Introduction to Phase Diagrams

A diagram representing the limits of  stability of the various phases in a chemical system at equilibrium, with respect to variables such as pressure, temperature and composition, is known as phase diagram.

Before we start this topic be sure that you have clear understanding about the following terms, if not then please refer them before moving ahead.
Do not ignore any one of them, since they are the guides to understand phase diagrams.

Basic Chemical Terms ( Click here to read )
  • Elements, 
  • Compounds,
  • Substances,
  • Mixtures,
  • System,
  • Component,
Solid Solutions ( Click here to read )
  • Solid solutions,
  • Hume-Rothery rules
  • Solubility,

  • Matter,
  • State of matter,
  • Phases
  • Phase equillibria


Polymorphism = Elements/Compounds have definite crystalline structure under equilibrium conditions.
As we know structure differs from materials to materials. But for some elements/compounds, their crystal structure transforms from one to another with changes in temperature.

This behavior is known as polymorphism and those elements/compounds who show these behavior are known as polymorphic substances.
The term allotropy is used to describe the polymorphic changes.

Metastable Phases =  Under some conditions metastable crystal structures can be formed instead of stable structure. This happens due to rapid cooling but some are formed on slow cooling, like Fe3C.
Metastable phase means it requires additional energy to reach complete/true stability.



Phase Diagrams

In order to record and visualize the results of studying the effects of state
variables(temperature,pressure,composition) on a system, diagrams were introduced to show the
relationships between the various phases that appear within the system under equilibrium conditions.

Those diagrams are called constitutional diagrams, equilibrium diagrams, or phase diagrams.

Requirement & applications of phase diagrams,
  • Development of new alloys for specific applications,
  • Fabrication of these alloys into useful configurations,
  • Design and control of heat treatment procedures for specific alloys that will produce the required mechanical, physical, and chemical properties
  • Solving problems that arise with specific alloys in their performance in commercial applications, thus improving product predictability.
Classification of phase diagrams depends upon,
  • Number of Components present ( Unary, Binary, Ternary, Quaternary and Multinary)
  • Number of Phases present ( One phase, Two phase, Three or Multi-phase )
  • Type of reaction occurring( Isomorphic, Eutectic, Eutectoid, Peritectic, etc )
Variables of Phase Diagrams

Depending upon requirement the axis can be either of three
  • Thermodynamic ( P, T, V )
  • Composition variables ( C, wt% )
  • Kinetic ( t )
- In single component system the usual variable is T & P.
- In phase diagrams used in material science the usual variables are T & wt%.
- In the study of phase transformation kinetics, TTT - Time Temperature Transformation diagrams or CCT - Continuous Cooling Transformation diagrams the usual variables are T & t.
Note : Phase diagrams are plotted by help of cooling/heating curves of substances at definite composition.

1) Unary Phase Diagrams ( One component system )


Eg. Water-( H2O ) system is a typical example of unary phase diagrams.
Water has three stable phases ( ice, water & vapor ) depending upon temperature & pressure.

We can not take composition into consideration here since it's a one component system.

Note : In unary phase diagrams we consider temperature and pressure as variables. Thus it is a T-P equilibrium diagram.



 2) Binary Phase Diagrams ( Two component system )

Two components can be either of three possibilities,

  • Metal-Metal system
  • Metal-Compound system
  • Compound-Compound system
Binary phase diagrams are classified according to the solubility of components in both solid and liquid state.

- Complete solubility in both solid & liquid state
  • Isomorphous system
- Complete solubility in liquid sate but limited solubility in solid state
  • Eutectic system ( liquid analogue )
  • Peritectic system ( liquid analogue )
  • Eutectoid system ( solid analogue )
  • Peritectoid system ( solid analogue )
- Limited solubility in both solid & liquid state
  • Monotectic system ( liquid analogue )
  • Syntectic system ( liquid analogue )
  • Monotectoid system ( solid analogue )
Note : Pressure changes often have little effect on the equilibrium of solid phases (unless of course we apply ‘huge’ pressures). Hence, binary phase diagrams are usually drawn at 1 atm pressure.
We draw binary phase diagram in terms of temperature and wright % composition.



Eg. Cu-Ni system is a typical example of isomorphous binary phase diagram.

Here, there are two components namely copper-Cu and nickel-Ni.

Both are completely soluble in each other and thus forms single solid solution ( alloy called Cupronickel )

No. of phases = 2
  • α solid-solution
  • L
No. of phase fields = 3
  • α solid-solution
  • α + L
  • L

3) Ternary phase diagrams ( Three component system)

Ternary phase diagrams represents the phase behavior of mixture containing three components in a triangular diagram.

Such diagrams are drawn based on the property of equilateral triangles that the sum of the perpendicular distance from any point to each side of the diagram is constant equal to the length of any of the sides.

Eg. Stainless-Steel is a typical example of ternary phase diagram.

Here, there are three components namely iron-Fe, chromium-Cr and nickel-Ni.

In some region we can observe complete solid solubility wheres in some we see limited solubility.