Grain Boundary Microstructure Research Articles

Local atomic structures in grain boundaries of bulk nanocrystalline aluminium: A molecular dynamics simulation study

The microstructures of grain boundaries (GBs) in bulk nanocrystalline aluminium have been investigated by a large-scale molecular dynamics method. Bulk nanocrystalline aluminium is obtained directly by liquid quenching at an appropriate cooling rate, which has narrow grain-size distribution and high-angle GBs. It is found that up to 89.75% GB atoms (named as GB1 atoms) are located at the nearest-neighbour coordination shell around nanograins; others (named as GB2 atoms) are mainly at triple junctions. Local atomic structures in the GBs are quantified in terms of a recently developed method, in which the neighbours of an atom are identified with a parameter-free topological criterion rather than a fixed cut-off distance rc. The results demonstrate that though there are a large number of different cluster types in both the GB1 and the GB2 regions, only a few ones with FCC-like order appear with high frequency in the GB1 region and play a crucial role in the microstructural feature of the GB1. The GB1 region displays short-to-long range order. The GB2 region presents ICO- and BCC-like short-range orders whose degrees are in between the liquid and amorphous, but the medium-range order at the cluster-scale is very weak.

Analysis of the Microstructure of Mg2Si Thermoelectric Devices

High-performance Mg2Si thermoelectric devices have been obtained by spark plasma sintering of high-purity, pre-synthesized, all-molten Mg2Si powder. We studied the effects of source powder particle size on thermoelectric performance. To improve the performance, further investigation of the microstructure of the devices is needed. In this work we studied the microstructure of grain boundaries and interfaces between electrodes and Mg2Si sintered bodies to increase understanding of Mg2Si thermoelectric devices.

Consideration of grain size distribution in the diffusion of fission gas to grain boundaries

We analyze the accumulation of fission gas on grain boundaries in a polycrystalline microstructure with a distribution of grain sizes. The diffusion equation is solved throughout the microstructure to evolve the gas concentration in space and time. Grain boundaries are treated as infinite sinks for the gas concentration, and we monitor the cumulative gas inventory on each grain boundary throughout time. We consider two important cases: first, a uniform initial distribution of gas concentration without gas production (correlating with post-irradiation annealing), and second, a constant gas production rate with no initial gas concentration (correlating with in-reactor conditions). The results show that a single-grain-size model, such as the Booth model, over predicts the gas accumulation on grain boundaries compared with a polycrystal with a grain size distribution. Also, a considerable degree of scatter, or variability, exists in the grain boundary gas accumulation when comparing all of the grain boundaries in the microstructure.

SbVO4 doped ZnO–V2O5-based varistor ceramics: microstructure, electrical properties and conductive mechanism

The effects of SbVO4 addition on the microstructure, electrical properties and characteristics of grain and grain boundary of ZnO–V2O5 based varistor ceramics were studied using SEM, E–J measurements and impedance spectroscopy. XRD analysis revealed that all the samples consist of main phase of ZnO and the second phase of BiVO4 and Zn7Sb2O12. The microstructural homogeneity of the ceramic was improved through adding SbVO4. With an increase of SbVO4, the average grain sizes decrease from 16.1 to 6.1 μm. The resistivity of grain boundary is approximately constant (~103 Ω). The ZnO–V2O5-based varistor ceramics added with 0.3 mol % SbVO4 sintered at 940 °C for 4 h exhibited good nonlinear properties of α = 51, J = 13.4 μA/mm2 and E 1mA = 416 V/mm.

The thermal stability of high-energy ball-milled nanostructured Cu

The thermal stability of nanostructured (NS) Cu prepared by high-energy ball milling was investigated. The as-prepared samples were isothermal annealed for 1h in the temperature range of 200–1000°C. Effects of annealing on NS Cu samples were studied by means of Vickers hardness test, differential scanning calorimetry (DSC) and stress relaxation test. The exceptional high microhardness of as-prepared Cu sample of 1.7GPa was not detected to decrease after annealing at 500°C for 1h with corresponding small value of activation volumes V* of 22.6b3 and high value of strain rate sensitivity m of 0.0176. A prominent decrease of microhardness was detected after higher temperature annealing with a rapidly increase of activation volume and decrease of strain rate sensitivity. The present investigation demonstrates that the thermal stability of NS Cu prepared by high-energy ball milling is determined by not only the grain size but also the microstructure of grain boundaries, and during annealing process, the strain release process occurred prior to the grain growth process, therefore, the NS Cu has a relatively high thermal stability.

