Phase Diagrams

Phase diagram

A phase diagram is a graphical representation of the equilibrium phases present in a material system at different temperatures, pressures, and compositions. It is widely used in metallurgy, materials science, and chemistry to understand phase transformations and stability.

1.Major Types of Phase Diagrams

(a) Unary (Single-Component) Phase Diagram

• Shows the phases of a pure substance (e.g., water, iron) under different temperatures and pressures.

• Example: Water phase diagram (solid, liquid, gas).

• Key Features:

  • Triple Point – Where solid, liquid, and gas coexist.

  • Critical Point – Beyond which the liquid and gas phases are indistinguishable.

(b) Binary Phase Diagrams

These diagrams represent two-component (alloy) systems and show phase stability as a function of composition and temperature.

(i) Isomorphous Phase Diagram (Complete Solubility)

• Single-phase solid solution exists across all compositions.

• Example: Cu-Ni system (complete solubility in both solid and liquid states).

• Key Features:

  • Liquidus Line – Above which only liquid exists.

  • Solidus Line – Below which only solid exists.

  • Two-phase Region – Mixture of liquid and solid.

(ii) Eutectic Phase Diagram

• Distinct melting points for different compositions.

• Example: Pb-Sn (Lead-Tin) alloy used in soldering.

• Key Features:

  • Eutectic Point – Lowest melting temperature for the alloy.

  • Eutectic Reaction: L→α+β (Liquid transforms into two solid phases at eutectic temperature). 

(iii) Peritectic Phase Diagram

• A liquid and a solid phase react to form a new solid phase.

• Example: Fe-Ni system.

• Reaction: L+α→β

(iv) Monotectic and Syntectic Systems

• Monotectic: Liquid separates into two immiscible liquid phases.

• Syntectic: Two liquid phases solidify into a single solid phase.

(c) Ternary Phase Diagram (Three-Component Systems)

• Used for ceramics, polymers, and complex alloy systems.

• Requires a 3D representation with three composition axes.

• Example: Al-Ni-Co system.

2.Applications of Phase Diagrams

✅ Metallurgy & Alloy Design – Predicts phase stability and transformation temperatures.
✅ Welding & Casting – Helps in selecting proper alloys for joining.
✅ Heat Treatment – Used in annealing, quenching, and tempering processes.
✅ Ceramics & Polymers – Guides material processing.
✅ Geology & Petrology – Determines mineral stability under Earth’s conditions.

References:

https://www.science.smith.edu/~jbrady/petrology/igrocks-diagrams