Bicrystalline Samples Research Articles

The Li2Ti6O13 and Li2Zr6O13 composite as a high-performance anode for alkali-ion batteries: a molecular dynamics study.

The current development of technology has highlighted the necessity of compounds that enhance the durability and performance of alkali-ion batteries. The anodes of these batteries need to overcome the challenges of low dc-conductivity at ambient temperatures and interfacial resistance between the solid-state electrolyte. By conducting large-scale molecular dynamics simulations, we investigated the transport properties of Li2Ti6O13 and Li2Zr6O13 mono- and bi-crystals, as well as Li2Ti6O13@Li2Zr6O13 composites. While the monocrystalline and bi-crystalline Li2Zr6O13 show similar transport properties, the composite materials, combining both compounds, exhibit the highest diffusion coefficients and dc-conductivity. The transport properties of the composite materials are found to be significantly higher than those mono- and bi-crystalline samples due to the Li interstitial mechanism and the presence of grain boundaries. Our study offers valuable insights for the development of high-performance energy storage materials.

Micro-beam bending of FCC bicrystals: A comparison between defect dynamics simulations and experiments

To understand the role of the grain boundary (GB) in plasticity at small scale, a concurrently coupled mesoscale plasticity model was developed to simulate micro-bending of bicrystalline micron-sized beams. By coupling dislocation dynamics (DD) with a finite element model (FEM), a novel defect dynamics model provides the means to investigate intricate interactions between dislocations and GBs under various loading conditions. Our simulations of micro-bending agree well with corresponding micro-bending experiments, and they show that mechanical response of bicrystals could have not only hardening but also softening depending on the characters of the GB. In addition, changing the location of the GB in the microbeams results in different mechanical responses; GBs located at the neutral plane show softening compared to single crystals, while inclined GBs located halfway along the length of the beam show little effect. Simulation results could provide a clear picture on detailed dislocation-GB interactions, and quantitative resolved shear stress analysis supplemented by dislocation density distribution is used to analyze the mechanical response of bicrystalline samples.

Dislocation interactions at the grain boundary in FCC bicrystals: An atomistically-informed dislocation dynamics study

To understand the underlying mechanisms that control the mechanical properties of nanostructured metals, an insight into the role of the grain boundary in dislocation-driven plastic deformation is vital. The grain boundary has been observed as a dislocation source, sink, or having no effect, which in turn, gives rise to different macroscopic mechanical responses. With this motivation, atomistic simulations and three-dimensional dislocation dynamics simulations were performed to investigate dislocation interactions at various grain boundaries and their role in the plastic deformation of face-centered cubic (FCC) bicrystalline micropillars. The atomistically-informed dislocation dynamics simulations show that bicrystalline samples containing a high angle grain boundary (HAGB) display hardening and higher flow stresses compared to single crystals, while micropillars with a coherent twin boundary (CTB) show similar flow stresses to the reference single crystalline samples. This is due to the transparency of the grain boundary to slip transmission, which is observed in the atomistic simulations. Interestingly, allowing dislocation glide on the grain boundary exhibits a decrease in flow stress as slip transmission becomes easier.

Size effect in bi-crystalline micropillars with a penetrable high angle grain boundary

The implications of various size effects on the deformation behavior of and near grain boundaries is not yet fully understood. In this manuscript, slip transfer mechanisms through a general high angle grain boundary (HAGB) allowing for easy transfer are investigated in order to understand the size dependence of the dislocation-grain-boundary interaction. Complementary in situ micro compression tests on copper single and bi-crystals in the scanning electron microscope and with x-ray Laue microdiffraction were used to correlate the mechanical response with the evolving microstructure. It is shown that no dislocation pile-up is formed at the boundary. The lack of pile-up stresses results in a deformation process which is dominated by the initial dislocation source statistics. This is evidenced by similar size scaling of the single and bi-crystalline samples with the grain size being the characteristic length scale.