Dielectric and magnetic properties of multiferroic BiFeO3 ceramics sintered with the powders prepared by hydrothermal method

BiFeO3 ceramics were sintered in the temperature range of 700–900 °C by using the pure BiFeO3 powders hydrothermally synthesized at 250 °C. The low reaction temperature and low sintering temperature prevent the element volatilization and phase decomposition. The ceramics sintered at 800 and 850 °C exhibit much dense microstructure with clear grains and grain boundaries. They also show high dielectric constant, dielectric dispersion and low loss tangent. At room temperature, the dielectric behaviors of BiFeO3 ceramics are mainly attributed to the transition of localized charge carriers and the microstructure of grains and grain boundaries. The temperature dependence of dielectric constant and loss tangent confirms that the localized charge carriers are a main contribution to the dielectric permittivity. Activation energy Eα of relaxation process for the BiFeO3 ceramic sintered at 850 °C is 0.397 eV. The obtained BiFeO3 ceramics show magnetic responses, which are relative to the grain size.

Microstructure of Carbides at Grain Boundaries in Nickel Based Superalloys

It is well known that carbides at grain boundaries play an important role in affecting mechanical properties of nickel based superalloys. In order to deeply understand the relationship between grain boundary structures and properties, in this work, we have investigated the microstructure of grain boundaries with different misorientation angles in bicrystals of nickel based superalloys. It is found that the bicrystals with smaller misorientation angles contain denser M23C6 but sparse MC particles at grain boundaries, and this kind of bicrystals presents longer stress rupture lives. It was observed that MC carbides were decorated by M23C6 and M6C particles at grain boundaries. The formation of these carbide particles, therefore, is likely due to the local fluctuation of chemical composition around MC carbides. In addition, the orientation relationships between MC carbides and gamma/gamma' matrix were also determined.

Evolution of Grain Boundary Precipitates in Al 7075 Upon Aging and Correlation with Stress Corrosion Cracking Behavior

Transmission electron microscopy (TEM) was employed to investigate the microchemistry and microstructure of grain boundary precipitates in Al 7075 aged at room temperature for several hours, at 393 K (120 °C) for 12 hours (under aged), at peak aged (T651) and over aged (T73) conditions. High resolution TEM analysis of precipitates at grain boundaries and fine probe energy dispersive spectrometry showed that the grain boundary precipitates at peak and over aged conditions are hexagonal η phase with stoichiometry Mg(Cu x Zn1−x )2. Considerable increase in Cu content in the grain boundary η in the over aged condition compared to the peak aged condition was observed. The average Cu content in the over aged condition was found to be 20 at. pct. The higher Cu content of the precipitate is associated with a lower stress corrosion cracking plateau velocity.

Slag Treatment Followed by Acid Leaching as a Route to Solar-Grade Silicon

Refining of metallurgical-grade silicon was studied using a process sequence of slag treatment, controlled cooling, and acid leaching. A slag of the Na2O-CaO-SiO2 system was used. The microstructure of grain boundaries in the treated silicon showed enhanced segregation of impurities, and the formation of CaSi2 and other Ca-rich phases. Boron and phosphorus were found in the grain boundary phases of silicon after the slag treatment and were successfully removed together with most of the metallic impurities by acid leaching. The interaction between silicon and slag and the distribution of impurities are discussed. A novel mechanism of the refining approach is proposed, based on the microstructure of silicon and the analysis of impurities at each refining step. Parallel processes of slag refining, segregation, and solvent refining were observed, which explains the relatively high efficiency of the proposed refining technology. The investigated combination of refining processes followed by acid leaching has great potential as an efficient and cost-saving route for upgrading metallurgical-grade to solar-grade silicon.

Enhancement of giant dielectric response in Ga-doped CaCu3Ti4O12 ceramics

The influences of Ga3+ doping ions on the microstructure, dielectric and electrical properties of CaCu3Ti4O12 ceramics were investigated systematically. Addition of Ga3+ ions can cause a great increase in the mean grain size of CaCu3Ti4O12 ceramics. This is ascribed to the ability of Ga3+ doping to enhance grain boundary mobility. Doping CaCu3Ti4O12 with 0.25mol% of Ga3+ caused a large increase in its dielectric constant from 5439 to 31,331. The loss tangent decreased from 0.153 to 0.044. The giant dielectric response and dielectric relaxation behavior can be well described by the internal barrier layer capacitor model based on Maxwell−Wagner polarization at grain boundaries. The nonlinear coefficient, breakdown field, and electrostatic potential barrier at grain boundaries decreased with increasing Ga3+ content. Our results demonstrated the importance of ceramic microstructure and electrical responses of grain and grain boundaries in controlling the giant dielectric response and dielectric relaxation behavior of CaCu3Ti4O12 ceramics.

