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)     

Grain-boundary sliding in elongated microstructures during diffusion creep

In diffusion creep, the contribution of grain-boundary sliding to the overall strain os can be evaluated in arbitrary polycrystals, if the angular distribution of grain boundaries is known. A os value of 0.5 is obtained for two-dimensional (2D) equiaxed microstructures consisting of regular hexagonal grains, equiaxed grains grown from a Voronoi structure or grains having a circular distribution of grain-boundary angles. The os value is also evaluated for uniaxially deformed 2D microstructures, both diffusionally and uniformly deformed. For the former, the deformed microstructure is obtained by the simulation of microstructural evolution in polycrystals with straight grain boundaries. The os value increases gradually with increasing or decreasing strain and is larger in the diffusionally deformed microstructures than in the uniformly deformed microstructures for a given grain aspect ratio. The os value for three-dimensional (3D) polycrystalline microstructures is also obtained from an ellipsoidal distribution of grain-boundary angles. The resultant os value is 0.60 for 3D equiaxed polycrystals and increases gradually with increasing strain.     

Growth processes, structure, and intergrain magnetic interaction in electrolytically deposited CoW films

This paper discusses the formation of columnar microstructure and the associated variations of the magnetic characteristics of electrolytically deposited CoW films with in-plane magnetic anisotropy. It is shown that the nature of the intergrain magnetic interaction is determined by the concentration of the products of the electrode reactions, their distribution in the film, the structure of the intercrystallite boundaries, and also the thickness and texture of the films. Fiz. Tverd. Tela (St. Petersburg) 39, 894–897 (May 1997)     

Experimental study of the built-in field in intergrain channels of thin ferroelectric Pb(Zr,Ti)O3 films

The experimentally measured dependence of the photo-emf on the remanent polarization in thin-film M/Pb(Zr,Ti)O₃(PZT)/M capacitors correlates well with the model developed by us for the intergrain photovoltaic effect for films with columnar structure of PZT grains and heterophase intergrain boundaries. In this case, the photo-emf is determined by the depolarization field generated by the uncompensated polarization charge at PZT grain boundaries. It is shown that the magnitude and orientation of the built-in field in an intergrain channel of such PZT films can be derived from measurements of the photo-emf at zero polarization with a sensitivity on the order of a few millivolts.     

Dislocation nucleation at amorphous intergrain boundaries in deformed nanoceramics

A theoretical model is proposed for lattice dislocation nucleation in deformed nanocrystalline ceramics with amorphous intergrain boundaries. According to the model, a lattice dislocation dipole nucleates at an amorphous intergrain boundary through a local plastic shear along the boundary cross section. The energy parameters of this nucleation process are calculated. It is demonstrated that the dislocation nucleation at amorphous intergrain boundaries is energetically favorable and can occur as an athermic process (without energy barrier) in the nanocrystalline phase of cubic silicon carbide 3C-SiC and in the TiN/a-Si₃N₄ nanocomposite over wide ranges of structural parameters and mechanical loads.     

Influence of a weak pulsed magnetic field on grain-boundary relaxation in aluminum

It is discovered that the treatment of technical-grade polycrystalline aluminum in a weak pulsed magnetic field during heating leads to displacement of the grain-boundary maximum of the low-frequency internal friction toward lower temperatures. This phenomenon is associated with a decrease in the degree of interaction of the grain boundaries with impurity atoms. Fiz. Tverd. Tela (St. Petersburg) 41, 1985–1987 (November 1999)     

Physical model of the interaction of grain-boundary and lattice dislocations

Processes of absorption and creation of lattice dislocations by grain boundaries are examined in a physical model of grain-boundary quasi-dislocations. The dissociation time and the power and energy conditions for generation of dislocations are found and compared to experiment. Fiz. Tverd. Tela (St. Petersburg) 41, 1772–1777 (October 1999)     

Phonon kinetics in solid solutions of ceramics in the ZrO2-Y2O3-MgO-Al2O3 system

The propagation of weakly nonequilibrium phonons is studied in ceramics based on ZrO₂-Y₂O₃-MgO-Al₂O₃. Under the assumption of a large dispersion in the ceramic grain sizes, the characteristics of phonon scattering are studied as a function of the temperature and duration of isothermal annealing of the samples and the contribution of the intergrain boundaries to the phonon kinetics is isolated. Fiz. Tverd. Tela (St. Petersburg) 39, 1091–1093 (June 1997)     

