Energetics of the fcc (γ) -bcc (α) lattice transformation by the Bain tetragonal deformation is calculated for both magnetically ordered and paramagnetic (disordered local moment) states of iron. The first-principle computational results manifest a relevance of the magnetic order in a scenario of the γ -α transition and reveal a special role of the Curie temperature of α-Fe, TC , where a character of the transformation is changed. At a cooling down to the temperatures T < TC one can expect that the transformation is developed as a lattice instability whereas for T > TC it follows a standard mechanism of creation and growth of an embryo of the new phase. It explains a closeness of TC to the temperature of start of the martensitic transformation, Ms.PACS numbers: 61.50. Ks, 64.70.kd, 75.30.Et, 75.50.Bb, 71.20.Be Deeper understanding of mechanisms of polymorphous γ -α transformation in iron and its alloys is of fundamental importance for both metallurgical technologies [1,2] and for a general theory of phase transitions in solids [3,4,5]. Despite numerous investigations an issue of a mechanism of a new phase nucleation in the course of γ -α transformation remains open (see, e.g., discussion in Ref. 4).

Diversity of mesostructures formed in steel at cooling from high temperature austenite (γ) phase is determined by interplay of shear reconstructions of crystal lattice and diffusion of carbon. Combining first-principle calculations with large-scale phase-field simulations we demonstrate a decisive role of magnetic degrees of freedom in the formation of energy relief along the Bain path of γ-α transformation and, thus, in this interplay. We show that there is the main factor, namely, magnetic state of iron and its evolution with temperature which controls the change in character of the transformation. Based on the computational results we propose a simple model which reproduces, in a good agreement with experiment, the most important curves of the phase transformation in Fe-C, namely, the lines relevant to a start of ferrite, bainite, and martensite transformations. Phase field simulations within the model describe qualitatively typical patterns at these transformations.

We describe our optical and electron-microscopy observations of pearlite structures in eutectoid steels which seem to imply that the mechanisms of formation of pearlite colonies in these steels differ from those observed earlier for non-eutectoid steels. A simple theoretical model to study kinetics of pearlite transformations is suggested. Simulations of growth of pearlite colonies based on this model reveal that for the volume carbon diffusion mechanism usually-supposed such growth is always unstable, and the steady-state growth can be realized only via the interfacial carbon diffusion mechanism. A model of formation of pearlite colonies based on the assumption of a strong enhancement of carbon diffusion near grain boundaries is also suggested. The model can be applicable to the plastically deformed steels, and the results of simulations based on this model qualitatively agree with some microstructural features of formation of pearlite colonies observed in such steels.

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