Title : Nanoscale aspects of the dynamic theory of martensitic transformations
Abstract:
Martensitic transformations (MTs) include structural transformations that proceed cooperatively, mainly as first-order phase transitions. In the dynamic theory of MT, the growth of a martensite crystal is interpreted as a process that unfolds sequentially in space and time. The beginning of crystal growth is associated with the appearance in the elastic field of the dislocation of the initial excited (vibrational) state (IES), which breaks the symmetry of the crystal lattice of the initial phase (austenite). The oscillatory process generates a control wave process (CWP), which carries a threshold deformation that retains the memory of the type of deformation in the region of localization of the IES. In solids, nanoscales include sizes from 1 nm to 100 nm. These scales are connected, firstly, with the processes of the nucleation of a new phase, which, for first-order phase transitions, must have a heterogeneous character. Second, martensitic crystals often have a fine structure (the so-called transformation twins) with characteristic thicknesses of twin microblades of 1–10 nm. Third, the development of materials with an ultrafine-grained structure (grain diameter 10–100 nm) is topical, for which it is fundamentally important to understand the factors that determine the critical grain size, which is still compatible with the onset of martensitic transformation. The focus is on γ-α (fcc-bcc or bct) MT in iron-based alloys. The observed features of the MF are described as the result of a concerted action of relatively long-wavelength and short-wavelength displacements in the composition of the CWP. The following are discussed: the mechanisms of formation of the fine structure of lamellar crystals, including the degenerate structure of transformation twins and the formation of dislocations; the possibility of transferring information about the nature of IES from a truly nanoscale level to a scale level of 100 nm - 1000 nm; the existence of a critical grain size for martensitic transformation, its dependence on the chemical composition of the alloy, and the possibility of influencing the critical size of external fields.
Audience Take Away Notes:
The audience will receive unique fundamental information necessary for current scientific research, applied development, pedagogical and educational activities.
- dynamic (wave) mechanism of formation of martensitic crystals
- the role of the electronic subsystem in the implementation of the martensitic transformation
- fine structure of twins of transformation and its degenerate variant
- the birth of transformation dislocations
- destabilization of austenite, previously stabilized by plastic deformation, by a strong magnetic field
- possibilities of transformation of nanograin as a whole