Intergranular Oxidation Effects During Dwell-Time Fatigue of High-Strength Superalloys

The present paper summarizes experimental work to identify the mechanisms of dwell-time cracking during service operation of polycrystalline nickel-base superalloys, such as Alloy 718 and AD730. By means of crack growth monitoring during various kinds of cyclic loading in vacuum and in air using the potential drop technique, it was shown that the combination of sustained tensile stress, load reversal, and oxidizing atmosphere leads to an increase in the crack propagation rate by orders of magnitude, as compared to cyclic reference tests without dwell time and/or under vacuum conditions. By careful metallographic and theoretical analysis, the embrittling effect was attributed to stress-induced oxygen diffusion ahead of the intergranular crack tip followed by decohesion in a nanometer scale and had been termed “dynamic embrittlement.” More recently, atom probe tomography of the near-crack tip region revealed that the damage zone consists of Cr-rich transition oxides rather than elemental oxygen. This is in qualitative agreement with TGA measurements on Alloy 718 specimens without mechanical loading, which shows that crack propagation velocities of 50 µm/s do not allow massive Cr2O3 or NiO scale formation. By means of a quantitative analysis of the fracture surface, it became evident that grain-boundary attack depends on the grain-boundary character. This observation was supported by four-point bending experiments on grain-boundary-engineered samples with a high fraction of coincident site lattice boundaries and bicrystalline samples with well-defined grain-boundary misorientation relationships with respect to the loading axis. Taking the experimental results into account, semiquantitative modeling concepts have been developed to correlate crack propagation rates with the oxygen grain-boundary diffusivity, the local microstructure, and the mechanical stress states. These concepts are discussed in terms to adapt grain size and precipitate microstructure of polycrystalline superalloys.

Grain growth competition during thin-sample directional solidification of dendritic microstructures: A phase-field study

We present the results of a comprehensive phase-field study of columnar grain growth competition in bi-crystalline samples in two dimensions (2D) and in three dimensions (3D) for small sample thicknesses allowing a single row of dendrites to form. We focus on the selection of grain boundary (GB) orientation during directional solidification in the steady-state dendritic regime, and study its dependence upon the orientation of two competing grains. In 2D, we map the entire orientation range for both grains, performing several simulations for each configuration to account for the stochasticity of GB orientation selection and to assess the average GB behavior. We find that GB orientation selection depends strongly on whether the primary dendrite growth directions have lateral components (i.e. components perpendicular to the axis of the temperature gradient) that point in the same or opposite directions in the two grains. We identify a range of grain orientations in which grain selection follows the classical description of Walton and Chalmers. We also identify conditions that favor unusual overgrowth of favorably-oriented dendrites at a converging GB. We propose a simple analytical description that reproduces the average GB orientation selection from 2D simulations within statistical fluctuations of a few degrees. In 3D, we find a similar GB orientation selection as in 2D when secondary branches grow in planes parallel and perpendicular to the sample walls. Remarkably, quasi-2D behavior is also observed even when those perpendicular sidebranching planes are rotated by a finite azimuthal angle about the primary dendrite growth axis as long as the absolute values of those azimuthal angles are equal in both grains. In contrast, when the absolute values of those azimuthal angles differ markedly, we find that unusual overgrowth events at a converging GB are promoted by a high azimuthal angle in the least-favorably-oriented grain. We also find that diverging GBs can be strongly affected by those azimuthal angles, while converging GBs exhibit a weak dependence on those angles. For diverging GBs, GB orientation is also strongly affected by the relative signs of the lateral components of the primary dendrite growth directions in both grains.

In Situ TEM Microcompression of Single and Bicrystalline Samples: Insights and Limitations

In situ micromechanical compression experiments in a transmission electron microscope enable the study and analysis of small-scale deformation behavior. The implementation of instrumented indenter systems allows measuring the force and displacement, providing additionally insights on sample strength and flow behavior. Using focused ion beam sample preparation, single- and bicrystalline specimens can be fabricated to study the influence of individual grain boundaries on the mechanical behavior. Taperless single crystalline and bicrystalline Cu compression pillars including a coherent twin boundary were deformed in scanning and conventional transmission electron microscopy mode to study the applicability of both techniques for examining dislocation dynamics and interaction with the boundary. Based on experimental results, possibilities and limitations of such experiments are critically discussed, including sample preparation, in situ annealing to remove ion beam-induced defects, imaging of dislocations, and acquisition of stress–strain data. Finally, an outlook is given on the potential of micromechanical in situ transmission electron microscopic experiments for analyzing the influence of grain boundaries on mechanical behavior.

