Grain Boundary Regions Research Articles

Effect of external electric field on diffusivity and flash sintering of 8YSZ: A molecular dynamics study

The atomistic structural modification and ionic diffusivity in 8YSZ at elevated temperature with the presence of an external electric field (E-field) were investigated by molecular dynamics (MD) simulations. As an example, a sufficiently large system of atomic configuration for a bi-crystal ∑5(310)/[001] grain boundary (GB) model was studied. MD results show that the E-field promotes the formation of Frenkel pair defects. Notably, some Zr ions diffuse out of GB regions at E-field>∼500-1000V/cm, resulting in an “avalanche” of cation vacancies at GBs and reduction of GB space charge. Consequently, the diffusivities of cations and anions are enhanced in the 8YSZ system with the presence of E-field. The atomistic level understanding of E-field induced structural modifications and ionic diffusivities provide an in-depth insight to unravel the flash sintering mechanisms of ionic ceramics, especially the coupled thermal-field effect during the flash sintering process.

Striped Non-Uniform Corrosion Behavior of Non-Equiatomic FeMnCoCr High-Entropy Alloy Prepared by Laser Melting Deposition in 0.1 M H2SO4 Solution.

The corrosion behavior of the Fe50Mn30Co10Cr10 high-entropy alloy (HEA) manufactured via laser melting deposition (LMD) was investigated using open circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy measurements. The microstructure and morphology of LMD samples before and after the electrochemical tests were compared using X-ray diffraction, optical microscopy, scanning electron microscopy, and electron backscatter diffraction techniques. After the corrosion tests, a striped morphology was observed on the surface of the LMD HEA, which is mainly caused by the interval distribution of high-density and low-density grain-boundary regions. The corrosion performances varied with different planes of the LMD HEA sample, which is mainly controlled by the grain size at each plane. Local corrosion in this HEA is concentrated at the melt pool boundary, which may be due to the abundant metallurgical defects and stress concentration at this location.

Local Inhibition by 2-mercaptobenzothiazole of Early Stage Intergranular Corrosion of Copper

Corrosion inhibition by 2-mercaptobenzothiazole (MBT) at the surface termination of various types of grain boundaries (GBs) was studied at the nanometer scale on microcrystalline copper in HCl acid solution using in situ electrochemical scanning tunneling microscopy (ECSTM). Macroscopic electrochemical analysis by cyclic voltammetry showed highly effective inhibition of Cu(I) active dissolution blocked by MBT pre-adsorption in a potential range of 0.15–0.2 V. ECSTM analysis of the initial stages of intergranular corrosion confirmed the mitigation of net intergranular dissolution by the pre-adsorbed MBT surface layer but also revealed the local accumulation of reaction products in the GB regions. For Coincidence Site Lattice boundaries other than coherent twins, intergranular dissolution, mitigated by the pre-adsorbed MBT layer, and protection by intergranular formation of a film of reaction products were observed. For random GBs, protection by reaction products was dominant, in agreement with their more reactive intrinsic character, generating more Cu(I) ions under anodic polarization and thus promoting the formation of a protective film of reaction products. Coherent twins did not show preferential intergranular reactivity compared to adjacent grains, indicating equally strong efficiency than on grains. These results bring new insight on how inhibition operates locally at various types of GBs.

Microstructure and creep anisotropy of Inconel 718 alloy processed by selective laser melting

In this study, Inconel 718 alloy cylindrical rods with their axes normal or parallel to the building directions were fabricated by selective laser melting (SLM), then homogenized and double aged. Creep tests of the heat-treated samples were conducted under a constant stress of 650 MPa at 650 °C. The microstructures of the as-built, heat-treated and post-creep horizontal and vertical samples were characterized using scanning and transmission electron microscopy and electron backscatter diffraction. The experimental results showed that both equiaxed grains and columnar dendrites exist in the horizontal and vertical samples before and after creep. The average grain sizes remained nearly unchanged during creep. The larger grain size and reduced regions of transverse grain boundaries, as well as the promoted precipitation of the γ″ phases at the creep test condition contributed to the longer creep rupture life (~120 h) of the vertical sample. Stress concentration occurring especially at the transverse grain boundaries led to the final failure by intergranular fracture for the two building orientational samples. The anisotropy of creep behavior of the SLMed Inconel 718 alloy could not be eliminated by the present heat treatment schedule.

