Grain Boundary Junctions Research Articles

The Causes of Formation of the Triple Junctions of Grain Boundaries Containing Excess Free Volume in fcc Metals at Crystallization

The formation conditions of strained (non-equilibrium) triple junctions of grain boundaries were studied by the method of molecular dynamics. It is shown that strained triple junctions, containing excess free volume, mainly forms during crystallization process in the result of "locking" of the liquid phase density at a meeting of the three crystallization fronts and, as a consequence, of the concentration of excess free volume in the triple junction after solidification.

Effects of electron-beam irradiation on the transition properties of YBa2Cu3O7−d grain boundary junctions

We have studied the effects of high-energy electron-beam irradiation on the transition properties of superconducting YBa2Cu3O7−d (YBCO) grain boundary junctions, bicrystal junctions and step-edge junctions, on SrTiO3 substrates. A uniform 1-MeV electron beam irradiated all over the samples. The irradiation doses were 0, 4.7 × 1014, 4.7 × 1015, and 4.7 × 1016 e/cm2. For each junction type, we used at least two samples for each dose level and compared the transition parameters before and after irradiation. For comparison, we also studied the same irradiation effects for YBCO microbridges. We measured the resistive transition temperature, the current-voltage characteristics, the normal-state resistance, and the critical current. The effect of irradiation was the most significant for the bicrystal grain-boundary junction and the least significant for the microbridges. The critical current data for the YBCO bicrystal grain-boundary junction showed a maximum at (0.47 ∼ 0.9) × 1015 e/cm2, and those for the microbridges showed a monotonic decrease with increasing dose. The normal-state resistance increased monotonically with increasing dose for all samples by up to ∼40% for the microbridges and ∼20% for the grain-boundary junctions at 4.7 × 1015 e/cm2. The change in the superconducting temperature (Tc) was negligible except for the bicrystal junction at 4.7 × 1016 e/cm2, which was not superconducting at 77 K. These results show that grain-boundary junctions are more susceptive to irradiation, indicating that their critical currents are controllable by using high-energy electron-beam irradiation.

Toughness and strength of nanocrystalline graphene.

Pristine monocrystalline graphene is claimed to be the strongest material known with remarkable mechanical and electrical properties. However, graphene made with scalable fabrication techniques is polycrystalline and contains inherent nanoscale line and point defects—grain boundaries and grain-boundary triple junctions—that lead to significant statistical fluctuations in toughness and strength. These fluctuations become particularly pronounced for nanocrystalline graphene where the density of defects is high. Here we use large-scale simulation and continuum modelling to show that the statistical variation in toughness and strength can be understood with ‘weakest-link' statistics. We develop the first statistical theory of toughness in polycrystalline graphene, and elucidate the nanoscale origins of the grain-size dependence of its strength and toughness. Our results should lead to more reliable graphene device design, and provide a framework to interpret experimental results in a broad class of two-dimensional materials.

Microstructure, Mechanical Properties and Electromagnetic Shielding Effectiveness of Mg-Y-Zr-Nd Alloy

Abstract The as-cast Mg-Y-Zr- x Nd alloys with different Nd contents were prepared, and the effects of Nd content on microstructure, mechanical properties and electromagnetic interference (EMI) shielding properties were investigated. The results show that the grain sizes are refined from 70.1 μm to 45.2 μm, discontinuous bone-shaped β phases are formed mostly in the triple junction of grain boundaries when the content of Nd increases to 2.63 wt%. The mechanical properties and EMI shielding capacity are both enhanced significantly by adding Nd element. The alloy with 2.63 wt% Nd has a good combination of mechanical properties and EMI properties. T6 heat treatment is able to further improve the EMI shielding effectiveness. The above-mentioned experiment results are attributed to the microstructural variation caused by adding different contents of Nd.

結晶粒界三重線における変形不適合の回位モデル

The geometrical concept of Volterra's disclination is applied to model the inhomogeneous deformation that occurs at the triple junction of grain boundaries in deformed polycrystal. It is shown that even though the deformation is restricted to maintain the strain compatibility on the three grain boundaries meeting at the triple junction, the misfit in the dihedral angles of the three grains would occur. The angular misfit gives the angle of the equivalent disclination that is accompanied by a logarithmic stress singularity at the origin, which is similar to the stress concentration at the triple junction in deformed tricrystals.

