Grain Boundary Volume Research Articles

Combined effects of nonmetallic impurities and planned metallic dopants on grain boundary energy and strength

Most research on nanocrystalline alloys has been focused on planned doping of metals with other metallic elements, but nonmetallic impurities are also prevalent in the real world. In this work, we report on the combined effects of metallic dopants and nonmetallic impurities on grain boundary energy and strength using first-principles calculations, with a Σ5 (310) grain boundary in Cu chosen as a model system. We find a clear correlation between the grain boundary energy and the change in excess free volume of doped grain boundaries. A combination of a larger substitutional dopant and an interstitial impurity can fill the excess free volume more efficiently and further reduce the grain boundary energy. We also find that the strengthening effects of dopants and impurities are dominated by the electronic interactions between the host Cu atoms and the two types of dopant elements. For example, the significant competing effects of metal dopants such as Zr, Nb, and Mo with impurities on the grain boundary strength are uncovered from the density of states of the d electrons. As a whole, this work deepens the field's understanding of the interaction between metallic dopants and nonmetallic impurities on grain boundary properties, providing a guide for improving the thermal stability of materials while avoiding embrittling effects.

Synthesis and electrical characterization of Ba5Nb4O15 and Ba5Nb3.9M0.1O(15-δ) (M = Ti, Zr) hexagonal perovskites

The 5H-type hexagonal perovskite Ba5Nb4O15 was synthesized by solid-state reaction and the Pechini method and acceptor-doped phases Ba5Nb3.9M0.1O15-δ (M = Ti, Zr) by the former route. Monophasic doped and undoped material was obtained on final sintering at 1450 °C for 4 h with the M-doped samples leading to a reduced c-parameter. Scanning electron microscopy revealed a morphology of interlacing elongated plaques for all compositions, but with reduced grain size for the doped compositions. The highest density (95.1%) was observed for the Pechini-synthesized phase, whereas Zr doping had a deleterious effect on densification. The electrical transport properties were measured in H2O- and D2O-humidified air in the temperature range 300–550 °C by impedance spectroscopy. The presence of a H+/D+ isotope effect confirms the presence of proton conductivity in this structure type. Ba5Nb4O15 synthesized by the Pechini method exhibits a conductivity of 8.4 × 10−6 S.cm−1 at 550 °C in humidified air with an activation energy of 0.75 eV. The M-doped samples presented lower total conductivities, most probably due to the larger grain-boundary volume associated with their smaller grain size.

Influence of microstructure on thermal stability of ultrafine-grained Cu processed by equal channel angular pressing

The well-established enhanced mechanical performance of ultrafine-grained (UFG) materials is often accompanied by poor thermal stability which precludes their use in some applications. To provide fundamental insight into the problem of thermal stability, we studied the microstructure of two different UFG microstructures in Cu: one with lamellar grains and low-angle grain boundaries (GBs) and another with equiaxed grains and high-angle GBs. These distinct microstructures were obtained using equal channel angular pressing (ECAP) for 2 and 16 passes, respectively. Our results show that both microstructures exhibited a similar yield strength, but the material processed using two passes (ECAP-2) exhibited a higher thermal stability when compared to that of ECAP-16. Thermodynamic calculations indicate that the ECAP-2 Cu has a lower stored energy (0.43 J/g) relative to that of the ECAP-16 Cu (0.66 J/g) because the former microstructure has lower GB energy and lower GB volume fraction relative to the latter one. Kinetic analysis revealed that the ECAP-2 Cu has higher activation energy of recrystallization (94 kJ/mol) as compared to that corresponding to the ECAP-16 Cu (72 kJ/mol).

Influence of oxygen vacancies on the structural, dielectric, and magnetic properties of (Mn, Co) co-doped ZnO nanostructures