3D arrangement of atomic polyhedra in tilt grain boundaries

Polycrystalline materials have been used throughout the recorded history, and their macroscopic properties strongly depend on the defects in their crystalline structures. Unique properties of the grain boundaries (GBs) should result from nearby nano-scale space, leading to the questions regarding the type of atomic polyhedra that pack the GB region as well as the universal set of structural units. It is essential to reveal the underlying mathematical framework for the polyhedral arrangement of GB structures and identify the origin of structure-property relationships in polycrystalline materials, which is a fundamental aspect but important for designing high-performance materials. Herein, 3D atomic structures of [001], [110], and [111] symmetrical tilt GBs are analyzed in the face-centered cubic (fcc) crystals, where the polyhedral units are rigorously defined in contrast to the polygonal arrangement within the 2D structural unit model. It is concluded that the GB region can only be packed by the bulk or GB-type polyhedral units with minor variations. In this study, the 3D arrangement of atomic polyhedra around the GBs is quantitatively determined and systematically described by numerical analysis. Moreover, the GB hierarchy directly follows the distribution of rational numbers that is represented by the modified Farey diagram, which represents the 3D atomic structure of the symmetrical tilt GBs in an fcc crystal. The definition of singular GB is proposed as the GB packed by at the most two types of polyhedral units, and any GB can be packed by the polyhedral units that form the singular GBs.

The effect of grain boundary on the local incipient plastic deformation of fcc metals during nanoindentation

The effect of grain boundaries (GBs) on deformation mechanisms becomes increasingly important as the volume of deformation reaches the submicrometer and nanoscale. The current work investigates the impact of grain boundaries on the incipient plasticity of small-scale deformations of fcc metals. For this purpose, the behavior of single and bi-crystal Au thin films during nanoindentation are studied, using large-scale atomistic simulations. Various symmetric ⟨110⟩ tilt GBs with a wide range of misorientation angles are included to analyze the effect of GB geometry on the nanoscale plasticity mechanisms. Potentially, GBs can act as a source, sink, or obstacle for lattice dislocation, depending on their geometry, energy level, and distance from the deformation zone. The role of the heterogeneous nucleation and emission of dislocations from GBs on the plasticity and hardness of bicrystals is analyzed. According to our results, the intrinsic free volume involved in the GB region is associated with dislocation nucleation at the GB. The volume of the plastic zone generated beneath the tip and the way it grows is strongly dependent on the GB structure. Dislocation nucleation occurs predominantly in the early stages of indentation at GBs with a dissociated interface structural unit, before the interaction of lattice dislocation and GB. Coherent twin boundaries display the lowest effect on the hardness. Based on our results, there is a strong correlation between the interfacial boundary energy and its effect on the bicrystal hardness. GBs with lower interfacial energy offer a higher barrier against slip transmission and nucleation at the GB.

A facile and cost-effective approach for the fabrication Bi0.5Na0.5TiO3 thick films on flexible substrate for energy storage capacitor applications

The current research reports a facile and cost-effective technique for the deposition of functional ceramics Bi0.5Na0.5TiO3 thick films on flexible, low cost, electrically integrateable Ni substrate by using electrophoretic deposition (EPD) technique. EPD is a well-known, eco-friendly process for depositing coatings on a conductive substrate from a colloidal suspension. A reliable approach was adopted for stable colloidal suspension of BNT particles during conventional solid-state synthesis process by using high energy ball milling in acetone medium without the complications of surfactants and expensive platinum substrate. BNT films with thickness ranging from ~55 to 165 μm were successfully obtained by exploring EPD parameters i.e. voltage from 100 to 125 V and coating time from 30 to 240 s. Thick coatings with comparatively better adhesiveness were deposited at 100 and 125 V with coating time up to 90 s. Thick films were densified by sintering at 1150 °C for 30 min. Phase analysis were conducted by using X-Ray diffraction (XRD), Fourier transformed infrared spectroscopy and Raman spectroscopy. XRD analysis revealed the formation of single (111) peak indicating the pure perovskite structure of BNT thick film. Scanning electron microscope studies indicated that dense and uniform microstructure was obtained by EPD coating with the film thickness of ~93 μm. Impedance spectroscopy confirmed two electrically active regions of grain interior and grain boundaries.