The inverse hall–petch relation in nanocrystalline metals: A discrete dislocation dynamics analysis

When the grain size in polycrystalline materials is reduced to the nanometer length scale (nanocrystallinity), observations from experiments and atomistic simulations suggest that the yield strength decreases (softening) as the grain size is decreased. This is in contrast to the Hall–Petch relation observed in larger sized grains. We incorporated grain boundary (GB) sliding and dislocation emission from GB junctions into the classical DDD framework, and recovered the smaller is weaker relationship observed in nanocrystalline materials. This current model shows that the inverse Hall–Petch behavior can be obtained through a relief of stress buildup at GB junctions from GB sliding by emitting dislocations from the junctions. The yield stress is shown to vary with grain size, d, by a d1/2 relationship when grain sizes are very small. However, pure GB sliding alone without further plastic accomodation by dislocation emission is grain size independent.

TiO2 doped UO2 fuels sintered by spark plasma sintering

UO2 fuels doped with oxide additives Cr2O3 and TiO2 display larger grain size, improving fission product retention capability and thus accident tolerance. Spark plasma sintering (SPS) was applied to consolidate TiO2-doped UO2 fuel pellets with 0.5 wt % dopant concentration, above its solubility, in order to induce eutectic phase formation and promote sintering kinetics. The grain size can reach 80 μm by sintering at 1700 °C for 20 min, and liquid U–Ti–O eutectic phase occurs at the triple junction of grain boundaries and significantly improves grain growth during sintering. The oxide additive also impedes the reduction of the initial hyperstoichiometric fuel powders to more stoichiometric fuel pellets upon SPS process. Thermal–mechanical properties of the sintered doped fuel pellets including thermal conductivity and hardness are measured and compared with undoped fuel pellets. The enlarged grain size (80 μm) and densification within short sintering duration highlight the immense possibility of SPS in fabricating large grained UO2 fuel pellets to improve fuel performance.

Crystallographic analysis of the solid-state dewetting of polycrystalline gold film using automated indexing in a transmission electron microscope

We analyzed the effect of crystallographic anisotropy on the morphological evolution of a 12-nm-thick gold film during solid-state dewetting at high temperatures using automated indexing tool in a transmission electron microscopy. Dewetting initiated at grain-boundary triple junctions adjacent to large grains resulting from abnormal grain growth driven by (111) texture development. Voids at the junctions developed shapes with faceted edges bounded by low-index crystal planes. The kinetic mobility of the edges varied with the crystal orientation normal to the edges, with a predominance of specific edges with the slowest retraction rates as the annealing time was increased.

Microstructure and tensile creep resistance of Mg-5.5%Zn-(0.7%, 1.5%, 3.5%, 7.5%)Y alloys

The tensile creep resistance of Mg-5.5%Zn-(0.7%, 1.5%, 3.5%, 7.5%)Y (mass fraction, %) gravity-casting alloys was investigated systematically. The corresponding physical models were established for analyzing the microstructure evolution and creep mechanism. The results show that four second phases are found in Mg-5.5%Zn-(0.7%, 1.5%, 3.5%, 7.5%)Y alloys, including Mg3ZnY, Mg3Zn6Y, Mg3Zn3Y2 and Mg7Zn3, where the rare earth rich phase (Mg3ZnY, Mg3Zn6Y, Mg3Zn3Y2) with high melting point can more effectively improve the creep resistance properties of alloys than Mg7Zn3. With the increasing of Y content, the creep resistance of alloys is improved correspondingly. The alloys with (1.5%, 3.5%)Y addition exhibit high creep resistance at temperatures from 175 °C to 200 °C and load from 55 MPa to 70 MPa. The 7.5%Y added alloy presents excellent creep resistance even at 275 °C and 55 MPa. The second phase which shows discontinuous distribution at the grain boundary of (0.7%, 1.5%, 3.5%)Y added alloys has preferred orientation and clogs in triple junctions of grain boundary. Simultaneously, the arrangement of second phase particles along tensile direction and the formation of denuded zones are observed during the creep process. Moreover, the crack initiates in these areas and propagates along grain boundary. Compared with discontinuous second phase, the continuous skeleton-like second phase of 7.5%Y added alloy at grain boundary has a better effect on improving the creep resistance properties of alloys.

