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).

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