Temperature and orientation dependences of the grain-boundary diffusion coefficient of antimony in copper bicrystals

An investigation is made of the diffusion of antimony through the bulk and along grain boundaries in copper bicrystals containing a symmetric 〈100〉 misorientation boundary with misorientation angles from 20 to 37.2°. The bicrystals are grown by the method of horizontal zone recrystallization. The temperature range for these studies is 480–580 °C, where the solubility of Sb in Cu is about 6 atomic % and practically temperature-independent. The concentration profiles are obtained by x-ray spectral microanalysis, and the grain-boundary diffusion parameters are computed by the method of Whipple and Suzuoka. The orientation dependence of the triple product P=sδD _b (where s is the segregation coefficient, δ the width of the grain boundary, and D _b the grain-boundary diffusion coefficient) is nonmonotonic, with a maximum for the special ∑5 misorientation boundary (36.9°). The effective activation energy for grain-boundary diffusion ranges from ∼70 kJ/mol for ∑5 to140 kJ/mol for general boundaries. Fiz. Tverd. Tela (St. Petersburg) 39, 1153–1157 (July 1997)     

Critical current of an inhomogeneous Josephson junction at an intergrain boundary with a random distribution of dislocations

The critical current J _c(θ) of an intergrain boundary is calculated as a function of the contact misorientation angle θ of the granules. It is assumed that the ordering parameter is suppressed in regions near boundaries with an enhanced mechanical stress induced by randomly distributed surface dislocations. The stress distribution function is determined using a probabilistic approach. Assuming that the weak coupling at the boundary is Josephson coupling, an analytic expression is found for the angular dependence J _c(θ) (for tilt and twist boundaries). The magnitude of the residual critical current of a boundary in a strong magnetic field is estimated. Fiz. Tverd. Tela (St. Petersburg) 40, 393–402 (March 1998)     

Disclination mechanism for plastic deformation of nanocrystalline materials

A model is developed for the plastic deformation of nanocrystalline materials in terms of the evolution of a spatial grid of disclinations located at the triple junctions of grains. Plastic deformation takes place as the result of plastic rotation of grains, the mismatch of whose rotations causes the nucleation of partial disclinations at the junctions of intergrain boundaries. It is shown that the distinctive feature of the mechanical behavior of nanocrystals is a deviation from the Hall-Petch law up to a critical grain size D _cr⩽25 nm. Fiz. Tverd. Tela (St. Petersburg) 39, 2023–2028 (November 1997)     

Structure of the mixed state induced in thin YBaCuO superconducting films by the field of a small ferromagnetic particle

The temperature dependence of the local energy barrier to formation of the mixed state in YBaCuO thin-film superconducting samples has been determined. The measurement technique is based on use of a small ferromagnetic particle as the magnetic field source. It is found that the energy barrier to creation of vortices (for the field oriented parallel to the CuO planes) is anomalously small while the dependence of the corresponding threshold current j _c(T) differs substantially from the temperature dependence of the pair-breaking current. The experimental results are interpreted in terms of the model of a Josephson medium. The observed temperature dependence of j _c points to a strong suppression of the superconducting order parameter at the intergrain boundaries, which for the most probable type of boundaries: superconductor-insulator-superconductor, is evidence of anisotropic pairing. Zh. éksp. Teor. Fiz. 116, 1735–1749 (November 1999)     

Connectivity and percolation in simulated grain-boundary networks

Random percolation theory is a common basis for modelling intergranular phenomena such as cracking, corrosion or diffusion. However, crystallographic constraints in real microstructures dictate that grain boundaries are not assembled at random. In this work a Monte Carlo method is used to construct physically realistic networks composed of high-angle grain boundaries that are susceptible to intergranular attack, as well as twin-variant boundaries that are damage resistant. When crystallographic constraints are enforced, the simulated networks exhibit triple-junction distributions that agree with experiment and reveal the non-random nature of grain-boundary connectivity. The percolation threshold has been determined for several constrained boundary networks and is substantially different from the classical result of percolation theory; compared with a randomly assembled network, about 50-75% more resistant boundaries are required to break up the network of susceptible boundaries. Triple-junction distributions are also shown to capture many details of the correlated percolation problem and to provide a simple means of ranking microstructures.     