Atomistic interpretation of microstrain in diffraction line profile analysis

A virtual X-ray powder diffraction experiment is conducted on a realistic computer-generated nano-polycrystalline microstructure. It is shown that the size and strain broadening contributions to the diffraction line profiles can be directly and reliably extracted from the atomistic model. It is also shown that current line profile analysis methods cannot fully interpret the observed patterns due to the peculiar microstructure of grain boundaries.

Effects of Ta5+ doping on microstructure evolution, dielectric properties and electrical response in CaCu3Ti4O12 ceramics

Abstract The effects of Ta 5+ substitution on the microstructure, electrical response of grain boundary, and dielectric properties of CaCu 3 Ti 4 O 12 ceramics were investigated. The mean grain size decreased with increasing Ta 5+ concentration, which was ascribed to the ability of Ta 5+ doping to inhibit grain boundary mobility. This can decrease dielectric constant values. Grain boundary resistance and potential barrier height of CaCu 3 Ti 4 O 12 ceramics were reduced by doping with Ta 5+ . This results in enhancement of dc conductivity and the related loss tangent. Influence of charge compensations on microstructure and intrinsic electrical properties of grain boundaries resulting from the effects of replacing Ti 4+ with Ta 5+ are discussed. The experimental data and variation caused by the substitution of Ta 5+ can be described well by the internal barrier layer capacitor model based on space charge polarization at the grain boundaries.

Grain-boundary structure and segregation behavior in a nickel-base stainless alloy

Atom-probe tomography (APT) is utilized to obtain three-dimensional chemical information concerning grain boundary (GB) segregation in a Ni–Cr–Fe alloy 600 with atomic spatial resolution. Detailed crystallography of GBs is determined using an approach that combines electron backscatter diffraction and focused ion-beam microscopy to establish a GB’s five macroscopic degrees of freedom, followed by an APT GB composition analysis. Characterizations of GB microstructure and microchemistry are performed to improve our understanding of mechanisms controlling intergranular attack and stress-corrosion cracking.

Microstructure and transport properties of [001]-tilt bicrystal grain boundaries in iron pnictide superconductor, cobalt-doped BaFe2As2

Abstract Relationships between microstructure and transport properties of bicrystal grain boundary (BGB) junctions were studied in cobalt-doped BaFe2As2 (BaFe2As2:Co) epitaxial films grown on [0 0 1]-tilt bicrystal substrates of MgO and (La, Sr)(Al, Ta)O3 with misorientation angles θGB = 3–45°. The θGB of BaFe2As2:Co BGBs were exactly transferred from those of the bicrystal substrates. No segregation of impurities was detected at the BGB junction interfaces, and the chemical compositions of the BGBs were uniform and the same as those in the bulk film regions. A transition from a strongly-coupled GB behavior to a weak-link behavior was observed in current density–voltage characteristics under self-field around θGB ∼ 9°. The critical current density decreased from (1.2–1.6) × 106 A/cm2 of the intragrain transport to (0.7–1.1) × 105 A/cm2 of θGB = 45° because supercurrent becomes more governed by Josephson current with increasing θGB.

Predicting Thermal Conductivity Evolution of Polycrystalline Materials Under Irradiation Using Multiscale Approach

A multiscale methodology was developed to predict the evolution of thermal conductivity of polycrystalline fuel under irradiation. At the mesoscale level, a phase field model was used to predict the evolution of gas bubble microstructure. Generation of gas atoms and vacancies was taken into consideration. Gas bubbles were predicted to form, grow, and coalesce around grain boundary (GB) areas. On the macroscopic scale, a statistical continuum mechanics model was applied to predict the anisotropic thermal conductivity evolution during irradiation. Microstructures predicted by the phase field model were fed into the statistical continuum mechanics model to predict properties and behavior. A decrease of thermal conductivity during irradiation was demonstrated. The influence of irradiation flux, the exposure time, and the grain microstructure were investigated. If the initial GB microstructure was isotropic, the thermal conductivity under irradiation would be similarly isotropic. If the initial GB configuration was anisotropic, anisotropy of thermal conductivity would intensify under irradiation as gas bubbles coalesce around GB areas. The prediction of microstructure and property evolution of polycrystalline materials under irradiation by bridging two models in different scales were demonstrated successfully. This approach provides a deep understanding from a basic scientific viewpoint.

