Critical Diameters of Grains or Individual Particles for the Start of a Martensitic Reaction from the Positions of Dynamic Theory

In the case of spontaneous (under cooling) γ–α martensitic transition in iron alloys, the formation of a martensitic crystal is determined by a wave controlling process and occurs at a supersonic velocity as compared to longitudinal waves. The initiation of three-dimensional threshold deformation is associated with the coordinated action of relatively long (Formula presented.)-wave and relatively short s-waves. The (Formula presented.)-waves appearing during the formation of an initial excited state (IES) are responsible for the description of habit planes. This approach is efficient both in the description of twinned crystals and in the limit case of a degenerate twin structure comparable to the formation of dislocation crystals at initial γ-phase grain sizes exceeding the critical values of Dc. In the case of a nanocrystalline state, the variants of grain transformation as a whole are observed and provisionally classified as an accommodative martensitic transition (AMT). The critical size Dac for AMT is less than Dc. In dynamic theory, Dac can be associated with the spatial IES scaling process compared with the propagation of cylindrical and spherical waves for a relative change in volume. In such a transformation scenario, Dac and the critical size for the transition of small free particles are determined by the threshold deformation value. © 2022, Pleiades Publishing, Ltd