Degenerate Structure of Transformation Twins and Estimation of Dislocation Density in Martensite Crystals

In the dynamic theory of martensitic transformations, the wave mechanism of controlling martensite crystal growth is determined by the superposition of wave beams of quasi-longitudinal (or longitudinal) waves carrying the "tensile-compression" deformation in the orthogonal directions. The wave beam formation is considered to be a result of the formation of excited (vibrational) states. The existence of transformation twins is interpreted as a result of a matched propagation with respect to long-wave (l waves) and short-wave (s waves) shifts. The matching condition is analyzed for the gamma-alpha martensitic transformation in iron-base alloys. It is shown for the first time that the transition to a degenerate twin structure with the allowance for the medium discreteness enables one to estimate the dislocation density in crystals with habit {557}, which agrees with that observed experimentally

Promising Versions of alpha-Martensite Rod-Like Crystal Initiation in Iron Alloys by Three Elastic Wave Sources

Versions are provided for alpha-martensite rod-shaped crystal initiation by three orthogonal longitudinal elastic waves propagating in orthogonal directions of the types aOE (c) 001 >(gamma) and aOE (c) 110 >(gamma) in Fe - 31% Ni single crystals. The probability is considered of direct initiation by Bain-type deformation waves. It is demonstrated within the scope of dynamic theory that the anticipated orientation of rod-like crystals is close to aOE (c) 111 >(gamma) or to aOE (c) 905 >(gamma). Possible features of martensite crystal tetragonality are discussed. Ultrasound source power required for initiating martensitic transformation is determined.Work was carried out with financial support of the RFFI (project number 14-08-00734)

The formation of martensite crystals with a degenerate structure of transformation twins

The dynamic theory of martensitic transformation explains the phenomenon of initiation of the fine structure of transformation twins as a result of the coordinated action of relatively long-wave displacements (ℓ-waves) with more short-wave displacements (s-waves). Being a part of a controlling wave process, ℓ-waves provide habit formation, whereas s-waves play the leading part in initiation of the main component of twin structure (TS). It was shown that the dynamic theory allows one to consider the degenerate TS (DTS) formation as a particular case of TS when the twin component volume is converted to zero. In this work the case of DTS is discussed by the example of crystals with habits {110}. The peculiarity of this variant consists in the fact that in order to describe the morphology of transformation it is enough to only consider longitudinal waves running along axes <100> as a part of the controlling wave process. In particular, habit (101) may be matched with a pair of ℓ-waves with velocities along [100] and [001] and a pair of s-waves with velocities along [100] and [010]. At the same time, condition d s = λ s / 4, where λ s is the wavelength of s-waves, and d s is a transversal (in directions [100] and [010]) size of the initial exited (oscillatory) s-ℓ-cell with longitudinal size d s << d ℓ < λ ℓ / 2, conforms to DTS formation. For martensite transformations fcc-bct, bcc-fct, fcc-fct, the transition to finishing deformations and the connection of the values of tetragonality of martensite and volume effect with one of characteristic main values of deformation tensor are discussed. © 2018, Institute for Metals Superplasticity Problems of Russian Academy of Sciences. All rights reserved

The formation of martensite crystals with a degenerate structure of transformation twins

The dynamic theory of martensitic transformation explains the phenomenon of initiation of the fine structure of transformation twins as a result of the coordinated action of relatively long-wave displacements (ℓ-waves) with more short-wave displacements (s-waves). Being a part of a controlling wave process, ℓ-waves provide habit formation, whereas s-waves play the leading part in initiation of the main component of twin structure (TS). It was shown that the dynamic theory allows one to consider the degenerate TS (DTS) formation as a particular case of TS when the twin component volume is converted to zero. In this work the case of DTS is discussed by the example of crystals with habits {110}. The peculiarity of this variant consists in the fact that in order to describe the morphology of transformation it is enough to only consider longitudinal waves running along axes <100> as a part of the controlling wave process. In particular, habit (101) may be matched with a pair of ℓ-waves with velocities along [100] and [001] and a pair of s-waves with velocities along [100] and [010]. At the same time, condition d s = λ s / 4, where λ s is the wavelength of s-waves, and d s is a transversal (in directions [100] and [010]) size of the initial exited (oscillatory) s-ℓ-cell with longitudinal size d s << d ℓ < λ ℓ / 2, conforms to DTS formation. For martensite transformations fcc-bct, bcc-fct, fcc-fct, the transition to finishing deformations and the connection of the values of tetragonality of martensite and volume effect with one of characteristic main values of deformation tensor are discussed. © 2018, Institute for Metals Superplasticity Problems of Russian Academy of Sciences. All rights reserved

The degenerate structure of transformation twins and the monocrystallinity of part of the thin-plate martensite initiated by a strong magnetic field

In the dynamic theory, the formation of twin martensite crystals is the result of a coordinated propagation of relatively long-wave (l -waves) and short-wave (s-waves) displacements. The matching condition is analyzed for the γ–α martensitic transformation in iron-based alloys, taking into account the quasi-longitudinalness of the l -wave carrying compression deformation. It has been shown for the first time that the previously established single-crystal effect of part of the crystals of thin-plate martensite, which arises upon cooling under the action of a strong magnetic field, can naturally be interpreted as a consequence of the formation of a degenerate structure of transformation twins. © 2020, Allerton Press, Inc

Dynamic Scenarios of the Formation of Martensite with the {110} Habits in the Ni50Mn50 Alloy

Martensitic transformation B2–L10 in the ordered alloy Ni50Mn50, which occurs at comparatively high temperatures (980–920 K), is discussed with the use of dynamic concepts of the wave control of the threshold deformation. The proximity of the observed orientations of martensite-crystal habits (and of twin boundaries) to the planes of the {110} family makes it possible to use the longitudinal waves along the axes 〈001〉 (in the basis of the initial phase) as the driving factors. It is shown that at temperatures of the onset of the transformation there is a satisfactory correspondence between the calculated and experimental data on the tetragonality of martensite and on the volume effect. The opportunity of different dynamic scenarios of the formation of the final phase is noted, namely, of separate crystals; layered structures, in which the crystals of martensite with the identical orientation relationships alternate with the untransformed regions of austenite; and packets of pairwise-twinned crystals. Examples are given of morpho-types corresponding to these scenarios. © 2019, Pleiades Publishing, Ltd