We analysed the variation and effect of oxygen vacancies on the structural, dielectric and magnetic properties in case of Mn (4%) and Co (1, 2 and 4%) co-doped ZnO nanoparticles (NPs), synthesized by chemical precipitation route and annealed at 750 °C for 2 h. From the XRD, the calculated average crystallite size increased from15.30 ± 0.73 nm to 16.71 ± 012 nm, when Co content is increased from 1 to 4%. Enhancement of dopants (Mn, Co) introduced more and more oxygen vacancies to ZnO lattice confirmed from EDX and XPS. The high-temperature annealing leads to reduction of the dielectric properties due to enhancement in grain growth (large grain volume and lesser number of grain boundaries) with the incorporation of Co and Mn ions into the ZnO lattice. The electrical conductivity of the Mn doped and (Mn, Co) co-doped ZnO samples were enhanced due to increase in the volume of conducting grains and charge density (liberation of trapped charge carriers in oxygen vacancies and free charge carriers at higher frequencies). The Mn-doped and (Mn, Co) co-doped ZnO NPs show ferromagnetic (FM) behaviour. The saturation and remnant magnetizations (Ms and Mr) elevates from (0.235 to 1.489) × 10−2 and (0.12 to 0.27) × 10−2 emu/g while Coercivity (Hc) reduced from 97 to 36 Oe with enhancement in the concentration of dopants in ZnO matrix. Oxygen vacancies were found to be the main reason for room-temperature ferromagnetism (RTFM) in the doped and co-doped ZnO NPs. The results show that the enhanced dielectric and magnetic properties of Mn doped and (Mn, Co) co-doped ZnO is strongly correlated with the concentration of oxygen vacancies. The observed enhanced RTFM, dielectric properties and electrical conductivity makes TM doped ZnO nanoparticles suitable for spintronics, microelectronics and optoelectronics based applications.

Σ3(111)grain boundary of body-centered cubic Ti-Mo and Ti-V alloys: First-principles and model calculations

The energetics and atomic structures of $\mathrm{\ensuremath{\Sigma}}3[1\overline{1}0](111)$ grain boundary (GB) of body-centered cubic (bcc) Ti-Mo and Ti-V alloys are investigated using density-functional-theory calculations and virtual crystal approximation. The electron density in bcc structure and the atomic displacements and excess energy of the GB are correlated to bcc-$\ensuremath{\omega}$ phase stability. Model calculations based on pairwise interplanar interactions successfully reproduce the chemical part of GB energy. The chemical GB energy can be expressed as a sum of excess pairwise interactions between bcc (111) layers, which are obtained from Gaussian elimination of the total energies of a number of periodic structures. The energy associated with the relaxation near the GB is solved by numerical minimization using the derivatives of the excess interactions. Anharmonic interlayer interactions are necessary for obtaining accurate relaxation energy and excess GB volume from model calculations. The effect of GB on vibrational spectrum is also investigated. Segregation energies of B and Y to a substitutional site on the GB plane are calculated. Preliminary results suggest that Y tends to segregate, while B tends to antisegregate.

The improvement of toughness and tribological properties of niobium nitride film by addition of copper

NbN/Cu composite films with different copper contents were synthesized by pulsed bias ion plating and the influence of copper contents on the crystal structure, toughness and tribological properties at room temperature was investigated. The results showed that three-phase of face-centered cubic (fcc) NbN, hexagonal close-packed (hcp) NbN and fcc-copper coexisted in the films. Addition of copper enhanced toughness from 0.14 MPa m1/2 at 0 at.% copper to 0.73 MPa m1/2 at 25.1 at.% copper due to the appearance a large volume of grain boundaries, while the hardness decreased gradually with the increase of copper contents. The average friction coefficient dropped from 0.81 at 0 at.% copper to 0.58 at 25.1 at.% copper, and the wear rate (WR) first decreased until a minimum, but a further increase in copper content led to the increase of the WR. The minimum value of WR was 1.2 × 10−8 mm3/N·mm at 5.2 at.% copper.

Positron Lifetime Calculation for Plastic Deformed Nanocrystalline Copper

Positron lifetime calculation has been performed on a computer-generated nanocrystalline copper with a mean grain size of 9.1 nm during its deformation. For the undeformed and deformed nanocrystalline copper, calculated positron lifetimes are around 157 ps which come from the positron annihilation in the free volume in grain boundaries. Due to the grain-boundary deformation mechanism, no vacancies or vacancy clusters will be induced in grains during the plastic deformation of the nanocrystalline copper, which is different to the deformation of the conventional polycrystal. From this point of view, in-situ positron annihilation measurements can provide important experimental information on the deformation mechanism of nanocrystalline metals.

Effect of sintering conditions on the electrical-transport properties of the SrZrO3-based protonic ceramic electrolyser membrane

Abstract The effects of sintering temperature and addition of 4 mol.% ZnO as sintering additive on the crystal structure, microstructure and electrical properties of SrZr 0.9 Y 0.1 O 3-δ are reported. The presence of ZnO as sintering aid brings about high densification at 1300 °C (relative density ∼97%); gas-tightness is not achieved for ZnO-free samples sintered below 1600 °C. Bulk conductivity (σ B ) is considerably higher in wet and dry O 2 on doping with ZnO, but only slight variations of σ B with sintering temperature are observed for the Zn-containing phases. Similarly, the apparent grain-boundary conductivities are much greater for the Zn-doped samples. The grain-boundary volume and accompanying resistances are much reduced on sintering at 1500 °C with ZnO addition in comparison to Zn-modified samples sintered below 1500 °C, with only minor changes in grain-boundary relaxation frequency observed. Conversely, in comparison to the undoped sample with sintering temperature of 1600 °C, there is an enormous improvement in the specific grain-boundary conductivity of two orders of magnitude for the ZnO-containing samples. Analysis on the basis of the core space-charge-layer model relates the enhancement of the grain-boundary transport to a higher concentration of charge carriers in the space-charge layer and associated lower potential barrier heights.

