Grain Boundary Network Research Articles

Spectrum of embrittling potencies and relation to properties of symmetric-tilt grain boundaries

Grain boundaries in metallic materials can exist in a wide range of stable and metastable structures. In addition, the properties of a grain boundary may be altered through solute segregation. In this work, we present a formulation that combines the spectrum of embrittling potencies associated with solute segregation with site-occupancy statistics. As a prototype problem, we illustrate the relation between segregation and embrittlement in the case of S segregation to grain boundaries in Ni. To obtain a population of site segregation energies, we perform molecular statics calculations on 378 different symmetric-tilt grain boundaries and their free surface equivalents, using an embedded-atom method interatomic potential developed specifically for studying embrittlement. Our results show that it is important to consider both the energies associated with embrittlement and the probability of occupancy to describe the general embrittling nature of a grain boundary. When analyzed in isolation, certain grain boundaries show large embrittling potencies; however, that effect is diminished when the probability of S segregation to that grain boundary is considered within a polycrystal. We propose a new quantity, the embrittling estimator, which not only categorizes grain boundaries as embrittling or strengthening, but also considers site occupancy probabilities, so that the embrittlement behavior of grain boundaries within a network of grain boundaries can be compared. Finally, we examine the relationship between embrittlement behavior and innate grain boundary properties, such as the free volume, and find statistical evidence that the complex nature of embrittlement cannot be explained by linear correlations with excess volumes or energies. Ultimately, this combined approach provides a theoretical tool to assist grain boundary engineering of metastable alloys.

Point defects and interstitial climb of 90° partial dislocations in brown type IIa natural diamond

Multiple electron microscopy techniques have been used to study a brown type IIa natural diamond. Electron backscatter diffraction shows evidence of plastic deformation in the form of slip bands, while cathodoluminescence reveals a network of low-angle grain boundaries, also observed in transmission electron microscopy together with long straight dislocations and dislocation dipoles. Aberration-corrected scanning transmission electron microscopy shows interstitial absorption on the 90° partial of both dissociated dislocations and Z-type faulted vacancy dipoles, forming structures similar to that observed in other fcc materials. The observations indicate an interstitial concentration of 1017 to 1019 cm−3 and calculations of point defect concentrations produced by plastic deformation show that this can be produced by strains of the order of 1%. Brown coloration in diamond has been previously attributed to vacancies and vacancy clusters with concentrations around 1018 cm−3, which suggests that roughly equal numbers of interstitials and vacancies are generated in diamond via plastic deformation. Atomic resolution images of Z-type faulted dipoles allowed a stacking fault energy of 472 ± 38 mJ m−2 to be determined.

Intra- and intergranular fission gas transport on large irregular hexagonal grain networks by an included phase model

The evolution of intergranular fission gas bubbles, including growth, percolation through the fuel matrix, venting and tunnel collapse is simulated using the Included Phase Model (IPM). The model accounts for the effects of vacancy and fission gas species on bubble growth and morphology, which ultimately lead to tunnel formation, venting of fission gas, and subsequent tunnel collapse. A simplified two-dimension bubble model is implemented on a randomly generated irregular, equiangular hexagonal network of grain boundaries and coupled to an intragranular diffusion model. A set of 28 simulations were conducted on a 20 by 15 grid of hexagons with an open surface at one end to permit venting and ensuing tunnel collapse. Results are post-processed and considered statistically to investigate macroscopic fission gas release parameters and the percolation of the network. The behaviors of individual grains and edges are further investigated as a function of edge length, equivalent grain size, and proximity to the free surface. This analysis revealed discrete growth modes corresponding to the number of bubbles on each edge. These modes emerge because edge lengths vary continuously but only an integer number of bubbles precipitate on them, leading to an unequal number of bubbles per unit length for different grains, and corresponding difference in vacancy potential. This result may improve edge percolation conditions utilized in other models. Limitations resulting from the two-dimensional model and avenues future development of more quantitative models are also discussed.