Uncovering the Zr segregation behavior and its effect on the fracture strength of Al Σ5 (210) [100] symmetrical tilt grain boundary: Insight from first principles calculation

Segregation behavior of Zr solute and its effect on the Al Σ5 (210) [100] symmetrical tilt grain boundary (GB) were investigated through first-principles calculations. Preferential sites and sequence for the replacement of Al by Zr in the GB were determined, so were the fracture energy and tensile strength of the GBs with the varied number of segregated Zr atoms. Segregation energies indicates the GB can accommodate no more than three Zr atoms. The calculation results about grain boundary and strengthening/embrittling energies suggest the strengthening effect of Zr solute on the GB, confirmed by higher fracture energy and peak theoretical tensile stress of the Zr-doped GBs than that of the clean GB, which is the result of the shorter length of the bond that holds the GB together, the contraction of the GB region and the shrink of charge depletion after the segregation of Zr. The conclusions of this investigation shed light on the design of aluminum alloys with high performance GB.

Impurity Effect of Light Elements on the Nickel Crystallization in the Triple Junction Region of Grain Boundaries: Molecular Dynamics Simulation

The impurity effect of light elements (carbon, nitrogen, and oxygen) on nickel crystallization in the triple junction region of grain boundaries is studied by the molecular dynamics method. The tilt grain boundaries with disorientation axis 〈111〉 are considered as grain boundaries. The computational cell is a cylinder, the axis of which coincided with the triple junction and the disorientation axis of grains. Periodic boundary conditions are imposed along the cylinder axis; the atoms on the lateral surface of the cylinder are stationary. To simulate crystallization, the computational cell was melted by heating to a temperature significantly higher than the melting point of nickel. After the simulated polycrystal became liquid, the thermostat was turned on and the polycrystal was kept at a constant temperature less than melting point. In this case, the rigid boundary conditions on the lateral surface of a cylindrical computational cell simulated the crystallization fronts from three crystallization centers. The area near the triple junction crystallized in the last turn. Defects and free volume are concentrated in this area. The impurities significantly slowed down the crystallization rate. The addition of 10% impurity atoms slowed down the rate of the crystallization front by several times. The impurity effect on the crystallization rate increased in the direction C–N–O. This is due to the difference in the deformation of the crystal lattice caused by impurity atoms: the greater this deformation, the more impurity atoms slow down the crystallization front. Impurity carbon atoms formed aggregates of crystal grains at rather high concentrations. Crystallization front remained on these aggregates. Oxygen atoms and nitrogen atoms did not form aggregates; nevertheless, they distorted the crystal lattice and significantly slowed down the crystallization front.

Influence of Bi3Zn2Sb3O14 pre-synthesis phase on electrical properties of the ZnO-Bi2O3 based varistor ceramics

Abstract The effects of the pre-synthesis phase Bi3Zn2Sb3O14 on microstructure and non-linear characteristic of ZnO varistor ceramics are investigated. The results are discussed and compared with previous studies of ZnO varistors with Sb2O3 additions. It is shown that adding Bi3Zn2Sb3O14 to ZnO varistor ceramics can effectively inhibit the growth of ZnO grains, which results in a smaller grain size and a significantly higher breakdown voltage comparing with Sb2O3 samples. On the other hand, Bi3Zn2Sb3O14 has a significant influence on the defect structure and component distribution of grain-boundary regions. When Bi3Zn2Sb3O14 content increases from 0 to 3 wt%, the value of the interface state density (Ns) increases sharply. At the same time, the electrical properties are improved gradually and reached an optimized value with the nonlinear coefficient (α) up to 43 corresponding to the high barrier height (ϕb) of 2.04 eV.