Grain size effect on stress hysteresis of nanocrystalline NiTi alloys

In this paper, the grain size effect on the stress hysteresis of a nanocrystalline NiTi alloy during superelastic cycling was investigated by testing two types of samples with 39% and 75% cold-drawn area reduction. The TEM microstructure and DSC analysis reveal that the sample with 75% area reduction experiences a crystallization process upon heating and contains few dislocations after annealing. The activation energy for grain boundary migration of the NiTi alloy was analysed and found to be 353.6 ± 15.3 kJ/mol. The hysteresis analysis shows that the stress hysteresis of the samples after annealing increases with decreasing grain size. We propose that the energy dissipation per volume of grain boundary is larger than that of the grain core in the nanocrystalline NiTi alloy, which contributes to this grain size effect. However, the sample with 39% area reduction does not experience the crystallization process upon heating, and many dislocations are encountered in the samples after annealing. The stress hysteresis of the samples with 39% area reduction after annealing decreases with increasing dislocation density. The results clarify the difference between the grain size effect and the dislocation density effect on the stress hysteresis of nanocrystalline NiTi alloys.

Improved dielectric properties and grain boundary response in neodymium-doped Y2/3Cu3Ti4O12 ceramics

Rare earth element neodymium was adopted to refine grain and in turn increase the volume of grain boundary of Y2/3Cu3Ti4O12 ceramics, which could strongly increase the resistance of grain boundary. Proper amount of Nd substitution in Y2/3−xNdxCu3Ti4O12 ceramics could significantly depress the low-frequency dielectric loss. When the doping level is 0.06 and 0.09, the samples exhibited a relatively low dielectric loss (below 0.050 between 0.3 and 50 kHz) and high dielectric constant above 11000 over a wide frequency range from 40 Hz to 100 kHz. Based on the ɛ′-T plots, dielectric relaxation intensity was substantially weakened by Nd doping so that the temperature stability of dielectric constant was improved obviously. The correlations between low-frequency dielectric loss and the resistance of grain boundary were revealed. After Nd doping, the activation energies for the conduction behavior in grain boundaries were significantly enhanced, and the activation energies for the dielectric relaxation process in grain boundaries were slightly influenced.

Origin of differences in the excess volume of copper and nickel grain boundaries

The excess volume associated with grain boundaries is one of the primary factors driving defect segregation and diffusion which controls the electronic, mechanical and chemical properties of many polycrystalline materials. Experimental measurements of the grain boundary excess volume of fcc metals Cu and Ni have shown a difference of over 40%. The difference in lattice constant between Cu and Ni is only 3%, therefore this substantial difference is currently lacking explanation. In this article we employ a high throughput computational approach to determine the atomic structure, formation energy and excess volume of a large number of tilt grain boundaries in Cu and Ni. By considering 400 distinct grain boundary orientations we confirm that theoretically there is a systematic difference between the excess volumes in the two materials and we provide atomistic insight into the origin of the effect.

Incorporating the grain boundary misorientation effects on slip activity into crystal plasticity

ABSTRACTThe role of grain boundary misorientation angle (GBMA) distribution on slip activity in a high-manganese austenitic steel was investigated through experiments and simulations. Crystal plasticity simulations incorporating the GBMA distribution and the corresponding dislocation–grain boundary interactions were conducted. The computational analysis revealed that the number of active slip systems decreased when GBMA distribution was taken into account owing to the larger volume of grain boundary–dislocation interactions. The current results demonstrate that the dislocation–grain boundary interactions significantly contribute to the overall hardening, and the GBMA distribution constitutes a key parameter dictating the slip activity.

