Note on phases and phase diagrams- one component system

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Phase diagram

To study the mechanical properties of solid as well as the microstructure of of solid we must understand the phase diagram for the mixture or alloy.

  1. We must know the terminology associated with phase diagram and phase diagram.
  2. Pressure-temperature phase diagram
  3. Interpretation of binary phase diagram
  4. Example of binary phase diagram (Iron-carbon)
  5. Development of microstructure

Definition and basis of concept of certain terms

  1. Alloy: It is the mixture of two or more elements (especially solids, metals).
  2. Solvent: Those elements which have large concentration in alloy or mixture is known as solvent.
  3. Solute: Those elements which have smaller concentration in alloy of mixture is known as solute.
  4. System: Alloy or mixtures which are taken under consideration called system.
  5. Solubility limit: For many alloy system and at specific temperature there is a maximum concentration of solute atoms that may dissolve in solvent to form a solid solution, this is called solubility limit.

solubility limit

The solubility limit that can be expressed in terms of weight percent or atom percent in solution. For example; in binary alloy if solubility limit of Nickel in copper is 31.5 wt % then concentration of copper in the solution is 68.5 wt %.

When sugar is added to water then a sugar–water solution forms. when more sugar is added to given water, the solution becomes more concentrated, until the solution becomes saturated with sugar. At this time the solution is not capable of dissolving any more sugar, and further additions simply settle to the bottom of the container. Thus, the system now consists of two separate substances: a sugar–water syrup liquid solution and solid crystals of undissolved sugar.


It is defined as the homogenous portion of system that has same physical and chemical characteristics. Every pure material is considered to be a phase. For example; copper rod, ice, water in a glass, a balloon filled with gas are all considered as phase. The homogenous solution of salt in water is also considered as phase. The mixture of polymeric form (having) both FCC and BCC structure of two metals having separate structure is considered to be mixture of two phase.

When there is present more than one phase is present in a given system, each will have its own distinct properties, and a boundary separating the phases will exist across which there will be abrupt change in physical or chemical characteristics. When two phases are present in a system, it is not necessary that there be a difference in both physical and chemical properties; there is dissimilar in one or the other set of properties is sufficient. When water and ice are present in a container, two separate phases exist; they are physically dissimilar (one is a solid, the other is a liquid) but identical in chemical composition.


In metals, alloy microstructure is related to the number of phase present their properties and the manner in which they are distributed or arranged. The microstructure depends upon the types of element present, concentration of element temperature, pressure and the process of formation.

The microstructure of an alloy depends on such variables as the alloying elements present, their concentrations, and the heat treatment of the alloy (i.e., the temperature, the heating time at temperature, and the rate of cooling to room temperature).

Equilibrium and metastable state:

fig: Meta stable state
fig: Meta stable state

A system is at equilibrium if at constant temperature, pressure and composition of system is stable not changing with time. Equilibrium atate is achieved after long time. The system may attain number of equilibrium state which appears to be stable but not actually stable. The state which is not minimum state is known as metastable state.

In thermodynamic sense equilibrium is describe as a state of system that corresponding to minimum free energy of system as shown in figure. The change is always towards equilibrium state. The state of system may be trapped in local minimum free energy which is not stable. The substance attains equilibrium state after it reach global minimum in free energy.

Phase diagram:

Phase diagram is a graphical representation of all the equilibrium phases as a function of temperature , pressure and composition. For one component system the equilibrium of a state is defined by two independent parameters,

  1. Pressure and temperature
  2. Pressure and volume
  3. Temperature and volume


Callister, W.D and D.G Rethwisch. Material Science and Engineering. 2nd. New Delhi: Wiley India, 2014.

Lindsay, S.M. Introduction of Nanoscience . New York : Oxford University Press, 2010.

Patton, W.J. Materials in industry . New Delhi : Prentice hall of India, 1975.

Poole, C.P. and F.J. Owens. Introduction To Nanotechnology. New Delhi: Wiley India , 2006.

Raghavan, V. Material Science and Engineering. 4th . New Delhi: Pretence-Hall of India, 2003.

Tiley, R.J.D. Understanding solids: The science of Materials. Engalnd : John wiley & Sons , 2004.


  • We must know the terminology associated with phase diagram and phase diagram.
  • Pressure-temperature phase diagram
  • Interpretation of binary phase diagram
  • Example of binary phase diagram (Iron-carbon)
  • Development of microstructure



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