Nanocrack nucleation in polycrystalline silicon during grain-boundary sliding

A theoretical model is proposed to describe nanocrack nucleation in polycrystalline silicon. In terms of this model, nanocrack nucleation is stimulated by grain-boundary sliding, which creates sources of local stresses in triple junctions of grain boundaries. The relaxation of these local stresses is the main driving force of nanocrack nucleation near triple junctions in polycrystalline silicon, in which grain-boundary sliding contributes substantially to plastic deformation under cyclic loading at room temperature. The model is used to calculate the critical external stress required for nanocrack nucleation in polycrystalline silicon.     

Grain-boundary fluctuations in two-dimensional colloidal crystals

We study grain-boundary fluctuations in two-dimensional colloidal crystals in real space and time using video microscopy. The experimentally obtained static and dynamic correlation functions are very well described by expressions obtained using capillary wave theory. This directly leads to values for the interfacial stiffness and the interface mobility, the key parameters in curvature-driven grain-boundary migration. Furthermore, we show that the average grain-boundary position exhibits a one-dimensional random walk as recently suggested by computer simulations [Z. T. Trautt, M. Upmanyu, and A. Karma, Science 314, 632 (2006)]. The interface mobility determined from the mean-square displacement of the average grain-boundary position is in good agreement with values inferred from grain-boundary fluctuations.     

Structure,energy and solute segregation behaviour of [110] symmetric tilt grain boundaries in yttria-stabilized cubic zirconia

Yttria-stabilized cubic zirconia bicrystals with [110] symmetric tilt grain boundaries are systematically fabricated by the diffusion bonding method. It is revealed that the grain-boundary atomistic structures, excess energies and solute segregation behaviours are strongly dependent on the macroscopic geometries of the boundaries. High-resolution transmission electron microscopy combined with lattice statics calculations suggests that the grain-boundary structures are characterized by the accumulation of coordination-deficient cation sites at their cores, whose densities have a clear correlation with excess energies and amounts of solute segregation. The orientation dependence of grain-boundary properties in cubic zirconia can thus be linked and understood via local grain-boundary atomistic structures with the characteristic miscoordinated cation sites.     

Mechanism of deformation-twin formation in nanocrystalline metals

A theoretical model is proposed to describe the nucleation of deformation twins at grain boundaries in nanocrystalline materials under the action of an applied stress and the stress field of a dipole of junction or grain-boundary wedge disclinations. The model is used to consider pure nanocrystalline aluminum and copper with an average grain size of about 30 nm. The conditions of barrier-free twinning-dislocation nucleation are studied. These conditions are shown to be realistic for the metals under study. As the twin-plate thickness increases, one observes two stages of local hardening and an intermediate stage of local flow of a nanocrystalline metal on the scale of one nanograin. In all stages, the critical stress increases with decreasing disclination-dipole strength. The equilibrium thickness and shape of the twin plate are analyzed and found to agree well with the well-known results of experimental observations.     

Changes in the fine structure of grain boundaries, induced by the absorption of helium, and helium embrittlement

The changes in the structure of grain boundaries in tungsten due to the absorption of helium atoms are investigated experimentally and theoretically. Intergranular dilatation localized in a plane layer of subatomic thickness is observed. It is established that dilatation is accompanied by splitting of the cores of grain-boundary dislocations and a decrease in the grain-boundary stacking fault energy. The relationship of intergranular damage to the changes induced in the parameters of grain-boundary dislocations by the absorption of helium is discussed. Zh. Tekh. Fiz. 68, 64–69 (July 1998)     

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)     

Grain-boundary diffusion in nanocrystals with a time-dependent diffusion coefficient

A solution to the equation of grain-boundary diffusion is obtained under conditions where migration of the diffusant from the boundaries into the grains is absent and the diffusion coefficient decreases with time from an increased value to a value characteristic of equilibrium grain boundaries. The specific features of the grain-boundary diffusion in nanocrystals are qualitatively analyzed in terms of this solution.     

