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

Abstract

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.