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Group for Mechanochemistry and Synthesis of New Materials

Background

Solid-solid phase transformations (PT) are phenomena that are very widespread in nature, physical experiments, and modern technologies. The practical significance of PT is connected to such technologies as the heat treatment of metals, high pressure synthesis of diamonds and other superhard materials, as well as to the new class of smart materials called shape memory alloys (SMA). One of the mechanisms of earthquakes is related to instability caused by shear strain-induced PT. Phase transformations play an important role in such phenomena as transformation-induced plasticity (TRIP), stress and strain induced PT, pseudoelasticity and pseudoplasticity, shape memory effect (SME), and plastic shear-induced PT under high pressure.  Various sciences consider the PT from their own point of view, including materials science, solid state physics, continuum mechanics, thermodynamics, crystallography, stability theory, and mathematics. That is why a multidisciplinary approach is necessary.  One of the classes of PT's which we study is martensitic or displacive PT. We study the displacive PT as a special type of deformation of a crystal lattice of parent phase (austenite) in a crystal lattice of product phase (martensite) which is accompanied by a jump in all the thermomechanical properties. This deformation is called the transformation strain. Reverse PT transforms martensite into austenite. Due to the symmetry there is a finite number (e.g. 12 for the PT from a cubic to monoclinic lattice) of crystallographically equivalent variants of martensite with the same (to within symmetry operations) transformation strain tensor. The reason for the occurrence of PT s the loss of stability of the crystal lattice of the parent phase accompanied by deformation toward a new stable crystal lattice. Typical values of components of transformation strain are the following: shear strain for shape memory alloys and steels reach 10 to 20%, volumetric strain is near zero for shape memory alloys, and varies from 1-4% for steels to 54% for the PT graphite to diamond and rhombohedric boron nitride to cubic boron nitride. Even for temperature-induced PT (no external stresses), the appearance of such a large transformation strain in some regions of a body, results in large stresses and accommodational inelastic strains in the transforming regions and surroundings. Practically all PT's with volumetric transformation strain exceeding 0.5% are accompanied by plastic strains.  As PT represents a deformation of crystal lattice, external stresses affect the PT significantly. PT's caused by external stresses lower than the yield stress are called stress-induced PT. Pressure-induced PT are a particular case of stress-induced PT. When plastic strain precedes or occurs simultaneously with the PT, one speaks about strain-induced PT. Strain-induced nucleation occurs at new defects generated during plastic flow, e.g. at slip bands, shear-band intersections, and dislocation pile-ups. Plastic strain significantly promotes various PT. It is known from numerous experiments that plastic shear leads to the following results: (a) to a significant (by factor 3-4) reduction of PT pressure and pressure hysteresis, (b) to an appearance of new phases, which were not obtained without additional shear, (c) to a substitution of a reversible PT by an irreversible PT, which allows production of phases that are metastable at normal pressure and can be used in engineering applications, and (d) to strain-controlled kinetics. A knowledge of the influence of plastic strain, applied and local stress fields on PT is very important for the understanding, simulation, and controlling the PT in known PT-related technologies, as well as for the development of new technologies and synthesis of new materials. Various spatial scales are involved in PT: