phases as functions of variables such as temperature, pressure and composition. It does not directly provide transformation rate, grain size or final mechanical properties.
How to read a binary diagram
Locate the alloy composition and temperature, identify the phase field, then use a tie line to obtain phase compositions and the lever rule to estimate phase fractions. Distinguish the overall alloy composition from the composition of each phase.
Key reactions
Eutectic, eutectoid and peritectic reactions are invariant transformations with characteristic phase changes. Their importance comes from the microstructures they create under real cooling conditions.
Equilibrium versus production
Industrial cooling is finite. Diffusion may be incomplete, creating segregation, metastable phases or retained high-temperature structures. Time–temperature–transformation and continuous-cooling diagrams are needed when kinetics matter.
Takeaway
Use phase diagrams to constrain what can exist. Use kinetics and processing knowledge to predict what will actually form.