A γ−α-martensitic transformation during the fastest rearrangement of the {110}γ planes

Abstract

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

Estimation of the effective growth rate of a plate of bainitic ferrite in dynamic theory

“…As was shown by careful measurements [11], ORs for the lath martensite are close to Nishiyama's values, which can naturally be explained in terms of the dynamic approach [12]. The habit planes of the laths are close to { 57} γ , where the tilde sign marks the index that, of the two close Miller indices, is arbitrary greater.…”

Morphological features in B2→B19 martensitic transformation and their description based on the concept of controlling wave process

“…This paper discusses the possibilities of applying the dynamic approach to the description of macroscopic morphological features in the B2→B19 transformation, using data for TiNiCu alloys. [3,4] and γ→α transformation [2]. It can thus be expected that the earlier made conclusions would, to a large extent, hold true for the B2→B19 transformation, e.g., in the TiNiCu system.…”

“…Today much progress has been achieved in the dynamic theory of reconstructive martensitic transformations, such as the fccbcc (γ→α) transformations in iron alloys [1,2] and bcchcp (α→ε) transformations 1 [3,4] in titanium. The core of the new paradigm that supplements the conventional scheme of equilibrium thermodynamics is to decode the dynamic structure of the excited lattice state in the nonequilibrium region of the nonlinear transformation wave front.…”

Formation of martensite crystals in the limiting case of a supersonic growth rate

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