Microwave Losses in YBCO Coplanar Waveguide Resonators at Low Power and Millikelvin Range

We have investigated the temperature dependence of microwave losses in the low-power limit and millikelvin temperature range of a thin-film YBa2Cu3O7-x (YBCO) λ/2 coplanar waveguide resonator patterned on a (La0.3Sr0.7) (Al0.65Ta0.35)O3 (LSAT) substrate covered by a CeO2 seed layer. The unloaded quality factor is mainly governed by the surface resistance of the YBCO film and the dielectric losses caused by resonant absorption due to a bath of two-level fluctuators in the dielectrics. The value of the unloaded quality factor at 20 mK and low power, i.e., Q0∼2000, allows for the realization of YBCO microwave quantum circuits implementing biepitaxial grain-boundary Josephson junctions.

Nanoanalytical TEM Studies and Micromagnetic Modelling of Nd-Fe-B Magnets

We have analysed the influence of the microstructural features, such as intergranular grain boundary (GB) phases and misalignment of the hard magnetic grains, on the optimization of magnetization reversal processes in order to improve the coercive field of Nd-Fe-B magnets.The microstructural model of the grains and intergranular phases, which is used for theoretical simulations, has been derived from a detailed nanoanalytical TEM/STEM study of a Dy/Tb free magnet and a high coercive (Nd,Tb)-Fe-B magnet. Special attention is laid on the EELS analysis of GB with a thickness ranging from 2 - 30nm. This analysis identified the majority of the GB phases to have about 50 -70 at.% of iron and only a few GBs, which are connecting two nearby grain boundary junctions (GBj), possess a similar chemical composition as the adjacent GBj with a low iron content (< 10 at. %) and a high rare earth and oxygen content.Finite element micromagnetic simulations have been carried out in order to study the influence of internal demagnetizing fields determined by the microstructure on the magnetization switching behaviour. Special emphasis was put on the influence of the GB and their magnetic properties, due to their substantial influence on the nucleation of reverse magnetic domains and the pinning of domain walls. The strongest reduction of the coercive field is caused by GB with soft ferromagnetic properties. Shielding the Nd-Fe-B grains from the nucleation sites at the GBj with Dy or Tb shells, leads to an increase of the coercivity from 2.5 to 3.6 T and 2.5 to 4.3 T, respectively.

Analysis of the study of the fine structure of the cold-deformed 20 steel before and after the pre-recrystallizational heat treatment

The article deals with the influence of the prerecrystallization heat treatment on the fine structure of cold-deformed mild steel 20 by the help of metallography and X-ray structure analysis. It was determined experimentally that the prerecrystallization heat treatment of the plastically deformed by 60 % steel 20 at 500°С for 1.5 minutes leads to the grinding of the blocks sizes (with the CSR reduced from 139 nm after deformation to 87 nm after the prerecrystallization heat treatment). At that, the reduction of the level of the internal stresses by 20 % occurs. The prerecrystallization heat treatment of steel 20 does not result in complete removal of the long-range stress fields formed in the structure during the plastic deformation. The sources of the long-range stress fields are boundaries and junctions of grain and subgrain boundaries. The dislocation density is reduced by 57 % due to the restructuring and ordering of the substructure, and relaxation processes which occur when heating the plastically deformed steel. It was determined that the prerecrystallization heat treatment at 500° C with a 1.5 minute hold enables grinding the plastically deformed substructure (by 60 %) of the steel 20 by ordering defects, forming dislocation networks and subgrains with low-angle boundaries, providing the enhancement of the physical and mechanical properties.