Grain-boundary anelastic relaxation and non-equilibrium dilution induced by compressive stress and its kinetic simulation

Grain boundaries are known to be sources and sinks for bulk vacancies, but the exchange that occurs between the grain boundary and the bulk under a low stress is still obscure. In the present paper, it is shown that grain boundaries may act as sources to emit vacancies when an anelastic deformation occurs under a compressive stress. These emitted supersaturated vacancies are combined with solute atoms to form complexes. Solute non-equilibrium grain-boundary dilution may be induced by the diffusion of complexes away from the boundary. An equation of solute concentration at grain boundary is derived under stress equilibrium during its anelastic relaxation. Furthermore, kinetic equations are also established to describe the non-equilibrium grain-boundary dilution. Additionally, an attempt is made to simulate experimental data to justify the present model.     

Kinetic properties of a first-order boundary ridge in Zn

The motion of a grain-boundary half-loop system with a first-order ridge is investigated and a theory of the steady-state motion of such a system is presented. The pattern of variation in the shape and properties of a moving boundary that contains various structural elements (curved segments, facets, first-order ridges) is presented. Important kinetic parameters (the mobility and the migration activation enthalpy) of a first-order grain-boundary ridge with curved segments are measured.     

Grain-Boundary Shear-Migration Coupling in Al Bicrystals. Atomistic Modeling

The energy of grain boundary shears is calculated for symmetric grain boundaries (GBs) using ab initio methods and molecular-dynamic modeling in order to elucidate mechanisms that control GB shear-migration coupling in typical symmetric GBs, such as Σ3 (111), Σ5 (012), Σ5 (013) and Σ11 (113) tilt GBs, in Al bicrystal. The energy of generalized grain-boundary stacking faults (GB–SF) is determined, and the preferred directions and the energy barrier are established for grain-boundary slippage. It is shown that the relative slippage of neighboring grains at certain directions of particle shears is accompanied by conservative migration of GB in the direction perpendicular to its plain. The modeling data are comparative to known grain-boundary shear-migration coupling mechanisms in Al.     

Model of grain-boundary self-diffusion in α- and β-phases of titanium and zirconium

A model of the grain-boundary self-diffusion process in metals undergoing phase transitions in the solid state is proposed. The model is based on the ideas and approaches of the theory of nonequilibrium grain boundaries. It is shown that the range of application of basic relations of this theory can be extended, and they can be used to calculate the parameters of grain-boundary self-diffusion in high-temperature and low-temperature phases of metals with phase transition. Based on the constructed model, activation energies of grainboundary self-diffusion in titanium and zirconium are calculated, and their anomalously low values in the low-temperature phase are explained. The calculated activation energies of grain-boundary self-diffusion are in good agreement with experimental data.     

Grain Boundary Diffusion in Polycrystalline Ferrites with Different Intergranular Potential Barrier

Using the method of layer-by-layer measurements of the electrical conductivity activation energy, an investigation is performed of grain-boundary diffusion of oxygen in polycrystalline Li–Ti-ferrites in the range of temperatures 873–1073 K. A correlation relation is established between the value of the intergranular potential barrier and the parameters of grain-boundary diffusion of oxygen in polycrystalline ferrites. An increase in this value gives rise to a growth in the diffusion activation energy. The grain-boundary diffusion coefficient increases with the value of the intergranular potential barrier.     

Nucleation of deformation twins on gliding grain-boundary dislocations in nanomaterials

A new micromechanism of nucleating deformation twins in nanocrystalline and ultrafine-grained materials under action of severe mechanical stresses has been proposed and theoretically described. The mechanism is a subsequent splitting of grain-boundary dislocations into lattice partial and sessile grain-boundary dislocations. Ensembles of gliding partial dislocation forms deformation twins. The energy characteristics of this process are calculated. The nucleation of the twins is shown to be energetically profitable and can be athermic (without an energy barrier) under conditions of severe mechanical stress. The dependence of a critical stress at which the barrier-less nucleation of twins took place on the widths of these twins is calculated.     

Effect of the microstructural porosity parameters on the fracture and deformation of copper during creep at 773 K

The parameters of intergranular fracture of copper during creep under tension at T = 773 K and σ = 12.5 MPa are determined, and the contribution of grain-boundary porosity to the increase in the creep rate at stage III is estimated. The increase in the creep rate is shown to occur due to the pore-induced decrease in the grain boundary area, an increase in the mobile-dislocation density, and the deformation of the material because of the formation of pores and cracks.     

