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 shortwave 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, fccfct, 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.

Nucleation centers for martensite with habitus {110} in the shape memory alloys

“…In the case of crystals with (110) habits, it is easy to associate the tetragonality of t martensite to one of the main values of the Bain strain tensor. Although we discussed this issue in [9,10], it is useful for the convenience of the reader to recall the arguments presented.…”

“…This option corresponds to the simplest version of inheriting the characteristics of the elastic field of the NC, in which the directions of the wave normals of the two ℓ-longitudinal waves responsible for describing the habit coincide with the orientations of the eigenvectors of the deformation tensor of the elastic field of the NC. It is the high symmetry of the habit planes that made it possible earlier in [9,10] to immediately use purely longitudinal waves in the analysis.…”

Вырожденная Структура Двойников Превращения И Оценка Плотности Дислокаций Мартенситных Кристаллов

“…Одним из принципиальных итогов такого анализа является вывод о фрагментации ДС, причем каждый фрагмент порождается единственной спонтанно возникшей возбужденной ячейкой, инициирующей s-волны. Динамическая теория позволяет рассмотреть и предельный случай вырожденной двойниковой структуры (ВДС), в которой доминирует основная компонента [14].…”