Effect of nanograin boundary sliding on nanovoid growth by dislocation shear loop emission in nanocrystalline materials

A theoretical model is suggested to describe the effect of nanograin boundary (NGB) sliding at triple junctions of grain boundaries on nanovoid growth by incipient emission of dislocation shear loop from its surface in mechanically loaded nanocrystalline materials. In the framework of the model, NGB sliding deformation represents generation of disclination dipoles at triple junctions of NGBs. The analytical expression of critical stress for the first dislocation emission is derived. The effects of NGB sliding deformation, the sizes of nanograin and nanovoid on critical stress and the corresponding most favorable slip plane for dislocation emission are evaluated quantitatively in the deformed nanocrystalline materials. The results indicate that NGB sliding deformation releases, in part, the high stresses near the nanovoid, thereby nanovoid growth is slowed down or even arrested with the increment of the strength of NGB sliding deformation in nanocrystalline solids. There exists a range of nanovoid size to make critical stress keep almost unchanged with the appearance of NGB sliding deformation. There is also a critical nanograin size to make critical stress reach the maximum value, at which dislocation emission and nanovoid growth occur most difficultly. • Suggest a theoretical model to describe effect of NGB sliding on nanovoid growth. • Derive analytical expression of critical stress for the first dislocation emission. • Nanovoid growth is slowed down or even arrested due to NGB sliding deformation. • Effect of NGB sliding deformation on critical stress is evaluated. • Effect of sizes of nanograin and nanovoid on critical stress is studied.

Void formation of nanocrystalline materials at the triple junction of grain boundaries

A theoretical model of void formation at the triple junction of grain boundaries is described. Based on the combined effects of grain boundary diffusional creep and triple junction diffusional creep as well as dislocation climb, void formation time and void growth rate are derived. The results indicate that vacancy concentration increases with increasing creep strain rate and the angle of the Burgers vector to its dislocation line, and with decreasing grain size. It sharply declines at low creep strain rates, then the asymptotic behavior approaches a constant at high rates. It is also found that the dislocation density is noticeable for small grain sizes in nanocrystalline Cu, and the void growth rate decreases with creep strain rate and time, which are qualitatively consistent with the conclusions in previous work (Dongare et al 2010 J. Appl. Phys. 108 113518; Du et al 2010 Mater. Sci. Eng. A 527 4837).

Kinetic Model of the γ to α Phase Transformation at Grain Boundaries in Boron-bearing Low-alloy Steel

The effect of boron on the nucleation and growth of ferrite at austenite grain boundaries is examined theoretically under the assumption that the junction of 4-austenite grain boundaries (i.e., the 4-grain junctions) are the dominant nucleation sites of ferrite. Boron segregates to the austenite grain boundaries and reduces the grain-boundary energy; it thereby retards ferrite nucleation at the grain boundary. The retardation is expressed as a decrease in nucleation density due to an increase in the critical activation energy for nucleation, and the calculated value of the fraction of active nucleation sites is in satisfactory agreement with the experimental results. The reduction of the austenite grain-boundary energy, which we obtained by applying the Gibbs isotherm for adsorption to the boron segregation, is of the same order of magnitude as the reduction is deduced from the results of calculations for a decrease in the nucleation density based on experimental results. The growth of ferrite was calculated using DICTRA, which yielded both the volume fraction and the grain size of transformed ferrite as functions of time; the results agreed with the experimental results. This agreement suggests that the influence of boron on the growth rate is negligible. However, the increase in the size of the diffusion cell due to the addition of boron is considered to be the main reason for the slightly larger grain size of ferrite compared with that in boron-free steel; this result is also in good agreement with experimental observations.

The effect of bias current asymmetry on the flux-flow steps in the grain boundary YBaCuO long Josephson junctions

The current-voltage characteristics and critical current versus magnetic field dependence of long 24°[001]-tilt YBa2Cu3O7−δ bicrystal grain-boundary junctions are studied both experimentally and theoretically. For the opposite magnetic field directions, the flux-flow steps with significantly different height and slope are observed. It is demonstrated that the most probable reason of this discrepancy is recently predicted asymmetry of spatial bias current distribution due to crystallographic anisotropy of bicrystal substrates [Kupriyanov et al., JETP Lett. 95, 289 (2012)].