Structural, microstructural, transport, and magnetotransport properties of nanostructured La0.7Sr0.3MnO3 manganites synthesized by coprecipitation

The structural, transport, and magnetotransport properties of single-phase, homogeneous nanostructured La0.7Sr0.3MnO3 (LSMO) manganites synthesized by the coprecipitation route were investigated and the effect of sintering temperature on the microstructure of LSMO compounds was studied. A strong dependence of transport and magnetotransport behavior on the microstructure and nature of grain boundaries has been observed in the single-phase LSMO sintered at various temperatures. High-field magnetoresistance (HFMR) at room temperature is found to increase [13% (LS6) to 25% (LS9)] while low temperature (5 K) magnetoresistance decreases [75% (LS6) to 46% (LS9)] under 9 T field with increase in sintering temperature, which has been attributed to the spin-polarized tunneling and spin-dependent scattering of charge carriers.

Influence of Nd Oxide Phase on the Coercivity of Nd-Fe-B Thin Films

To understand the coercivity mechanism of Nd-Fe-B sintered magnets, the microstructure of grain boundary composed of Nd2Fe14B and Nd-rich phases has been studied. However, the influence of Nd-rich phase, which contains some amount of oxygen, on microstructure and coercivity has not been clear. In this study, the influence and the interfacial microstructure between the Nd2Fe14B phase and the Nd-rich phase were investigated using Nd-Fe-B thin films. Furthermore, the microstructural change of Nd-oxide (Nd-O) phase was investigated using oxidized Nd thin films. The coercivity (HcJ) of the Nd-Fe-B thin films decreased by about 80% from the level of as-deposited film (HcJ(as-depo)) after oxidation and annealing at low temperature (∼350°C). From TEM observation of the Nd-Fe-B film, some steps along the surface of the Nd2Fe14B phase contacting with the hcp Nd2O3 phase were observed. Investigation of the microstructural change of Nd oxide phase was carried out using Nd thin films. The as-deposited Nd film was composed of the dhcp Nd (α-Nd) phase, and the fcc NdOx phase formed at the surface of α-Nd phase after oxidation. After annealing at 350°C, the hcp Nd2O3 phase crystallized from the fcc NdOx phase, and it resulted in large roughness at the boundary with the α-Nd phase. From the results described above, the crystallization of hcp Nd2O3 phase causes damage at the surface of Nd2Fe14B phase during the annealing at low temperature, which results in the decrease of coercivity.

Evolution of Grain-Boundary Microstructure and Texture in Interstitial-Free Steel Processed by Equal-Channel Angular Extrusion

The equal-channel angular extrusion (ECAE) of Ti-bearing interstitial-free (IF) steel was performed following two different routes, up to four passes, at a temperature of 300 °C. The ECAE led to a grain refinement to submicron size. After the second pass, the grain size attained saturation thereafter. The microstructural analysis indicated the presence of coincident-site lattice (CSL) boundaries in significant fraction, in addition to a high volume fraction of high-angle random boundaries and some low-angle boundaries after the deformation. Among the special boundaries, Σ3 and Σ13 were the most prominent ones and their fraction depended on the processing route followed. A deviation in the misorientation angle distribution from the Mackenzie distribution was noticed. The crystallographic texture after the first pass resembled that of simple shear, with the {112}, {110}, and {123} aligned to the macroscopic shear plane.

Fundamental interaction process between pure edge dislocation and energetically stable grain boundary

The interaction between dislocations and grain boundaries is the principal factor for determining the mechanical properties and the plastic deformation behavior of metals. It is possible to control the grain-boundary microstructure and the macroscopic behavior has been widely exploited for scientific and industrial applications. In atomic scale, however, specific interaction characteristics such as the reaction energy and pathway have yet to be revealed. We have investigated the interaction process between a dislocation and an energetically stable grain boundary, and the quantitative characteristics were determined via atomistic transition state analysis. As a result, the interaction energy is found to be $1.16\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}1}\text{ }\text{eV}/\text{\AA{}}$, which is ${10}^{4}$ times higher than the Peierls potential. The lattice dislocations subsequently experience anomalous dissociations on the grain boundary, which becomes a key factor for the previously unexplained dislocation disappearance and grain-boundary migration.

Effects of Fe, Ti, and V doping on the microstructure and electrical properties of grain and grain boundary of giant dielectric NiO-based ceramics

We report the giant dielectric response and electrical properties of Li0.05B0.02Ni0.93O (B=Fe, Ti, and V) ceramics prepared by a polymer pyrolysis route. The giant dielectric response in these materials can be ascribed based on the Maxwell–Wagner polarization and thermally activated mechanisms. It is found that Fe, Ti, and V doping has a strong effect on the microstructure and the conduction of grains and grain boundaries of these NiO-based ceramic systems, which make large contribution to their dielectric properties.