Thermodynamic and Kinetic Properties of Grain-Boundary Ridges on Tilt $$[11\bar 20]$$ Grain Boundaries in Zinc

Thermodynamic and kinetic properties characteristic of grain-boundary ridges on tilt \([11\bar 20]\) grain boundaries in zinc are studied experimentally. Temperatures are determined for faceting–defaceting and roughening phase transitions in grain-boundary ridges for tilt \([11\bar 20]\) grain boundaries with 35°, 57°, and 85° angles of misorientation. Mobilities and enthalpies of activation are obtained for grain-boundary ridges on the same boundaries. Parameters of inhibition are also calculated for grain-boundary ridges, according to the observed shapes of grain-boundary loops.     

Micromechanics of the transition from intergrain to intragrain deformation in nanomaterials

A micromechanism of the transition from intergrain sliding to intragrain glide by nucleation and emission of lattice partial dislocations at grain-boundary dislocations is proposed and described theoretically. The energy characteristics of this process are calculated. It is shown that the nucleation of lattice partial dislocations is energetically efficient and can occur athermally (without the energy barrier) under conditions of the action of ultrahigh mechanical stresses. The critical stresses required for the athermal nucleation and emission of dislocations are calculated.     

Creating and destroying vacancies in solids and non-equilibrium grain-boundary segregation

Determining how the vacancies in excess of equilibrium concentration are created and destroyed in solids is crucial for understanding many of their physical characteristics and processes. Grain boundaries are known to be sources and sinks for bulk vacancies, but the exchange that will occur between the grain boundary and the bulk under a stress is still obscure. In the present paper, we show that grain boundaries will work as sources to emit vacancies when a compressive stress is exerted on them and as sinks to absorb vacancies when a tensile stress is exerted. At the same time, this physical process will produce solute non-equilibrium grain-boundary segregation and dilution. A set of kinetic equations is established to describe this physical process. Additionally an attempt has been made to simulate the experimental data with the kinetic equations to justify the physical process.     

Field percolation and high current density in 80/20 DyBa2Cu3O7 − x/Dy2BaCuO5 bulk magnetically textured composite ceramics

We measured the AC susceptibility of magnetically textured (123) 80%/211(20%) DyBaCuO composite in a special set-up in order to enhance the intergrain contribution. The synthesis process led to very clean “weak links” at grain boundaries. At the percolation threshold bulk shielding paths were such that the intergrain critical current density J_c was above 10⁵ A/cm². The field dependence of J_c was understood through an analytical form indicating a distribution of currents similar to the law of clusters at fracture/percolation thresholds.     

Magnetoresistance effect in A 2(FeMo)Ox double perovskites (A = Sr, Ca; 5.90≤x≤6.05)

The dependences of magnetic, electric, and magnetotransport properties on oxygen non stoichiometry were investigated in compounds of Ca₂(FeMo)O_x and Sr₂(FeMo)O_x (5.90≤x≤6.05). The regular trends in behavior of the magnetization, resistance, and magnetoresistance of samples of these series are determined. It is established that the magnetoresistance is composed of two parts that appear as a result of magnetic ordering in grain-boundary layers and of the intergrain transport of spin-polarized charge carriers. The electronic transport in the samples is assumed to be governed by percolation processes between grains which have a metallic type of conductivity and are separated by insulating spacers.     

Defect chemistry of grain boundaries in proton-conducting solid oxides

The defect chemistry of charged grain boundaries in an acceptor-doped oxide in equilibrium with water vapour is examined theoretically. The basis of the theoretical approach is that the formation of charged grain boundaries and attendant space-charge zones is governed by differences in the standard chemical potentials of oxygen vacancies and hydroxide ions between bulk and grain-boundary core, that is, by the thermodynamic driving energies for defect redistribution. A one-dimensional continuum treatment is used to predict the space-charge potential and defect concentrations in the grain-boundary core as a function of water partial pressure, temperature and acceptor dopant concentration for various values of the two thermodynamic driving energies. The results are discussed with respect to experimental data in the literature for acceptor-doped perovskite oxides (e.g. BaZrO₃) and fluorite oxides (e.g. CeO₂).