CRITICAL DIMENSIONS OF GRAIN DIAMETERS OR INDIVIDUAL PARTICLES FOR THE START OF A MARTENSITIC REACTION FROM THE DYNAMIC THEORY POSITION

10.25712/ASTU.1811-1416.2022.02.007

Abstract

In the case of spontaneous (upon cooling) γ – α martensitic transformation in iron alloys, the formation of a martensitic crystal is determined by a controlling wave process and proceeds at supersonic speed with respect to longitudinal waves. The initiation of three-dimensional threshold deformation is associated with the coordinated action of relatively long-wavelength ℓ-wave and relatively short-wavelength s-waves. For the description of habitus planes, ℓ-waves that arise during the formation of the initial excited state (IES) are responsible. This approach is effective both in describing twinned crystals and in the limiting case of a degenerate twin structure comparable to the formation of dislocation crystals at grain sizes of the initial γ-phase exceeding critical D_c values. In the case of the nanocrystalline state, variants of the transformation of grains as a whole are observed, provisionally associated to the accommodative martensitic transformation (AMT). The critical D_ac size for AMT is less than D_c. In dynamic theory, the value of D_ac can be associated with the process of spatial scaling of the IES, which is compared with the propagation of cylindrical and spherical waves for a relative change in volume. In such a transformation scenario, the size of the D_ac, as well as the critical size for the transformation of small free particles, is determined by the threshold deformation value.