Near Grain Boundary Behavior of Aluminum Bicrystals Deformed in Plane Strain Conditions

The paper describes the mechanism of deformation at 77 K of pure aluminum bicrystals of different grain orientations. The following orientations were selected: {100}/{110} (cube/Goss) and - {100}/{100} (cube/shear) to represent the unstable vs. stable and the unstable vs. unstable behaviours, respectively. The bicrystalline samples were deformed in the plane strain conditions with the use of a channel-die immersed inside a reservoir with liquid nitrogen. The low temperature deformation increases the tendency to form plain strain inhomogeneities of the deformation in the grains with an unstable orientation. In both sets of crystallite compositions, the grain boundary was situated perpendicularly to the compression plane. A particular interest was paid to the analysis of the tendencies of the crystal lattice rotations near the grain boundary and the description of the deformation behaviour of the material in the macro- scale (hardening behaviour). A detailed analysis of the crystal lattice rotations was possible with the application of the local orientation measurements by means of scanning and transmission electron microscopes, equipped with the electron backscattered diffraction and convergent beam electron diffraction facility, respectively. The experimental results of the local orientation measurements were used to evaluate the accuracy of the numerical prediction of the macro-scale behaviour of bi-crystalline samples by a single Cristal Plasticity Model. The investigation shows that the crystallites behave essentially as single crystals in the same deformation conditions. Due to the similar hardening behaviour of the investigated crystallites (similar values of the Taylor factors) the grain boundary remains unchanged. The calculated lattice rotations are similar to those observed experimentally. Key words: aluminium bi-crystals, texture, microstructure, single crystal plasticity model

Superconducting weak bonds at grain boundaries in MgB2

The possibility of preparing bicrystalline Josephson junctions and bolometers based on superconducting MgB2 on specially prepared bicrystalline MgO substrates is investigated. Microbridges 0.85–6.00 μm in width, intersecting the bicrystalline interface, are formed in epitaxial bicrystalline MgB2 films grown on these substrates. It is found that annealing of bicrystalline samples in oxygen leads to a systematic decrease in the critical current, an increase in the temperature width of the superconducting transition region, and to an improvement of the current-voltage (IV) characteristic, which becomes close in shape to the IV characteristic of a Josephson junction. The response of such a junction to radiation at a frequency of 110 GHz with an amplitude attaining 0.5 mV is measured.

Molecular dynamics simulation of crack tip blunting in opposing directions along a symmetrical tilt grain boundary of copper bicrystal

ABSTRACTMode I crack growth along some grain boundaries of copper embrittled by solute segregation shows strong anisotropy. For instance, growth along the direction on the symmetrical tilt boundary has been reported to occur by intergranular brittle fracture, whereas growth along the opposite sense occurs in a ductile manner. In this paper, we simulate such crack configurations using molecular dynamics (embedded atom method [EAM]) in 3‐dimensional perfect bicrystalline samples of pure copper of the aforementioned orientation at room temperature. In both cases the response is ductile, crack opening taking place by dislocation emission from the crack tip. The critical stress intensity factors (SIFs) for dislocation emission have been calculated by matching the displacement fields of the atoms in the tip neighbourhood with the continuum elastic fields. They are of the same order of magnitude for both growth senses despite the different morphology of their respective blunted crack tips and of the patterns of dislocations constituting their plastic zones. Thus, it seems that, in agreement with published results of continuum crystalline plasticity for the same problem, the plastic anisotropy associated with the different orientation of the slip systems with respect to the crack cannot in this case explain the experimental behaviour observed with solute embrittled bicrystals.

