Bicrystal Grain Boundary Research Articles

Critical current density behaviors across a grain boundary inclined to current with different angles in YBa2Cu3O7−δ bicrystal junctions**Project supported by the National Natural Science Foundation of China (Grant Nos. 61501222, 61371036, and 61571219) and the School Scientific Research Fund of Nanjing Institute of Technology, China (Grant Nos. YKJ201418).

The critical current density behaviors across a bicrystal grain boundary (GB) inclined to the current direction with different angles in YBa2Cu3O7−δ bicrystal junctions in magnetic fields are investigated. There are two main reasons for the difference in critical current density in junctions at different GB inclined angles in the same magnetic field: (i) the GB plane area determines the current carrying cross section; (ii) the vortex motion dynamics at the GB affects the critical current value when the vortex starts to move along the GB by Lorentz force. Furthermore, the vortex motion in a bicrystal GB is studied by investigating transverse (Hall) and longitudinal current–voltage characteristics (I–Vxx and I–Vxy). It is found that the I–Vxx curve diverges from linearity at a high driving current, while the I–Vxy curve keeps nearly linear, which indicates the vortices inside the GB break out of the GB by Lorentz force.

An atomic scale characterization of coupled grain boundary motion in silicon bicrystals

The mechanical response of symmetric tilt grain boundaries (GBs) in silicon bicrystals under shear loading are characterized using molecular dynamics simulations. It is seen that under shear, high-angle GBs namely Σ5 and Σ13 having a rotation axis [0 0 1] demonstrate coupled GB motion, such that the displacement of grains parallel to the GB interface is accompanied by normal GB motion. An atomic-scale characterization revealed that concerted rotations of silicon tetrahedra within the GB are the primary mechanisms leading to the coupled GB motion. Interestingly, so far, this phenomenon has only been examined in detail for metallic systems. A distinguishing feature of the coupled GB motion observed for the silicon symmetric tilt bicrystals as compared to metallic bicrystals is the fact that in the absence of shear, spontaneous coupled motion is not observed at high temperatures.

Atomic dynamics of grain boundaries in bulk nanocrystalline aluminium: A molecular dynamics simulation study

Dynamics of grain boundary (GB) atoms in the bulk nanocrystalline aluminium is investigated by means of a large-scale molecular dynamics simulation. It is found that the GB atoms in the nanocrystalline aluminium display the glassy dynamics. Their dynamic features are, on one hand, similar to those in the glass systems and bicrystal GBs, in terms of the time-correlation functions of mean-square displacement (MSD) and non-Gaussian parameter (NGP). But on the other hand, these GB atoms also demonstrate some different dynamic behaviors due to the more complicated microstructure. In particular, the immobile GB atoms are localized into their equilibrium positions due to the strong cage effect, and they are mainly at the surfaces of grains, especially the large grains, while the mobile GB atoms are active and can easily hop away from the cage around them. These mobile GB atoms gather together at the surfaces of small grains and the triple junction regions of GBs, and they form some abnormally big clusters. The size distribution of these clusters significantly deviates from the power law.

Atomic structure and chemistry of dislocation cores at low-angle tilt grain boundary in SrTiO3 bicrystals

Dislocations in perovskite oxides have been found to have important impacts on their electronic and ionic transportation properties, which are believed to be related to the structure and chemistry of dislocation cores. For dislocation cores at a 6° low-angle [001] tilt grain boundary in SrTiO3, an embedded TiOx rocksalt-like structure has been suggested, consistent with a deficiency of Sr. However, direct evidence supporting these suggestions has not been obtained up to now. In this work, we reveal the atomic structure and chemistry of edge dislocation cores at a low-angle [001] symmetric tilt-boundary in SrTiO3 bicrystals by imaging techniques of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and electron energy loss spectroscopy (EELS) with an FEI Titan cube3 60–300 (PICO) microscope operated at 80kV. The experimental results demonstrate direct evidence for a local coordination of edge-sharing TiO6 octahedra at the dislocation cores, which can be understood as the result of strain. The local coordination of edge-sharing TiO6 octahedra is associated with the face-centered cubic (FCC) NaCl-type TiO phase. The present study therefore provides a solid structural and chemical basis for understanding the properties of dislocations.

Thermally driven grain boundary migration and melting in Cu.

With molecular dynamics simulations, we systematically investigate melting of a set of Σ3〈110〉70.53° tilt grain boundaries (GB) in Cu bicrystals, including coherent twin boundaries (CTBs), 12 asymmetric tilt grain boundaries (ATGBs), and symmetric incoherent twin boundaries (SITBs), in the order of increasing length weight of SITB or GB energy. ATGBs decompose into CTBs and SITBs, which migrate and coalesce as a result of internal stress relaxation. GBs can be superheated or premelted, and GB melting temperature decreases exponentially with increasing SITB weight, owing to the systematics in GB microstructure. GB melting nucleates at disordered CTB-SITB junctions, and grows along SITBs and then into grain interiors, with the solid-liquid interfaces preferentially aligned with {111}.