Large piezoelectricity and high transparency in fine-grained BaTiO3 ceramics

Grain size effect is considered as an important factor for the enhancement of dielectric and piezoelectric properties of the barium titanate (BT) ceramics. In this work, we prepared a dense fine-grained BT ceramic using the hot-press sintering method. This fine-grained BT not only exhibits superior electrical properties with a dielectric permittivity of 7200, piezoelectric coefficient of 420 pC/N, and field-induced electrostrain 0.2% at 30 kV/cm but also possesses high transmittance of 30%–60% over a wide range of wavelength from visible to middle infrared radiation spectrum. By a comparative study with the coarse-grained BT, the fine-grained BT is found to have a slight relaxation property due to its large volume fraction of grain boundaries. The coexistence of the fine domains in small grains and the relaxor regions of grain boundaries is responsible for the synchronous enhancement of electrical and optical properties of the fine-grained BT.

Zr segregation in Ni–Zr alloy: implication on deformation mechanism during shear loading and bending creep

Grain boundary (GB) migration and grain growth in nanocrystalline metals are major obstructions to exploit these materials to their fullest potential, thus limiting their utility as advanced structural materials. Here, we investigate the effect of Zr segregation to a Ni GB on migration and subsequent strengthening during shear deformation and high-temperature bending creep. Specifically, using molecular dynamics simulation, we simulate deformation of a Ni bicrystal specimen having a symmetric ∑5 grain boundary with and without segregated Zr. It is found that GB segregation up to 0.4 at.% pins the CSL boundary resulting in increased shear strength. Similar behavior is also observed in the case of bending creep deformation where the creep resistance increases only up to 0.4 at.%. This study indicates that larger amounts of GB segregation cause destabilization of the local atomic structure and consequently affect the GB stiffness. Moreover, we investigate the effect of Zr segregation on the interaction between the GB and a dislocation. We find that the preexisting dislocation is absorbed in the GB region for both clean and segregated specimens; however, the absorption path of the dislocation is different due to the change in GB stiffness caused by the Zr segregation.

Dielectric behavior of gamma-cyclodextrin polymer and carboxymethyl gamma-cyclodextrin polymer cross-linked both by epichlorohydrin

The γ-cyclodextrin polymer (γ-CDP) and carboxymethyl-γ-cyclodextrin polymer (CM-γ-CDP) cross-linked both by epichlorohydrin were investigated via dielectric spectroscopy over the frequency range 0.1–100 kHz and the temperature ranges 122.0–472.4 K and 123.6–460.3 K, respectively. The γ-CDP reveals a thermally reversible transformation of some normal hydrogen bonds to flip-flop type at Ttrans = 199.1 K, whereas the CM-γ-CDP doesn’t show any transformation of hydrogen bonds. Both systems present in the lnσ vs 1/T plot two linear parts following the Arrhenius equation. In the case of γ-CDP, the first linear part during cooling (a) has activation energy Eα = 0.41 eV and during heating (b) has Eα = 0.42 eV in the range 259.7–310.0 K. The second linear part (c) in the range 316.5–357.1 K has Eα = 0.36 eV. The corresponding values of CM-γ-CDP are Eα = 0.32 eV (a), Eα = 0.33 eV (b) in the range 242.1–295.1 K and Eα = 0.91 eV (c) in the range 307–366.1 K. The linear parts (a) and (b) of both polymers are related to the accumulation of protons in the grain-boundary regions as it is depicted in the Cole-Cole plots. The linear part (c) is related to the proton migration from the grain-boundary region to the electrodes and the dehydration process taking place. The ac-conductivity of CM-γ-CDP is considerably higher than that of γ-CDP as the partial hydrogen replacement of some hydroxyl groups by carboxymethyl moiety, renders numerous protons more flexible to move along the hydrogen-bonded network, contributing to the conductivity. Both systems after the dehydration process, presented a newly-formed hydrogen bonded network due to the reorientation of hydrogen bonded chains at T > 397.0 K for γ-CDP and T > 420.9 K for CM-γ-CDP.

