New phase nucleus appearance on a twin boundary, which is with an invaribe velocity or oscillating

Abstract

The occurrence and dynamics of a nucleus of a new phase on a moving twin boundary are described. The phase diagram of a nucleus on a moving twin boundary is obtained. It is shown that the nucleus can arise on the moving twin boundary only if its velocity v is smaller than a certain value v~. The existence of two kinds of low-symmetry phase trails (unstable and metastable) is described. The friction force, resulting from a trail and acting on a twin boundary is shown to arise. It is of a viscous type in the case of an unstable trail, and of a « dry » friction force type in the case of metastable trail. The oscillation of the twin boundary with the nucleus in the extemal periodic field, conjugated to the order parameter is described. The phase transition (PT) process was first considered by Frenkel in the framework of liquid- vapor transformation. According to Frenkel's mechanism the heterophase fluctuations give rise to nuclei of, say, vapor phase on the background of the superheated liquid phase. Being at nonequilibrium, this nucleus either disappears (if the size of the nucleus is smaller than the critical value), or rapidly increases unlimitedly (in the opposite case) and thus the crystal transforms into the vapor phase (Il. Frenkel's mechanism was generalized for solids, taking into account the elastic properties and anisotropy and making it possible to describe some phenomena of PTS in crystals (2). However, another mechanism of PTS in crystals has attracted considerable interest of late. Real crystals always contain defects : dislocations and dislocation clusters, cracks, inclusions, twin (domain) boundaries and so on. Even the perfect monocrystal has at least a dislocation of growth as well as edges, botli of which can be regarded as defects in the sense considered below. These inhomogeneities change the local value of a transition temperature. Thus the nucleus of a new phase must appear in the vicinity of the defect under a temperature different from that of a bulk PT. This nucleus is stable in contrast to Frenkel's nucleus. It is stabilized by the inhomogeneity. Stable nuclei were observed experimentally in various crystals under PTS of different nature in the vicinity of defects of different types.