Determination of the Oxygen Diffusion Coeficient in Polycrystalline Li-Ti Ferrites

Two independent methods, the isotope method based on nuclear microanalysis and the method based on measuring the electronic-conductivity activation energy, are used to determine the grain-boundary diffusion and volume diffusion of an oxygen isotope ¹⁸O in a polycrystalline lithium-titanium ferrite at the thermal annealing temperature 1073 K. A comparative analysis is conducted of the potential of the methods in studying oxygen diffusion in the material concerned. It is shown that the technique for obtaining the diffusion parameters from the electronic conductivity measurements allows a comparatively precise determination of both the volume and grain-boundary diffusion coefficients of oxygen in polycrystalline ferrites.     

Changes in the diffusion properties of nonequilibrium grain boundaries upon recrystallization and superplastic deformation of submicrocrystalline metals and alloys

The effect of an increase in the coefficient of the grain-boundary diffusion upon recrystallization and superplastic deformation of submicrocrystalline (SMC) materials prepared by severe plastic deformation has been studied. It is shown that the coefficient of the grain-boundary diffusion of the SMC materials is dependent on the intensity of the lattice dislocation flow whose value is proportional to the rate of the grain boundary migration upon annealing of SMC metals or the rate of the intragrain deformation under conditions of superplastic deformation of SMC alloys. It is found that, at a high rate of grain boundary migrations and high rates of superplastic deformation, the intensity of the lattice dislocation flow bombarding grain boundaries of SMC materials is higher than the intensity of their diffusion accommodation, which leads to an increase in the coefficient of the grain-boundary diffusion and a decrease in the activation energy. The results of the numerical calculations agree well with the experimental data.     

Radiation-induced changes in the atomic structure of grain boundaries in tungsten

Field-ion microscopy is used to study changes in the structure of the grain boundaries induced by intergrain adsorption of point defects created by ion bombardment of tungsten bicrystals. It is found that irradiation at temperatures below the threshold of grain-boundary relaxation causes a local expansion of the boundaries. Computer simulation using molecular dynamics shows that intergrain adsorption of vacancies can lead to the formation of three-dimensional grain-boundary structures. Fiz. Tverd. Tela (St. Petersburg) 41, 383–385 (March 1999)     

Measuring relative grain-boundary energies in block-copolymer microstructures

The (relative) energies of symmetric tilt grain boundaries in a strongly segregated lamellar block copolymer are determined by analysis of the dihedral angles at grain-boundary triple junctions. The analysis reveals two regimes: at low and intermediate misorientations (corresponding to a tilt-angle range 0≤θ≤85°) the grain-boundary energy is found to depend on the tilt angle as E(θ)～θ(x), with 2.5>x≥0. At large misorientations the grain-boundary energy is found to be independent (within the experimental uncertainty) of the angle of tilt. The transition between the two scaling regimes is accompanied by the transition of the grain-boundary structure from the chevron to the omega morphology. Grain-boundary energy and frequency are found to be inversely related, thus suggesting boundary energy to be an important parameter during grain coarsening in block-copolymer microstructures, as it is in inorganic polycrystalline microstructures.     

Conductivity of single-crystal and polycrystalline bilayer metal films under the conditions of interdiffusion

The electrical conductivity of a bilayer film with a single-crystal or polycrystalline structure is theoretically investigated under the conditions of metal interdiffusion at an arbitrary ratio between the thickness of layers and the mean free path of electrons in the layers. It is shown that analysis of the changes in the electrical conductivity of the bilayer film due to diffusion annealing allows one to elucidate the nature of the processes of bulk and grain-boundary diffusion, to determine the effective depth of penetration of impurity atoms into the bulk and grain boundaries of the sample, and to obtain information on the bulk and grain-boundary diffusion coefficient.     

String order and symmetries in quantum spin lattices

We show that the existence of string order in a given quantum state is intimately related to the presence of a local symmetry by proving that both concepts are equivalent within the framework of finitely correlated states. Once this connection is established, we provide a complete characterization of local symmetries in these states. The results allow us to understand in a straightforward way many of the properties of string order parameters, like their robustness or fragility under perturbations and their typical disappearance beyond strictly one-dimensional lattices. We propose and discuss an alternative definition, ideally suited for detecting phase transitions, and generalizations to two and more spatial dimensions.