Bicrystalline Grain Boundary and Hybrid SNS Junctions Based on Ba-122 Thin Films

To investigate the properties of iron-based superconductors, we examined different kinds of Josephson junctions using Co-doped Ba-122 thin films. Based on bicrystalline SrTiO3 substrates we prepared grain boundary (GB) junctions, which showed clear Josephson effects. We present electrical measurements at varying temperatures and magnetic fields. Furthermore, we prepared hybrid junctions using thin film technique. Confined by a Ba-122 base electrode (S) and a PbIn counter electrode (S') two different geometries of hybrid junctions are formed: a planar SNS' junction with a gold barrier (N) in between and an edge-type junction with an interface-engineered barrier. All three kinds of junctions showed asymmetric resistively shunted junction-like (RSJ) behavior with ICRN products of 20.2 /mu V (GB junctions), 18.4 /mu V (planar SNS' junctions) and 12.3 /mu V (edge-type junctions), respectively. An excess current could be observed at all junctions.

Influence of grain boundary sliding and grain size on dislocation emission from a crack tip

This article establishes a theoretical model to study the effect of grain boundary sliding deformation on the dislocation emission from the crack tip in nanocrystalline solids. Within our description, stress concentration near the crack initiates grain boundary sliding which results in the formation the dipole of disclinations at triple junctions of grain boundaries. The grain size-dependent criterion for the dislocation emission from a semi-infinite crack tip is derived. The influence of the grain size and the features of dipole of disclinations on the critical stress intensity factors for dislocation emission is evaluated. It is shown that grain boundary sliding deformation enhances the critical stress intensity factor for dislocation emission. The critical stress intensity factor will increase with the decrement of the grain size, which indicates that the emission of the dislocation becomes more difficult for smaller size of grain due to the effect of grain boundary sliding deformation.

Atomistic simulation study of brittle failure in nanocrystalline graphene under uniaxial tension

We show that, using molecular dynamic simulations, nanocrystalline (NC) graphene fails by brittle fracture along grain boundaries under uniaxial tension at room temperature. Initiated from either a grain-boundary triple junction or an array of vacancies on a preferential grain boundary, fracture occurs by unzipping atomic bonds along a preferential grain boundary. In sharp contrast to NC metals, no mobile dislocations are generated throughout the entire loading process, and the deformation remains fully elastic (albeit nonlinear) until the breaking of the first atomic bond due to high local stress near the initiation defect sites. Breaking of the first atomic bond triggers a cascade of bond breaking events along a preferential grain boundary that leads to the final brittle fracture failure. For the NC graphene monolayer sheet with an average grain size of ∼25 nm considered here, the predicted uniaxial tensile strength is 96.2 ± 4.2 GPa, which is one of the highest among all polycrystalline materials.

Study on Dependence of Ductile Fracture Progress on Grain-Boundary Due to FCC Tri-Crystal Model

Based on shock wave theory one of the authors derived the criterion of the micro-crack nucleation, namely the micro-crack nucleation is caused by the jump of the velocity along the intersected crossing line between two different stationary discontinuity bands characterized by vanishing velocity of an acceleration wave. In the previous paper, to consider dependence of the progress of ductile fracture of crystal solids on crystal orientations, the algorithm of acoustic tensors derived from the proposed model was built into finite element crystal plasticity model (FEPM) and the progresses of the ductile fracture in FCC bi-crystal and tri-crystal were analyzed. Then, the role of the crystal orientation and the grain boundary in the trigger of the transcrystalline fracture or intercrystalline fracture in the bi-crystal and also the role of the grain-boundary triple junction in the trigger of the ductile fracture in the tri-crystal were studied. In the present paper, the dependence of the ductile fracture progress on the grain-boundary is examined due to the FCC tri-crystal model consisted with 3 grains which are chacterized by single slip system, in-plane 4 slip systems and out-of-plane 4 slip systems.