Ca-doping-induced enhancement of the critical currents of coated conductors grown by ion-beam-assisted deposition

One of the most promising technologies for the fabrication of high-Tc cables is the ion-beam-assisted deposition (IBAD) technique. The performance of the superconductors fabricated by IBAD, and the fabrication costs, are to a great extent determined by the critical current densities of the superconductors’ grain boundaries. Since, in bicrystalline samples, overdoping has been found to improve the transport properties of grain boundaries in high-Tc superconductors, we have explored whether overdoping also enhances the critical currents of IBAD samples. The measurements show that, depending on the critical current density of the superconducting film, Jc (77 K) is increased by factors up to 2.2, also in applied magnetic fields of several tesla.

Metallic and non-metallic longitudinal conductance of grain boundaries inbicrystalline and polycrystalline germanium doped with mercury andantimony

The influence of temperature and magnetic and electric fieldson the grain boundary (GB) longitudinal conductance σat low temperatures in mercury-doped polycrystals and bicrystalsof germanium was studied. It was shown that the σ(T), σ(E) andσ(H)dependences for GBs in bicrystals are determined by metallicand hopping mechanisms, whereas we could not find a hoppingchannel in bicrystalline samples. The behaviour of the σ(H) dependencesfor low magnetic fields (super-linear) and the linearity of the current–voltage characteristicsat T < 100K indicate merely the presence of a metallic mechanism in bicrystalline samples.All of the results obtained indicate that during the zone-melting process, thesegregation of impurities at grain boundaries develops differently in bicrystallineand polycrystalline ingots.

The surfaces of Mo bicrystals studied by low-energy ion scattering

Molybdenum bicrystals having two crystallographic orientations at the surface were studied by low-energy ion scattering (LEIS). Since the surfaces are part of the same specimen, their history and treatments are identical and thus provide an ideal possibility to compare segregation and annealing processes and to verify the quantification of LEIS. The crystallographic orientation of the seeds, the grown bicrystals and the bicrystalline samples was checked by means of X-ray Laue patterns as well as LEED patterns. The misorientation angles were about 1–2°. Because of the high surface sensitivity of LEIS, the scattered ion signals for the Mo(1 1 0)/Mo(1 0 0) bicrystal grains should reflect the atomic densities of the outermost atomic layers of their surfaces. The atomic density of the (1 0 0) surface is found to be 76% of that of the (1 1 0) surface. The difference with the theoretical value for the outer surface (71%) is ascribed to a small contribution of the second atomic layer for the open (1 0 0) surface. For the faces of a Mo(1 1 0)/Mo(1 1 0) bicrystal identical signals are found. At higher ion doses the bombardment leads to (partial) amorphization of the surface. It is found that the annealing of this layer starts already at 700 K for the (1 1 0), but at 1300 K for the (1 0 0) surface. A strong carbon segregation is already found at 1100 K for Mo(1 0 0), while there is still no segregation for the (1 1 0) surface. The results are explained in terms of differences in surface free energies, atomic mobilities and crystal structure.

Electrical activity of grain boundaries in silicon bicrystals and its modification by hydrogen plasma treatment

Electrical activity of grain boundaries (GBs) and its transformation under the influence of low-energy hydrogen plasma treatment in p-type silicon bicrystalline samples cut from EFG silicon polycrystals were investigated. Comprehensive studies have enabled one to investigate the electrical activity of GBs relative to the minority (MIC) and majority (MAC) carriers and to demonstrate the possibility of controlling this activity by different processing methods. These studies allowed for establishing the correlation between the type, structure and individual electrical activity of GBs and also thermal pre-history of samples. Among the tested modes, hydrogenation was found to be the most radical method of electrical activity modification for all types of GBs. In the process, results on hydrogenation of GBs in EFG silicon crystals depend essentially on three factors: type of GBs, state of ribbons (as-grown or annealed) and concurrence of grain boundary dangling bonds and boron passivation effects.