Reactivity of materials towards carbon of graphite foil during Spark Plasma Sintering: A case study using Ni–W powders

In this study, the issue of reactivity of materials towards carbon of graphite foil during Spark Plasma Sintering has been addressed through a case study using Ni-15at% W powders prepared by mechanical alloying or simple mixing. Tungsten carbide WC was found at the flat ends of the compacts obtained from both mixtures. In case of the mechanically alloyed powder, carbon diffusion into the sintered compact resulted in the formation of Ni2W4C particles in a Ni-based matrix throughout the compact volume. In contrast to the compact obtained from the mechanically alloyed powder, no carbon-containing phases were detected in the interior of the compacts produced from mixed Ni+W powders sintered in the same conditions. The uptake of carbon caused by its inward diffusion was facilitated by a much higher concentration of defects and a greater volume of grain boundaries in the mechanically alloyed powder. The compacts Spark Plasma Sintered from the mechanically alloyed powders were much harder (620HV) than the compacts produced from the mixed powders (315–370HV) due to the presence of submicron Ni2W4C reinforcing particles distributed in a Ni-based solid solution-strengthened matrix.

Effect of low-temperature annealing on the creep of 1570 aluminum alloy

The effect of preliminary low-temperature annealing on the creep of a submicrocrystalline 1570 aluminum alloy fabricated by severe plastic deformation is studied. The creep rate is found to increase with the annealing time, but long-term annealing for 4 h decreases the creep rate to the value characteristic of the alloy not subjected to preliminary annealing. The increase in the creep rate of the alloy subjected to preliminary annealing is likely to be caused by an increase in the nonequilibrium excess volume in grain boundaries as a result of the dissolution of grain-boundary nanopores upon annealing and, hence, by an increase in the grain-boundary diffusion rate and the grain-boundary sliding rate.

Study of magnetic behavior in ball-milled nanocrystalline Fe-50 at.%Al alloy as a function of milling time

Ball milling technique has been extensively used to prepare different metastable states with nanocrystalline microstructures from intermetallic compounds. The present study was made on the identification of the changes in magnetic and electronic properties as a result of high-energy ball milling of Fe -50 at.% Al alloy samples. The phase formation and physical properties of the alloys were determined as a function of milling time by means of Mössbauer and X-ray photoelectron spectroscopy (XPS). The Mössbauer results show the formation of nanostructured body-centered cubic (BCC) FeAl alloy only after 5 h of mechanical milling and the same is also confirmed by Scanning electron microscope (SEM) and Transmission electron microscopy (TEM) studies. Mössbauer studies further confirm that there is magnetic behavior retention in the FeAl alloy samples even after 5 h of milling but magnetization decreases as the milling time increases. The reason for the same is due to the shocks and fracturing of the Al atoms embedded in the sites of Fe and as a result of which Fe – Fe nearest neighbors decreases. Secondly, with the increase in milling time, the particle size and the number density of equiatomic BCC Fe 50 Al 50 grains decrease while the volume of grain boundary containing a solid solution of BCC FeAl and concentration of Al in a solid solution of BCC FeAl at the grain boundary increases as a result of which magnetization decreases. The shift in the binding energy of Fe 2p and Al 2p core level towards higher binding energy also supports the alloy formation after milling.

Electrical properties of nanometric CGO-thin films prepared by electron-beam physical vapour deposition

Thin-film oxide-ion-conducting electrolytes are of considerable interest for the development of micro-solid oxide fuel cells (μ-SOFCs). In this study, nanocrystalline CGO thin films (with thickness ranging from 0.3 to 1.35 μm) were deposited from a Ce0.9Gd0.1O2−δ target by electron-beam physical vapour deposition onto a selection of substrates (Si, glass, fused silica and Al2O3) which were either maintained at room temperature or at 700 °C during deposition. Films have a columnar microstructure with (111) texture in the surface plane. The crystallinity of the films deposited at a substrate temperature of 700 °C was found to be much superior to those deposited at RT and subsequently heated. Impedance spectroscopy revealed that the in-plane resistivity of the films in air at 900 °C is greater than that of bulk CGO by around two orders of magnitude, with this difference increasing with decreasing temperature. The poor electrical properties of the nanocrystalline films may be considerably affected by the expected higher concentration of strains in comparison with the micrometric sample. The composition of the thin films was also affected by a de-doping process. The in-plane charge transport in films is limited by grain boundaries, mainly due to both a greater grain-boundary volume and higher specific grain-boundary resistivity. Conductivity measurements as a function of oxygen partial pressure reveal a considerably narrower electrolytic domain for the thin films than that of bulk CGO.