Molecular dynamics simulation of fracture behaviors of 〈110〈 tilt grain boundaries in γ-TiAl

Molecular dynamics (MD) simulations were carried out to study the fracture behaviors of several symmetric tilt grain boundaries in γ-TiAl bicrystals with 〈110〈 misorientation axes. Tensile deformation along direction perpendicular to grain boundary was simulated under various strain rates and temperatures. The results indicate that the relative orientation of the grains and the presence of certain atom units are two critical factors of the interface structure affecting the stress required for dislocation nucleation. Dislocations nucleate and extend at or near the symmetric tilt grain boundaries during the tensile deformation of Σ3 (111) 109.5°, Σ9 (221) 141.1° and Σ27 (552) 148.4° interfaces. For Σ27 (115) 31.6° and Σ11 (113) 50.5° interfaces, the interfaces fractured directly in a cleavage manner due to no dislocation emitted from the boundary. The tensile fracture mechanisms of the bicrystals are that micro-cracks nucleate at the grain boundary and propagate along the interface. The variance of crack propagation is whether there is accommodation of plastic region at the crack tips.

Two-dimensional phase-field simulations of dendrite competitive growth during the directional solidification of a binary alloy bicrystal

We investigated the competitive growth of dendrites at the converging grain boundaries (GBs) of bicrystals during the directional solidification of an Al–Cu alloy by means of two-dimensional phase-field simulations. In particular, the focus was on the recently observed phenomenon of unusual overgrowth during the directional solidification of a Ni-based superalloy, where the favorably oriented (FO) dendrites are overgrown by the unfavorably oriented (UO) ones. The phase-field simulations were accelerated by parallel computations on graphics processing units. The simulation results showed that unusual overgrowth occurs in Al–Cu alloys, indicating that this phenomenon is a common one in metallic materials. It was also concluded that the differences in the diffusion layers in front of the FO and UO dendrites had a dominant effect on the competitive growth of dendrites at the converging GB as well as on the unusual overgrowth. In addition, unusual overgrowth was observed in all the FO dendrites with a spacing that allowed the dendrite array to grow stably without necessitating a change in the number of dendrites. The FO dendrite at the GB is overgrown by the UO dendrite when the spacing between the FO dendrite at the GB and the next FO dendrite is approximately equal to the critical minimum spacing. However, the unusual overgrowth was not observed for UO dendrites with a large inclination angle. In this case, all the UO dendrites are blocked by the FO dendrite at the GB, and the FO dendrites migrate toward the UO dendrites.

Molecular statics simulations of intergranular fracture along Σ11 tilt grain boundaries in copper bicrystals

The intrinsic brittleness and ductility of intergranular cracks in Cu bicrystals under quasi-static loading conditions are investigated using molecular statics simulations. Central cracks are inserted into nine Σ11\( [1 \, 1 \, 0] \) tilt grain boundaries with different inclination angles, seven of which are asymmetric, and two of which are symmetric. On the basis of results of molecular statics simulations, critical energy release rates at material failure are calculated using the corresponding continuum counterparts with the finite element method. Simulation results are compared to predictions made on the basis of Rice’s model, and the applicable range of Rice’s model is given in terms of inclination angle. The effects of inclination angle on the mechanical properties of grain boundaries and fracture mechanisms of the corresponding intergranular cracks are analyzed in detail.

Effect of Grain Boundary Energy Anisotropy on Faceting and Migration of Low Angle Grain Boundaries

The faceting and migration behavior of low angle <100> grain boundaries in high purity aluminum bicrystals was investigated. In-situ technique based on orientation contrast imaging was applied. In contrast to the pure tilt boundaries, which remained straight/flat and immobile during annealing at elevated temperatures, mixed tilt-twist boundaries readily assumed a curved shape and steadily moved under the capillary force. Computational analysis revealed that this behavior is due to the inclinational anisotropy of grain boundary energy, which in turn depends on boundary geometry – the energy of pure tilt low angle <100> boundaries is anisotropic, whereas that of mixed tilt-twist boundaries isotropic with respect to boundary inclination.

