Effect of magnetism on kinetics of γ–α transformation and pattern formation in iron

The kinetics of polymorphous γ–α transformation in Fe is studied numerically within a model taking into account both the lattice and the magnetic degrees of freedom, based on first-principle calculations of the total energy for different magnetic states. It is shown that a magnetoelastic phenomenon, namely the strong sensitivity of the potential relief along the Bain deformation path to the magnetic state, is crucial for a picture of the transformation. With increasing temperature, a scenario for the phase transformation evolves from a homogeneous lattice instability at T < Ms (Ms is the temperature of the beginning of the martensitic transformation) to the growth and nucleation of embryos of the new phase at T > Ms. In the latter case, the formation of a tweed-like structure occurs, with a strong short-range order and slow interphase fluctuations. (paper)