Pre-transition Low-Stability States and Concepts of the Influence of Point and Planar Defects and Their Complexes to Structural-Phase Transformations in Condensed Systems

An analysis of the regularities of martensitic transformations in FCC materials has been carried out, and the features of structural-phase transformations in weakly stable states of condensed systems have been considered. An original physical point of view is presented, based on a new understanding of the state of the system, in which the traditional point of phase transition takes the form of an interval of values of the controlling transition parameter. In this interval, the state of the material is structurally weakly stable relative to the influence of small changes in the controlling parameter. Original physical views on the emergence of the martensitic phase in FCC systems, in which a transition close to the second kind is implemented, are presented. In these views, the tendency towards lattice instability in relation to infinitesimal deformations (soft lattice condition), the presence of lattice instability in relation to short-wave displacements, and the presence of defects in the system play a significant role. The role of defects (vacancies, their complexes, and packing defects) is considered on a model FCC system. The analysis conducted allows us to assume that in weakly stable condensed systems, having a spectrum of structural states near the boundary of stability loss, the interaction of structural defects may have a significant impact on structural-phase transformations. It can be assumed that this is characteristic of a wide range of different systems (ordered alloys with periodic structural defects, metallic FCC systems with pre-transition states, materials with polytypic structure, etc.). It can be expected that structural defects (in the traditional sense) can act static structural defects of various types: point defects and their complexes (vacancies and their complexes, substitution and interstitial atoms, etc.), planar defects (APB, SF and their complexes) and, possibly, others.