Deformation study of bicrystalline and nano-polycrystalline structures using phase field crystal method

Deformation behaviors of bicrystalline and nano-polycrystalline structures of various tilt angles and inclination angles in two dimensions are investigated in detail using a two-mode phase field crystal model. The interaction between grain boundary (GB) and dislocation is also examined in bicrystals and nano-polycrystals that both contain asymmetric and symmetric tilt GBs, with energy analysis being carried out to analyze these processes. During deformation simulations, we assume the volume of each simulation cell at every time step is coincident with that of the initial state just before deformation. Our simulation results show that the behaviors of symmetric and asymmetric GBs in bicrystals and nano-polycrystals differ from each other depending on tilt angle and inclination angle. A new dislocation emission mechanism of interest is observed in bicrystals which contain low angle symmetric tilt GBs. Low angle GB has a higher mobility relative to high angle GB in both bicrystalline and nano-polycrystalline structures, as does asymmetric GB to symmetric GB. The generation, motion, pileup and annihilation of dislocations, grain rotation and grain coalescence are observed, which is consistent with the simulation results obtained by molecular dynamics. These simulation results can provide strong guidelines for experimentation.

Microscale Fracture Toughness of Bismuth Doped Copper Bicrystals Using Double Edge Notched Microtensile Tests

The availability of focused ion beam (FIB) milling, nanoindentation, and microelectromechanical systems (MEMS) based test platforms has enabled small-scale mechanical testing to become an increasingly popular approach for measuring material properties. While great emphasis has been placed on measuring plastic properties at the micro- and nanoscale [1, 2], an area that has received significantly less consideration is the measurement of fracture toughness. A technique for performing small-scale, in situ fracture toughness tests using double edge notched tensile (DENT) specimens has been developed and used to measure a nearly 40 % reduction in toughness associated with the addition of Bi to the grain boundary of a Cu bicrystal. That Bi embrittles Cu grain boundaries is well known [3, 4, 5, 6, 7, 8, 9, 10], however, as shown herein, the DENT technique offers certain advantages over existing boundary fracture tests, especially when used with ductile materials.

Atomic Scale Verification of Oxide-Ion Vacancy Distribution near a Single Grain Boundary in YSZ

This study presents atomic scale characterization of grain boundary defect structure in a functional oxide with implications for a wide range of electrochemical and electronic behavior. Indeed, grain boundary engineering can alter transport and kinetic properties by several orders of magnitude. Here we report experimental observation and determination of oxide-ion vacancy concentration near the Σ13 (510)/[001] symmetric tilt grain-boundary of YSZ bicrystal using aberration-corrected TEM operated under negative spherical aberration coefficient imaging condition. We show significant oxygen deficiency due to segregation of oxide-ion vacancies near the grain-boundary core with half-width < 0.6 nm. Electron energy loss spectroscopy measurements with scanning TEM indicated increased oxide-ion vacancy concentration at the grain boundary core. Oxide-ion density distribution near a grain boundary simulated by molecular dynamics corroborated well with experimental results. Such column-by-column quantification of defect concentration in functional materials can provide new insights that may lead to engineered grain boundaries designed for specific functionalities.

Grain boundary motion and grain growth in zinc in a high magnetic field

The motion of grain boundaries in zinc bicrystals (99.995 %) driven by the “magnetic” driving force was measured. An in situ technique for observations and continuous recording the boundary migration was applied. Planar symmetrical and asymmetrical \( \left\langle {10\overline{1} 0} \right\rangle \) tilt grain boundaries with rotation angles in the range between 60° and 90° were studied. The boundary migration was measured in the temperature regime between 330 and 415 °C. The mobility of \( \left\langle {10\overline{1} 0} \right\rangle \) tilt boundaries in zinc and its temperature dependence were found to depend on the misorientation angle and the inclination of the boundary plane. An application of a magnetic field during the annealing of cold rolled (90 %) zinc–1.1 % aluminum alloy sheet specimens substantially affected the texture and microstructure evolution. This effect is attributed to the additional magnetic driving force for grain growth arising due to the magnetic anisotropy of zinc.

Motion of a grain boundary facet in aluminum

The shape and migration of a capillary-driven 20.8°〈100〉 tilt grain boundary in aluminum bicrystals were investigated in situ in a scanning electron microscope at elevated temperatures. The moving boundary assumed a semi-faceted configuration composed of a singular facet connected to a curved boundary section joined at a distinct edge. At constant temperature the boundary with a facet moved steadily and its shape remained self-similar. Both the facet length and the connecting angle between the facet and the curved boundary section did not change over the investigated temperature range. The capillary-driven migration of the semi-faceted grain boundary in “quarter-loop” geometry was analyzed. The obtained results revealed that the major factor responsible for the formation of the mobile facet was the energy difference between the facet and the curved boundary. The temperature dependence of the facet mobility was determined. The migration activation enthalpy of the investigated capillary-driven planar boundary/facet was a factor of two higher than the activation enthalpy for migration of a geometrically similar boundary driven by an applied stress.