Comparison study of the magnetic permeability and dc conductivity of Co–Ni–Li ferrite nanoparticles and their bulk counterparts

The temperature dependence of relative permeability and dc electrical conductivity of nanosamples and their bulk counterparts of Co0.5Ni0.5−2xLixFe2+xO4 (from x=0.00 to 0.25 in step of 0.05) was investigated. The values of the relative permeability of the nano-samples are lower than their bulk counterparts as a result of porous and nano-grained structure besides the effect of the larger volume of grain boundaries in the nanosamples. Moreover, the dc conductivity of the nanosamples is higher than their bulk counterparts. This is probable explained according to the shorter metal–oxygen bonding length and higher lattice vibrations of the nanosamples. Also, the values of the relative permeability of both nano and bulk samples exhibit stability over a considerable range of temperatures. This may make them useful in practical applications that require stability. All the nanosamples show high rising Curie temperature values with increasing the Li content up to the sample of x=0.15 thereafter a decrease of the Curie temperature occurs while the inverse behavior was observed in their bulk counterparts. The interpretation of these findings is explained in the discussion. Moreover, in general doping Co–Ni ferrites with Li ions improves their electrical and magnetic properties and this is clearly observed in the nanosample of x=0.15 which can be regarded as the most promising sample for microwave applications.

Consolidation of cryomilled Al–Si using spark plasma sintering

Cryomilled eutectic aluminum–12% silicon powder was sintered using spark plasma sintering (SPS) to create bulk compacts. The cryomilling serves to break up and disperse the eutectic phase in the powder to create a well-distributed Si phase throughout the matrix and to modify the morphology of the Si phase from plate-like to spherical, whilst refining the aluminium grain size to the nanometric level. The effects of different sintering times and temperatures using SPS on the densification of the powder, the aluminium grain size evolution, the growth of the Si phase and the morphology change of the Si phase were investigated. The compacts were analysed using X-ray diffraction, scanning electron microscopy and optical microscopy. The initial stages of densification appear to be highly dependent on the yield strength of the powder. An estimate of the temperature gradient seen in the powder bed was made and calculated to be near 200 °C at the highest point. The Al and Si phase growth was investigated and it was observed that the Si coarsening rate is increased due to the increased volume of grain boundaries. As the Si coarsens, any pinning effect on the Al grains is lost, resulting in a highly unstable microstructure that coarsens rapidly.

Investigation on effects of CuO secondary phase on dielectric properties of CaCu3Ti4O12 ceramics

Two CaCu3Ti4O12 (CCTO) ceramics are separately sintered for 10h at 975 and 1000°C. The correlation between the microstructures and dielectric properties of the ceramics has been explored in this paper. The CuO secondary phase is separated out and locates at grain boundaries of the surface layer when the sintering temperature is raised up to 1000°C. The presence of the CuO phase highly reduces the grain-boundary thickness and helps the grain growth, which together result in the increase of the dielectric constant. The low-frequency dielectric loss decreases due to the density improvement of the CCTO ceramics, and the high-frequency dielectric loss increases due to the reduction of the grain-boundary volume. The dielectric loss as low as 0.074 is obtained from the ceramic pellet sintered at 1000°C.

Designing hierarchical porous features of ZSM-5 zeolites via Si/Al ratio and their dynamic behavior in seawater ion complexes

MFI-type zeolites were characterized using powder X-ray diffraction (XRD), and positron annihilation lifetime spectroscopy (PALS) to uncover the hierarchical porous properties with Si/Al ratios varying from 30 to >1000 (high silica MFI). Structural dynamics were observed in response to exposure of the zeolites to seawater solutions, mimicking service conditions for use in membrane desalination or other ionic solution applications (e.g. sensors). PALS results indicated MFI channels between 0.27 and 0.36nm. All materials had microporous grain boundaries of 1.1nm. Mesopores were also detected at approximately 8nm in size for all materials. However the volume of microporous grain boundaries reduced with increasing alumina as explained by increasing presence of cations. Seawater exposure had a strong and complex effect on all materials. Initially widely variable crystal dimensions were stabilized by seawater exposure. The highest alumina content ZSM-5 (Si/Al=30) demonstrated net exchange, replacing K+ from synthesis impurities with Na+ leading to little overall change in porous features. Higher silica materials, including high silica MFI, demonstrated an overall release of ions, principally Na+ and K+, which were liberated mainly from the zeolite channels, yet these materials adsorbed Ca2+ in the microporous grain boundaries and Mg2+ in the mesoporous grain boundaries. Si/Al=100 appeared to be a unique ratio, allowing the material to possess the same Na+ adsorption features (and hydrophilicity) of high alumina materials, but ion interactions resembling high silica materials. MFI-type zeolite is therefore shown to be highly configurable using Si/Al ratio for a wide variety of applications in the presence of ions.