Phase transition

Definition

A phase transition is a metallurgical transformation of the material produced by a modification of a particular exterior parameter (such as temperature, magnetic field…).

Such a transformation can be:

• a modification of the state of the material:
  • the transitions between the solid, liquid and gaseous phases

• a modification of the crystalline microstructure (see the example of iron below)
• a modification of the magnetic behavior:
  • the transition from a ferromagnetic to the paramagnetic behavior of magnetic materials at the Curie point.

• a modification of the electric behavior
  • certain ceramics become superconducting below a critical temperature

• …

Addition

The notion of order of a phase transition.

• First-order phase transitions are those that involve a latent heat. During such a transition, the system either absorbs, or releases a fixed quantity of energy.
• Second-order phase transitions, also called “continuous phase” transitions, which are not associated with latent heat.

Example of an iron body

Let us look at the dependence on temperature of the heat capacity of an iron body in the figure below.

This figure demonstrates:

• a second-order phase transition:
  • T= 1033 K: ferromagnetic iron / paramagnetic iron transformation (at the Curie point)

• first-order phase transitions:
  • T= 1183 K: iron α (cc) / iron γ (cfc) transformation
  • T= 1673 K: iron γ (cfc) / iron α (cc) transformation
  • T= 1812 K: iron α (cc) / liquid iron transformation

Dependence of the heat capacity of an iron body on temperature: