Preferential penetration path of gallium into grain boundary in practical aluminium alloy

The preferential penetration of a liquid metal along grain boundaries (GBs) in polycrystalline metals is a well-known phenomenon. Gallium-decorated GB networks in rolled aluminium alloys have been visualized three-dimensionally using the high-resolution synchrotron radiation computer tomography (SRCT). The distribution of gallium concentration along GBs was measured using X-ray absorption. Statistical correction for blurring was performed to raise accuracy of the measurement, and then compared with orientation mapping by the SEM/EBSP method on the surface of a tomographic specimen. The pancake-like grain microstructure formed by a rolling process causes the anisotropy of penetration direction. Although the gallium penetrated into high-angle GBs, all of the high-angle GBs are not necessarily decorated by the gallium. The reason for this may be explained by considering geometrically possible penetration paths that seem to be dependent on local grain arrangement and GB structure through each path. The dependence of the gallium concentration on the rotation axis of misorientation has been found along the high-angle GBs. Especially, GBs with a specific misorientation (〈221〉 as a misorientation axis) showed high gallium concentration.     

Cellular dislocation substructures in polycrystals of FCC solid solutions based on copper: Holt’s relation and the size effect

The cellular dislocation substructures (CDSes) of Cu-Mn polycrystalline solid solutions upon plastic deformation is studied by means of TEM. It is determined that Holt??s relation holds for alloys with different grain sizes, solid solution concentrations, and deformation temperatures. The dependence of Holt??s coefficient C on the main polycrystalline parameters and deformation conditions is studied. Regularities of the fraction of closed cell boundaries and disoriented boundaries in CDS that emerge upon deformation are determined.     

Grain-boundary hardening in polycrystalline copper-based solid solutions

A study is made of the contribution of grain-boundary hardening to the overall hardening in a polycrystalline material on the basis of Ashby's model. Yield curves are used for copper-based solid solutions in polycrystalline and singlecrystal forms. It is shown that the contribution from statistically accumulated dislocations to the yield stress in a polycrystalline specimen reflects the behavior of the corresponding single crystal. The contribution from grain boundaries to the yield stress can be described in terms of the additional dislocation density due to the joint grain deformation in the aggregate up to high strains. At low strains, the main role in hardening of a polycrystalline material is played by the grain boundaries. This extends up to larger strains as the strain temperature is reduced and the alloy-element concentration increases.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 47–52, January, 1984.     

Solid-phase wetting at grain boundaries in the Zr-Nb system

The microstructure of Zr-Nb polycrystalline alloys with niobium concentrations of 1, 2.5, 4, and 8 wt % is investigated in the temperature interval of 620–840°C. It is revealed that the second solid phase β-Nb forms either a chain of separate lens-like precipitates or continuous homogeneous layers at grain boundaries in zirconium, depending on the annealing temperature and the energy of the Zr/Zr grain boundary. It is shown that the greater the quantity of the second solid phase, the lower is the temperature of the termination of grain-boundary wetting. A model is constructed that explains the dependence of the temperature of grain boundary wetting on the amount of wetting phase. It is found that the complete wetting of all grain boundaries in zirconium by the second solid phase does not occur in Zr-Nb alloys.     

The Effect of Severe Plastic Deformation on the Structure and Mechanical Properties of Al–Mg–Li Alloys

The structural-phase state and mechanical properties of commercial aluminum alloys produced by severe plastic deformation are studied and compared to the initial polycrystalline state. This kind of treatment is found to give rise to the formation of a homogeneous ultrafine-grained structure with second-phase particles occurring predominantly along grain boundaries. With this structure, the strain-temperature and strain-rate intervals wherein the superplastic properties of the alloys under study are observed are shifted to lower temperatures and higher rates.     

Molecular dynamics simulation of Ga penetration along grain boundaries in Al: a dislocation climb mechanism

Many systems where a liquid metal is in contact with a polycrystalline solid exhibit deep liquid grooves where the grain boundary meets the solid-liquid interface. For example, liquid Ga quickly penetrates deep into grain boundaries in Al, leading to intergranular fracture under very small stresses. We report on a series of molecular dynamics simulations of liquid Ga in contact with an Al bicrystal. We identify the mechanism for liquid metal embrittlement, develop a new model for it, and show that is in excellent agreement with both simulation and experimental data.     

