Kinetic effects of magnetic field on the $γ/α$ interface controlled reaction in iron

The effect of magnetic field on the austenite $(γ)$ to ferrite $(α)$ transformation kinetics is studied in iron by means of dilatation measurements under magnetic field. In the frame of the Johnson–Mehl–Avrami-Kolmogorov analysis and considering a spherical growth mode for the ferrite grains, the Avrami exponent and the activation energy for the interface mobility are calculated. These parameters are found to be weakly influenced by the application of a 16 T magnetic field. This indicates that they are related mainly to the total amount of energy available for the transformation whatever its origin (whether magnetic or chemical). Whereas the magnetic field strongly affects the $α/γ$ thermodynamic equilibrium through the Gibbs free energy of phases, it has a non noticeable influence on the transformation kinetics itself.