During marensitic transformations (MP) it is reasonable to identify crystals pointing out their morphological features related to each other due to the action of a single governing process. In a dynamic approach for the first time used to describe γ-α MPs in iron alloys [1, 2], a description of habitus planes (HP) seems most convenient and illustrative.Martensite is developed as plates with a low thickness-to-linear dimensions ratio and as lens-like crystallites, in whose central part (midrib) of the plate formed in the first stage of the MP is well defined. The plate thickness varies within 10 -7 −10 -6 m. The HP (phase boundary or midrib plane boundaries) possesses several (depending on composition) stable orientations. In the Fe-С and Fе-Ni systems, there exist habitus planes close to {557} γ (up to 0.6 wt. % С, up to 29% Ni),In a dynamic interpretation [1, 2], the habitus is "swept" by a moving line of intersection of overlapping fronts of wave beams travelling in the orthogonal direction and carrying plane deformation of the tension (ε 1 > 0)compression (ε 2 < 0) type, which possesses invariant planes (strain value ε 3 ≈ 0). It is critical that at low strain values (threshold strain values ε 1th , │ε 2th │ less than the elastic limit e ε ~ (10 ─4 -10 ─3 ) << 1) their ratio is close to that of the squared wave beam velocitieswhere the velocities v 2 and v 1 can be calculated using the Christoffel equation [3]. Investigation of bcc-hcp (α-ε)transformations [4,5] demonstrated that the ratio of strains, prescribed in the threshold mode, is retained during the development of finite deformations in the lattice that lost its stability. It is important that during an α−ε-transformation, plane deformation ensures the fastest transformation of the {110} α planes in {0001} ε , which is followed by plane shuffling. The symmetry of atomic arrangement gives rise to a relationship between tensile and compressive strains, which in combination with (1) provides finite values of the two transformation deformations. The deformation along the direction orthogonal to the transformed plane is induced by electronic correlations that set a new symmetry of the electron density distribution [6]. These conclusions provide for a possibility of a new [7,8] description of a γ-α MP due to a fast rearrangement of the {111} γ planes. Note that in [7,8] there is an inaccuracy: in the calculations of HP the velocity along <112> γ is substituted for the velocity along <111> γ . As a result, instead of the habitus close to {5 13 18} γ , they obtained {3 7 10} γ separated from {5 13 18} γ by 1.4º. Formation of crystals with the habitus {557} γ with the fastest rearrangement of the {1 1 0} γ planes. According to [1,2], the habitus {557} γ refers to the pairs of quasi-longitudinal waves with the wave vectors in the vicinity of the orthogonal axes of symmetry <110> γ , <001> γ . The wave velocity ratio is expressed via the elastic moduli C 11 , C 12 , C 44 of the γ-phase

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536.421.1 In terms of the concepts of heterogeneous nucleation and the related driving wave process, a version of the dynamic theory of the formation of martensite crystals is stated in which the wave process initiates the fastest transformation of close-packed {111} γ atomic planes of a parent fcc phase into {110} α planes of a bcc phase. The lattice parameter ratio and the orientations of habit planes are analytically related with the elastic properties of the γ -phase. Quantitative estimation performed with the use of elastic moduli for an iron-nickel alloy yields habitus orientations close to {10 3 7} γ . An experiment is proposed to verify the theoretical predictions. A new pattern of the short-wave correction that finishes the fcc-bcc rearrangement is set forth.The growth of crystals in the (γ-α) fcc-bcc martensitic transformation is a fast process with strongly pronounced indications of the first-order transition. The nucleation is heterogeneous and the growth is controlled by a driving wave process (DWP). Recall [1, 2] that a DWP which forms a plate-shaped region bounded with habit planes (HP's), carries nearly plane and nearly homogeneous threshold deformation (of the tension-compression type) on the mesoscopic scale (of the order of the crystal thickness). As a result, the HP appears to be an invariant (or slightly distorted) plane. The wave normals n 1 and n 2 of the wave beams that describe, respectively, tensile deformation (ε 1 > 0) and compressive deformation (ε 2 < 0) in the superposition region are collinear to the eigenvectors ξ i (i = 1, 2) of the deformation tensor of the elastic field of a defect in the nucleation region (DWP inherits the information on the directions of ξ i ): n 1 || ξ 1 , n 2 || ξ 2 , n 1 ⊥ n 2 | n i | = | ξ i | = ||ξ⊥|||ξ|1.(1)These processes are sketched in Fig. 1. It can easily be shown [1−3] that the normal N w to a habitus plane that is related to the propagation of a driving wave is set by the kinematic relation N w || n 2 ± n 1 ae, ae = 2where v 1 and v 2 are the moduluses of the wave velocity. On the other hand, for a tension-compression deformation the normals to invariant planes are given by

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