Model of formation of broken dislocation boundaries at joint disclinations

We propose a physical model of formation of broken dislocation boundaries (partial disclinations of deformation origin) at the joints of large-angle grain boundaries. The model explains why and how rotational-type defects are necessarily formed in polycrystals in which plastic deformation at the microscopic level occurs exclusively via translational slips for strains ε > 0.2.     

Model of local melting of grain boundaries containing impurity atom clusters

A model describing the initial stages of the local melting of grain boundaries containing impurity atom clusters is developed. The local melting process is considered as the formation of liquid zones whose geometry and number depend on the initial concentration and thermodynamic characteristics of the impurity atoms, as well as on the material parameters and temperature. The evolution of the liquid-solid interface structure with increasing temperature is discussed. The dependences of the liquid-solid interface parameters on temperature are obtained at various initial concentrations and thermodynamic characteristics of the impurity atoms.     

Kinetics and mechanisms of the UV-radiation-assisted formation of gold nanoparticles in HAuCl<Subscript>4</Subscript>-doped chitosan solutions

Using methods of optical spectroscopy and small-angle X-ray scattering, the kinetics of the UV radiation-induced formation of gold nanoparticles in HAuCl₄-doped water–acid solutions of chitosan has been studied from the very beginning of the reaction. It has been shown that, during synthesis, as the mean size of nanoparticles grows from 2.9 to 6.3 nm, the maximum of the plasmon resonance shifts toward shorter waves (535–523 nm), whereas for a fully formed ensemble of nanoparticles, the reverse trend is observed. It has been found experimentally that the particle size distribution curve changes during synthesis. Based on the inverse problem analysis, conclusions have been drawn regarding the dominant mechanisms behind nanoparticle growth.     

Concept of the Fuel Cycle of the IGNITOR Tokamak

Physics of Atomic Nuclei - In 2015, the development of the conceptual design of IGNITOR, a Russian–Italian experimental thermonuclear tokamak, was completed, and the parameters of all the...     

Structural changes during annealing of submicro- and nanocrystalline aluminum alloys

The annealing-induced evolution of the structure and microhardness of submicro-and nanocrystalline Al—3% Mg and Al 1570 alloys produced by torsional severe plastic deformation are studied. Annealing of the Al-3% Mg alloy at 373–423 K and annealing of the Al 1570 alloy at 373–473 K are shown to result in the relaxation of internal stresses and subsequent normal grain growth. As the annealing temperature increases, the microhardness decreases. At higher temperatures (473 K for the Al—3% Mg alloy and 573 K for the Al 1570 alloy), anomalous grain growth takes place. This growth is accompanied by the appearance of numerous grains with a high dislocation density, a high concentration of impurity atoms in grain boundaries, and an increase in the microhardness. These effects are explained.     

Effects of self-consistent dynamics of dislocations in the elastic field of a planar mesodefect

The self-consistent dynamics of a dislocation ensemble in an elastic field of a planar mesodefect has been considered within the kinetic approach. The effective Airy stress function for a planar mesodefect, which takes into account the screening effect of a system of distributed dislocation charges, has been obtained. Coordinate dependences of the stress tensor components and the dislocation charge density for the considered screened system have been found. Its elastic energy and misorientation of the regions adjacent to the planar mesodefect have been calculated.     

Structural transformations at the initial stages of fragmentation of plastically deformed polycrystals: A computer experiment

Results have been presented for a computer experiment on concurrent micro-, meso-, and macroscopic studies of the evolution of dislocation structure in a large (adjacent to one of the junctions) domain of a grain after its constant-rate macroplastic deformation to an extent that corresponds to the onset of the stage of developed plastic deformation. The type of dislocation-density and dislocation-charge distributions, as well as amounts and degrees of inhomogeneity in local plastic deformation, have been analyzed. The type of dislocation rearrangements at the junctions and fractures of high-angle grain boundaries has been established, which is responsible for the formation of the first dangling dislocation boundaries, which are mesodefects that trigger fragmentation.     

High-speed superplasticity of microcrystalline alloys under conditions of local grain boundary melting

A model is proposed for the high-speed superplasticity of materials under conditions of local grain boundary melting at temperatures close to solidus. It is shown that the local melting of grain boundaries containing segregations of impurity atoms, results in the formation of a structure consisting of liquid-phase regions and solid intergranular bridges which provide cohesion of the grains during the deformation process. The equilibrium concentration, dimensions, and activation energy for the formation of solid bridges are determined as a function of the temperature, initial impurity concentration in the boundary, and the boundary thickness. A mechanism is proposed for grain-boundary slip under conditions of local grain boundary at anomalously high strain rates. Zh. Tekh. Fiz. 68, 38–42 (December 1998)     

Formation of a misorientation band in the elastic field of a disclination dipole

The formation of a misorientation band in the elastic field of a disclination dipole is considered and simulated using a kinetic approach. The dipole is artificially located on a grain boundary or induced by plastic deformation on a double ledge in a grain boundary in a bicrystal. The simulation results agree well with the results obtained earlier in a continuum approximation. The dislocation structure of the misorientation band that forms during accommodation slip is shown to be a dynamic steady-state structure, and its appearance has a kinetic nature.