Effect of External Electric Field on Diffusivity and Flash Sintering of 8ysz: A Molecular Dynamics Study

The atomistic structural modification and ionic diffusivity in 8YSZ at elevated temperature with the presence of an external electric field (E-field) were investigated by molecular dynamics (MD) simulations. As an example, a sufficiently large system of atomic configuration for a bi-crystal Σ5(310)/[001] grain boundary (GB) model was studied. MD results show that the E-field promotes the formation of Frenkel pair defects. Notably, some Zr ions diffuse out of GB regions at E-field≥~500-1000V/cm, resulting in an “avalanche” of cation vacancies at GBs and reduction of GB space charge. Consequently, the diffusivities of cations and anions are enhanced in the 8YSZ system with the presence of E-field. The atomistic level understanding of E-field induced structural modifications and ionic diffusivities provide an in-depth insight to unravel the flash sintering mechanisms of ionic ceramics, especially the non-thermal effect during the flash sintering process.

Migration Pinning and Roughening Transition of a Ni Grain Boundary

To date, much research has been conducted into the effect of migration pinning on the grain size in polycrystalline materials. However, effects of migration pinning on the grain-boundary structure and its transition have not been illuminated. Here, using transmission electron microscopy (TEM) we have explored the pinning effects for the grain boundary in a Ni bicrystal. During TEM specimen preparation, a hole was intentionally drilled in the middle of the grain boundary as a pinning point against grain-boundary migration. The specimen was heated to 600 °C. The grain boundary is driven to migrate by both the surface energy anisotropy and the total strain energy reduction. Grain-boundary facets with a plane orientation of {0 3 2}//{1 1 1} appear near the hole. The facets undergo a structural transition from atomically flat to rough with increasing distance from the hole. A pinning force exerted by the hole suppresses the migration of the grain boundary near the hole, indicating that the grain-boundary region away from the hole is subjected to a higher driving force. It certainly appears that the phenomenon originates from a change in driving force with the distance from the hole, being a signature of kinetic roughening.

Atomistic simulations of the interaction between transmutation-produced Re and grain boundaries in tungsten

High energy neutron irradiation not only causes the transmutation of tungsten (W), but also induces transmutation elements to segregate and precipitate at grain boundaries. In this work, the segregation of transmutation-produced rhenium (Re) and their clusters at a Σ5(130) grain boundary (GB) in bulk W has been investigated by using both molecular dynamics and molecular statics methods. It is found that, for single interstitial, Re atom diffuses from the bulk to the GB region via a three-dimensional rotation and migration in the form of an interstitial 〈1 1 1〉 Re-W mixed dumbbell, because the rotation and migration barriers of a Re-W dumbbell are both small. When Re atoms are absorbed by the GB, it is noteworthy that Re atoms tend to occupy the substitutional sites near the GB. The segregation energy of interstitial Re is ranged from −7.67 to −6.12 eV, and the effective interaction distance between the GB and Re interstitials is about 7 Å, which increases with increasing the cluster size. By modeling the uniaxial tensile test of W-GB structure containing Re substitution clusters, the fracture strength and elongation of the GB are significantly reduced. The present results provide important insights into the detailed mechanisms of Re segregation at GBs and its possible effects on the mechanical properties of W-based materials.

Exploring the intricacies in the conduction mechanism of the perovskite series Ba2HoSb1−xRuxO6: A conductivity scaling approach

A composition-dependent investigation of the electrical conductivity and electric modulus formalisms was performed for complex perovskites of the form Ba2HoSb1-xRuxO6 (x = 0.25, 0.50, 0.75, 1.0). The combination of X-ray diffraction and Rietveld refinement methods confirmed the structure of all the synthesized samples to be cubic. The lattice parameters and cell volumes of the synthesized materials satisfy the linear Vegard law, confirming the formation of complete solid solutions. The strength of bonding in the materials was analysed through the bond valence sum approach as a function of changing composition. The electrical conductivity trends were investigated as a function of Ru substitution. A scaling formalism satisfying the time–temperature superposition principle as functions of temperature and composition was sought. The activation energies extracted from the dc conductivity and hopping frequency show positive correlation. The scaled conductivity and modulus curves do not form a master curve in the grain and grain-boundary regions because of the dissimilar activation kinetics in the microstructures. Thus, the scaling remains inconsistent in these microstructural domains. However, a perfect scaling of the conductivity is found as a function of composition. Thus, the conduction mechanism follows a mobility scaling rather than a temperature scaling. A perfect scaling of the electric modulus as a function of composition is not found, showing the pronounced compositional dependence of the relaxation mechanism. Thus, the scaling behaviour has threefold implications: (i) there is no single universal scaling parameter for the conductivity spectra in the different electroactive microstructural domains, (ii) there is a unique scaling function over composition which can account for both temperature-independent and composition-independent conduction processes and (iii) it points towards a composition-independent conduction mechanism in contrast to a composition-dependent relaxation mechanism.