Electric field distribution at low-angle grain boundaries in high-temperature superconductors

Electric fields $\mathbf{E}$ are created in type-II superconductors by thermally activated flux creep or flux flow, driven by electric currents through the specimen. Usually, an average value $〈E〉=V/L$ is determined in resistive four-terminal-transport measurements by measuring the voltage V between two contacts with distance L as a function of the applied current. However, this average value can deviate by orders of magnitude of the true local value $\mathbf{E}(x,y),$ if inhomogeneities like grain boundaries are present in the specimen. In this article, we show the spatial distribution of $\mathbf{E}(x,y)$ of high-temperature superconducting bicrystalline films with a low-angle grain boundary in the flux-creep state. Even in a somewhat ``relaxed state,'' the electric-field value in bicrystalline samples varies by about two orders of magnitude.

An experimental method to study grain boundary diffusivity in anisotropic materials

It has been shown that the theoretical treatment developed by Di Prinzio et al. to study the grain boundary diffusivity in anisotropic material can be experimentally implemented. The method was applied to ice bicrystalline samples. The results obtained show that the diffusivity of tilt boundaries 〈101̄0〉/21 ° and 〈101̄0〉/90 ° depends on the grain boundary inclination. As a rule, the authors suggest that when Sun and Bauer's method is followed to study grain boundary migration b and F( α) may always be determined; it should be kept in mind that, neither values of the Mγ( β) function nor the mean value of Mγ were obtained; instead Mγ eff was found.

Electromagnetic coupling character of [001] twist boundaries in sintered Bi 2Sr 2CaCu 2O 8+ x bicrystals

The electromagnetic characteristics of [001] twist grain boundaries in Bi 2Sr 2CaCu 2O 8+ x (BSCCO-2212) have been deduced from measurements of the resistive transitions, voltage-current ( V- I) characteristics, and field-dependent transport critical current densities ( J ct( H)) of single- and bicrystalline samples. The bicrystals were prepared by solid-state sintering of (001) faces of freshly-cleaved, bulk-scale single crystals placed at a pre-determined misorientation angle, θ. A low-angle 2° bicrystal was strongly coupled with V- I and J ct( H) characteristics that were indistinguishable from those of the single crystals, indicating that the sintering process itself does not produce weak coupling. 30° and 36° [001] bicrystals were weakly coupled. 23° and 88° twist bicrystals were strongly coupled, even though their boundary T c values were reduced. This behavior contrasts starkly with that of [001] tilt boundaries in YBa 2Cu 3O 7 − δ, where a reduced boundary T c is always associated with weak coupling.

TEM observations of electromagnetically characterized YBa2Cu3O7-x grain boundaries in flux-grown bicrystals

The original thin-film bicrystal studies strongly indicated that all high-angle grain boundaries in YBa2Cu3O7-x are intrinsically weak-linked and exhibit Josephson junction characteristics. More recent investigations of individual grain boundaries in both bulk-scale bicrystals and textured thin films have shown that the Jct properties of high-angle grain boundaries can vary in important ways. Specifically, high-angle grain boundaries with Jct behavior characteristic of a flux pinning mechanism have been discovered. These results are encouraging from the view point of large-scale applications. However, further engineering of polycrystalline materials can first be conducted in a deliberate way when the microstructural origin of the observed behaviors is understood in detail.In these studies, light and transmission electron microscopy techniques are used to investigate the microstructure of the very same bulk bicrystalline samples whose electromagnetic properties have been characterized. The three-dimensional macroscopic features of the grain boundary are deduced from light microscopy observations as the entire bicrystal is mechanically ground and polished to approximately 20 microns in thickness.

Experimental Study of the Relative Energy of Symmetrical &lt;110&gt; Tilt Boundaries in a Semi-Conducting F.C.C. Oxide (NiO).

ABSTRACTLarge size bicrystalline samples of NiO corresponding to tilt configurations around a &lt; 110 &gt; axis have been grown by the flame fusion technique, the misorientation angles varying from 0 to 180°. The variation of the relative energy of the grain boundary with misorientation has been studied and has indicated a very different behaviour from that previously observed by the authors on &lt;100&gt; tilt boundaries. High relative energy values and deep cusps on the energy curve at high coincidence angles are observed. The results are discussed in terms of interface energy.