A genetic algorithm for generating grain boundaries

Generation of realistic grain boundaries (GBs) in bicrystals for atomistic simulations requires a search in a multidimensional space for a configuration with the atoms at the GB organized in such a way that the energy of the whole system is minimized. This paper presents a genetic algorithm that allows one to find low-energy GB configurations by optimizing three main criteria: the local arrangement of the atoms, the relative translation between the two grains that compose the GB and an overall expansion/contraction of the system. It is designed to make a wider and more effective search through the energy landscape compared with other traditional methods, giving access to more configurations and increasing the possibility of finding the global minimum in energy.

Magnetically Controlled Grain Boundary Motion and Grain Growth in Zinc

The motion of grain boundaries in zinc bicrystals (99.995%) driven by the “magnetic” driving force was investigated. Planar symmetrical and asymmetrical tilt grain boundaries with rotation angles in the range between 60° and 90° were examined. At a given temperature the boundary migration rate was found to increase linearly with an applied driving force. The absolute grain boundary mobility was determined. The boundary mobility and its temperature dependence were found to depend on the misorientation angle and the inclination of the boundary plane. An application of a magnetic field during the annealing of cold rolled (90%) Zn-1.1%Al sheet specimens resulted in an asymmetry of the two major texture components. This is interpreted in terms of magnetically affected grain growth kinetics.

Grain Boundary Dynamics under an Applied Stress

The response of various structurally different planar grain boundaries in aluminum bicrystals to an applied stress was experimentally investigated. Stress induced boundary migration was observed to be coupled to a tangential translation of the grains for symmetrical and asymmetrical and tilt boundaries with both low and high misorientation angles. The activation enthalpy of high angle boundary migration was found to vary non-monotonously with misorientation angle, whereas for low angle boundaries the migration activation enthalpy was virtually the same. The Σ7 CSL boundaries in bicrystals of different geometry were observed to move under an applied stress, but their migration did not produce shear. These crystallographically equivalent boundaries, however, were found to behave different with respect to migration rate and its temperature dependence. The stress driven migration of the mixed tilt-twist boundary was observed to be accompanied by both the translation of adjacent grains parallel to the boundary plane and their rotation around the boundary plane normal. This behavior was interpreted in terms of the structure of the investigated tilt-twist boundary.

Aberration-Corrected TEM Imaging of Oxygen Occupancy in YSZ.

We present atomic-scale imaging of oxygen columns and show quantitative analysis on the occupancy of the columns in yttria-stabilized zirconia (YSZ) using aberration-corrected TEM operated under the negative Cs condition. Also, individual contributions both from oxygen column occupancy and the static displacement of oxygen atoms due to occupancy change to the observed column intensities of TEM images were systematically investigated using HRTEM simulation. We found that oxygen column intensity is governed primarily by column occupancy rather than by static displacement of oxygen atoms. Utilizing the aberration-corrected TEM capability and HRTEM simulation results, we experimentally verified that oxygen vacancies segregate near the single grain boundary of a YSZ bicrystal. The methodology and the high spatial resolution characterization tool employed in the present study provide insights into the distribution of oxygen vacancies in the bulk as well as near grain boundaries and pave the way for further investigation and atomic-scale analysis in other important oxide materials.

Segregation to the grain boundaries in YSZ bicrystals: A Molecular Dynamics study

A Molecular Dynamics study about the segregation of yttrium at 1500K to a Σ5 grain boundary in 8mol% YSZ has been performed. Segregation has been induced by explicitly taking into account the excess energy associated to the elastic misfit effect for yttrium cations located nearby the grain boundary planes. After an initial transient, a steady regime is reached, in which the number of yttrium cations does not increase with time. Accumulation of yttrium cations is accompanied by that of zirconium ones and oxygen vacancies at some distance of the grain boundary planes. The changes in the radial distribution functions for different ionic pairs are discussed, as also the effect of segregation on oxygen diffusion along the grain boundaries and in volume. Finally, the possibility that segregated yttrium located at available free sites at the grain boundaries is pointed out.

Nanoscale Plasticity at Grain Boundaries in Face-centered Cubic Copper Under Shock Loading

We investigate the responses of four representative grain boundaries in face-centered cubic Cu bicrystals to shock compression as a function of the loading direction. Two loading directions are considered, either parallel or perpendicular to the grain boundary plane, representing the extremes that a polycrystalline sample will ordinarily experience under the uniaxial strain conditions of planar shock loading. Using molecular dynamics simulations, we demonstrate that the deformation processes during shock compression of the same boundary are altered measurably by changing the loading direction. The Majority of the differences in the nanoscale deformation processes were related to the activation of varying slip systems in the same boundary under the two loading conditions. This change in deformation processes, and hence the plastic response, might eventually affect the failure stress for a grain boundary.