When graphical diagrams deals with phases which are in equilibrium with the surroundings it is called an equilibrium diagram, otherwise it is called a phase diagram. eg: Fe – Fe3C system is phase diagram (or meta stable), Fe – graphite is an equilibrium diagram.
Gibbs phase rule: The changes in the number of phases in alloy under equilibrium conditions are expressed by the Gibbs phase rule F = C – P + 2 ; where F – degrees of freedom, C – number of components, P – number of phases in system. The minimum value for F is zero and this sets an upper limit on the number of phases that can be exist in a system under equilibrium.
Unary diagrams These are for single component systems and thus there is no composition variable. The variables are pressure and temperature. Temperature on ordinate (Y – axis ) and pressure on abscissa (X – axis). In single phase region both temperature and pressure can be varied independently. In two phase region either pressure or temperature can be varied independently. Both can’t be varied simultaneously. Three phase equilibrium exists only for fixed value of pressure and temperature.
Binary diagrams These depict the equilibrium between two components. Two components can be mixed in an infinite number of different proportions, that is composition also becomes a variable. These are drawn usually at one atmospheric pressure i.e. pressure is made constant, because the variation in pressure results in insignificant effect on the equilibrium. Hence F = C – P + 1 : phase rule for condensed phases. Temperature in degrees or Fahrenheit as ordinate ((Y – axis ) and composition in weight or atom percentage as the abscissa (X – axis)
Most common Binary diagrams types
1. Two components completely soluble in the liquid state
a) completely soluble in solid state (Isomorphous)
b) completely insoluble in solid state (Eutectic)
c) Partially soluble in solid state (Eutectic)
d) Peritectic reaction
e) Formation of intermediate phases (congruent and incongruent)
2. Partially soluble in liquid state (Monotectic)
3. Components insoluble in liquid as well in solid state
4. Transformations in the solid state
a) Allotropic change
b) Order – disorder
c) Eutectoid
d) Peritectoid
Methods for determination of phase diagrams
1. Thermal Analysis 2. Dilatometry 3. Metallographic methods 4. X-ray diffraction 5. Electrical – resistivity methods.