Dislocations in Grain Boundary Regions: The Origin of Heterogeneous Microstrains in Nanocrystalline Materials

Nanocrystalline materials reveal excellent mechanical properties but the mechanism by which they deform is still debated. X-ray line broadening indicates the presence of large heterogeneous strains even when the average grain size is smaller than 10 nm. Although the primary sources of heterogeneous strains are dislocations, their direct observation in nanocrystalline materials is challenging. In order to identify the source of heterogeneous strains in nanocrystalline materials, we prepared Pd-10 pct Au specimens by inert gas condensation and applied high-pressure torsion (HPT) up to γ ≅ 21. High-resolution transmission electron microscopy (HRTEM) and molecular dynamic (MD) simulations are used to investigate the dislocation structure in the grain interiors and in the grain boundary (GB) regions in the as-prepared and HPT-deformed specimens. Our results show that most of the GBs contain lattice dislocations with high densities. The average dislocation densities determined by HRTEM and MD simulation are in good correlation with the values provided by X-ray line profile analysis. Strain distribution determined by MD simulation is shown to follow the Krivoglaz–Wilkens strain function of dislocations. Experiments, MD simulations, and theoretical analysis all prove that the sources of strain broadening in X-ray diffraction of nanocrystalline materials are lattice dislocations in the GB region. The results are discussed in terms of misfit dislocations emanating in the GB regions reducing elastic strain compatibility. The results provide fundamental new insight for understanding the role of GBs in plastic deformation in both nanograin and coarse grain materials of any grain size.

Spreading Kinetics of a Destructive Melt Over a Solid

The paper proposes an approach to studying the mechanism and kinetics whereby a metallic melt of specific composition penetrates into the grain boundaries in unstressed metal samples. The melt penetration rate is controlled by processes that occur in the liquid phase: dissolution of grain-boundary regions, recrystallization to form solid-solution crystals, and opening of new crack regions under the pressure of growing crystals.

Defect Engineering of Palladium-Tin Nanowires Enables Efficient Electrocatalysts for Fuel Cell Reactions.

The defect engineering of noble metal nanostructures is of vital importance because it can provide an additional yet advanced tier to further boost catalysis, especially for one-dimensional (1D) noble metal nanostructures with a high surface to bulk ratio and more importantly the ability to engineer the defect along the longitudinal direction of the 1D nanostructures. Herein, for the first time, we report that the defect in 1D noble metal nanostructures is a largely unrevealed yet essential factor in achieving highly active and stable electrocatalysts toward fuel cell reactions. The detailed electrocatalytic results show that the Pd-Sn nanowires (NWs) exhibit interesting defect-dependent performance, in which the defect-rich Pd4Sn wavy NWs display the highest activity and durability for both the methanol oxidation reaction (MOR) and the oxygen reduction reaction (ORR). Density functional theory (DFT) calculations reveal that a large number of surface vacancies/agglomerated voids are the driving forces for forming surface grain boundaries (GBs) within Pd4Sn WNWs. These electronic active GB regions are the key factors in preserving the number of Pd0 sites, which are critical for minimizing the intrinsic site-to-site electron-transfer barriers. Through this defect engineering, the Pd4Sn WNWs ultimately yield highly efficient alkaline ORR and MOR. The present work highlights the importance of defect engineering in boosting the performance of electrocatalysts for potentially practical fuel cells and energy applications.