The Interaction Mechanism between Solid and Liquid Metals under Ultrasonic Action

At various structural-scale levels, the interaction between the aluminum alloy DT1 and a liquid eutectoid mixture of gallium and indium under the action of mechanical waves of the ultrasonic range is investigated. It is established that the deep penetration of the eutectoid mixture into the solid metal under ultrasonic action is due to the formation of channels along which the mixture moves. These channels are observed in both the axial and radial directions.     

Defect structure near grain boundaries in deformed polycrystalline Cu–Al alloys

The evolution of defect structure upon the deformation of Cu–Al polycrystalline solid solutions with grain sizes of 10, 100, and 200 μm is studied by means of transmission diffraction electron microscopy. Alloys are deformed by tension at a rate of 10^?2 s^?1 at room temperature. Different parameters that characterize defect structure are measured. Patterns of changes in them are revealed upon moving away from grain boundaries. Analysis of the results testifies to the presence of a reinforced zone near the grain boundaries.     

Diffusion through a polycrystalline thin film

Analytical solutions to Fick’s second law of diffusion have been simultaneously derived without the restrictions of parabolic profiles along the x-axis in grain boundaries and expressed in a series for both grain interior and grain boundary diffusing through a polycrystalline thin film. The analysis takes segregation of diffusion species at grain boundaries into account. The analytic solutions lead to the concentration profiles in the grain interior and in the grain boundary, to the average-integrated amount of diffusion species at the exit surface, and to the time lag, which can be technologically used for depth profile studies and kinetic accumulation measurements.     

Ionic conductivity of ThO2- and ZrO2-based electrolytes between 300 and 2000 K

A conductivity vs temperature relationship is derived which results from simplified geometrical considerations of a polycrystalline solid electrolyte, assuming that oxygen ion diffusion only takes place through the grains or along the grain boundaries. The relationship proves to be capable of describing conductivity data of YDT, CDT, YSZ and CSZ between 300 and 2000 K. The data analysis reveals that the activation enthalpy as well as the activation entropy of grain conductivity are significantly higher in comparison with grain-boundary conductivity. It may be concluded that BAUERLE's electrical equivalent circuit which underlies the interpretation of dispersive type measurements aiming at separate determination of grain and grain-boundary conductivity ought to be revised.     

Special interaction between quasiperiodic grain boundaries and lattice dislocations in crystalline solids

A theoretical model is suggested which describes phason imperfections (specific excitations) in a quasiperiodic grain boundary in a polycrystalline solid as dilatation flexes. In the framework of the model, an elastic stress field of the quasiperiodic grain boundary is calculated as a stress field created by an ensemble of dilatation flexes (phason imperfections) located in the boundary. It is shown that there is a special elastic interaction between crystal lattice defects and quasiperiodic grain boundaries comprising phason imperfections. The strengthening effect in plastically deformed polycrystalline solids is quantitatively described which is related to the special elastic interaction between lattice dislocations and quasiperiodic grain boundaries. Received: 29 October 1996 / Revised: 22 August 1997 / Accepted: 13 November 1997     

Nonclassical rotational inertia in helium crystals

It has been proposed that the observed nonclassical rotational inertia (NCRI) in solid helium results from the superflow of thin liquid films along interconnected grain boundaries within the sample. We have observed NCRI in large (4)He crystals grown at constant temperature and pressure, demonstrating that the superfluid grain boundary model cannot explain the phenomenon.     

Sonochemistry of molten gallium

This review article summarizes the comprehensive work that was done in our laboratory in recent years, as-well-as other reports, on the various aspects of sonochemistry of molten gallium. The low m_p (29.8 °C) of gallium enables its melting in warm water, aqueous solutions and organic liquids. This opened a new research direction that focused on the chemical and physical properties of gallium particles that were formed in such media. It includes their interactions with water and with organic and inorganic solutes in aqueous solutions and with carbon nanoparticles. Formation of nanoparticles of liquid gallium alloys was also reported.     

Grain Boundary Diffusion in C<Subscript>60</Subscript> Thin Films

This paper focuses on the effect of grain boundaries on the diffusion processes in polycrystalline C₆₀ thin films. Electrically induced diffusion of Au was investigated by in situ measurements of the film conductivity. Electron Paramagnetic Resonance (EPR) spectroscopy was used to study diffusion of oxygen. Increase in grain sizes in polycrystalline C₆₀ thin films was found to result in the acceleration of gold and oxygen diffusion. The results are interpreted assuming that these impurities diffuse in C₆₀ films dominantly along grain boundaries.     

The Region of gallium solubility in lead telluride films grown on silicon substrates

This work is aimed at finding the boundaries of the region of the existence of the solid solutions of Ga in PbTe films prepared on Si substrates with the use of the modified “hot wall” technique. The quantitative results on the composition of the Pb1-y GayTe films with a thickness of more than 1 μm indicate that the gallium concentration yGa in our samples ranges from 0.0004 yGa0.045, depending on the ratio of the partial pressures of the metallic components, which maybe specified by the temperature and composition of the Ga1–xPbx liquid melts. By varying the partial pressures of the metallic components and the chalcogen, we succeeded in synthesizing Pb1-yGayTe layers with a Te concentration ranging from 0.495 to 0.515 mol %. According to the results of a complex investigation with the use of X-ray diffraction, scan ning electron microscopy, and local X-ray spectrum analysis, the region of the existence of homogeneous Pb1 - y GaTe films is substantially narrower than the specified composition range: the limiting Ga concentration in these samples is no more than yGa = 0.011 ± 0.0005. It has been shown that the study of gallium solubility in PbTe should include not merely a quasi-binary PbTe-GaTe section, but also PbTe-Ga2Te3 and PbTe-Ga2Te5 polythermic sections. As indicated by the experimental data, the homogeneity region of the solid solutions of gallium in lead telluride is asymmetric with respect to the quasi-binary PbTe-GaTe section. It has been found that the deviation of the Pb1-yGayTe films from stoichiometry toward excessive concentration of tellurium promotes increased solubility of gallium in the lead telluride matrix.     

Effect of the wetting of grain boundaries on the formation of a solid solution in the Al-Zn system

Phase transitions in the bulk and at grain boundaries in the (Al-20 wt % Zn) alloy have been studied by means of differential scanning calorimetry and transmission electron microscopy. Polycrystals with a high specific area of grain boundaries have been obtained using severe plastic deformation (high-pressure torsion). It has been shown that the Zn-based solid phase completely wets the grain boundaries in aluminum at a temperature of 200°C. The position of the grain boundary solvus line (solubility line), which is above the bulk solvus by 40?C45 K, has been determined.     

基于阻抗模型解析的氧化锆固体电解质组织结构演变模型

晶界对多元多晶电解质材料电导率的影响, 已成为制约高温固体电解质材料发展的瓶颈. 传统的晶界观察方法难以将高温下材料的组织结构与电导性能相对应. 鉴于此, 本文研究了部分稳定氧化锆(PSZ) 固体电解质材料的变温交流阻抗特性, 并对交流阻抗谱进行了拟合分析, 发现等效拟合电路随温度的上升而发生变化. 通过对不同等效电路模型的物理解析, 得出PSZ电解质材料显微结构在高温下的演变模型. 经进一步分析, 演绎出一种'短程有序'的'晶界桥接'组织模型, 为改善PSZ电解质材料的晶界电导提供了参考.     

On the micromechanical modelling of the effective diffusion coefficient of a polycrystalline material

This study focuses on calculation of the effective diffusion coefficient of a polycrystalline material accounting for the grain size and shapes. Polycrystal is modelled as a composite consisting of a matrix with high diffusivity (grain boundaries and triple junctions) and inhomogeneities with low diffusivity (bulk grains including crystal defects like dislocations). The segregation at the grain boundaries is accounted for. Typical micromechanical models are re-written for diffusivity assuming that the grains have the shape of ellipsoids of revolution (spheroids). The results are compared with (1) numerical results for hydrogen diffusion in an imaginary polycrystalline material and (2) experimental results for diffusion of hydrogen in nickel polycrystal available in the literature. The approach can be used for extraction of information on diffusivity along the grain boundaries.     

Investigating the grain structure of Cu-Al and Cu-Mn alloys via electron backscatter diffraction and optical metallography

Parameters of grain-boundary ensembles of Cu-Al and Cu-Mn solid solutions are analyzed quantitatively by optical metallography and scanning electron microscopy with the use of backscattering electron diffraction. It is established by both methods that the fraction of all special boundaries in a grain boundary ensemble and the fraction of twin boundaries Σ3 in the spectrum of special boundaries increase as the tacking fault energy in substitutional solid solutions diminishes.     

The photoelectric properties of polycrystalline films of gallium arsenide

Certain photoelectric properties of polycrystalline films of gallium arsenide at room temperature obtained by a gas deposition method are studied. It is found that film specimens of GaAs have much greater photosensitivity than the solid material for the same current carrier density.The barrier theory of conductivity is invoked in order to explain the relationships observed.