Enhanced Photovoltaic and Photosensing Performances in Gadolinium-Doped Bismuth Ferrite

Recent emerging developments have demonstrated that bismuth ferrite is one of the promising lead-free perovskite materials for solar-energy harvesting, photoelectrochemical conversion, and photodetector. This work reports high short-circuit photocurrent densities ~1.2×1033 µA/cm2 in p-type gadolinium (Gd)-doped BiFeO3 ceramic with n-type indium-tin-oxide under 405 nm irradiation and sunlight at 102 mW/cm2 intensity, respectively. A polarization-enhanced photosensing responsivity (R) of ~5.4×10-2 A/W and detectivity (D*) of ~1.5×1011 Jones were achieved at low 405 nm irradiation. Enhanced photovoltaic conversion via a prior electric-field poling can be attributed to the p-n junction and field-modulated Schottky barrier in conjunction with domain nucleation, ordered polar nano-regions (PNRs), and increased O 2p-Fe 3d orbital hybridization. The network of domain walls and grain boundaries serves as conduction pathways for the photo-generated charge carriers. The remarkable improvement of photocurrent in polycrystalline Gd-doped BiFeO3 opens a window for using bismuth ferrite materials in self-powered UV-visible photodetector.

Blocking of Conducting Channels Widens Window for Ferroelectric Resistive Switching in Interface‐Engineered Hf0.5Zr0.5O2 Tunnel Devices

AbstractFilms of Hf0.5Z0.5O2 (HZO) contain a network of grain boundaries. In (111) HZO epitaxial films on (001) SrTiO3, for instance, twinned orthorhombic (o‐HZO) ferroelectric crystallites coexist with grain boundaries between o‐HZO and a residual paraelectric monoclinic (m‐HZO) phase. These grain boundaries contribute to the resistive switching response in addition to the genuine ferroelectric polarization switching and have detrimental effects on device performance. Here, it is shown that, by using suitable nanometric capping layer deposited on HZO film, a radical improvement of the operation window of the tunnel device can be achieved. Crystalline SrTiO3 and amorphous AlOx are explored as capping layers. It is observed that these layers conformally coat the HZO surface and allow to increase the yield and homogeneity of ferroelectric junctions while strengthening endurance. Data show that the capping layers block ionic‐like transport channels across grain boundaries. It is suggested that they act as oxygen suppliers to the oxygen‐getters grain boundaries in HZO. In this scenario it could be envisaged that these and other oxides could also be explored and tested for fully compatible CMOS technologies.

Failure mechanism of lead-free component boards in thermomechanical test based on recrystallization enhanced cracking

The purpose of this paper is to make a comprehensive understanding of the trends between the lifetime and the thermomechanical test setup parameters, based on the physical failure mechanism of SnAgCu solder interconnections in the component boards. The thermomechanical tests, including thermal cycling and thermal shock, were employed to investigate the failure of the component boards under different loading conditions. The in situ observation during the thermomechanical tests was employed to study the microstructure evolution and the crack propagation of solder interconnections. Electrical failures of the component boards under all setup conditions are mainly caused by the cracks, which propagate along the continuous network of the high-angle grain boundaries produced by the distinguished recrystallization of solder interconnections. The propagation of cracks is highly dependent on the expansion of the recrystallized volume in the solder interconnections. In addition, the expansion of the recrystallization would be influenced by the setup parameters of the thermomechanical test. The paper discusses the relationship among the four factors of the lifetime, the crack growth, the restoration phenomenon and the test setup parameters, in order to explain the complicated trend between the component board lifetime and the setup parameters.

Investigating the Origin of Enhanced C2+ Selectivity in Oxide-/Hydroxide-Derived Copper Electrodes during CO2 Electroreduction.

Oxide-/hydroxide-derived copper electrodes exhibit excellent selectivity toward C2+ products during the electrocatalytic CO2 reduction reaction (CO2RR). However, the origin of such enhanced selectivity remains controversial. Here, we prepared two Cu-based electrodes with mixed oxidation states, namely, HQ-Cu (containing Cu, Cu2O, CuO) and AN-Cu (containing Cu, Cu(OH)2). We extracted an ultrathin specimen from the electrodes using a focused ion beam to investigate the distribution and evolution of various Cu species by electron microscopy and electron energy loss spectroscopy. We found that at the steady stage of the CO2RR, the electrodes have all been reduced to Cu0, regardless of the initial states, suggesting that the high C2+ selectivities are not associated with specific oxidation states of Cu. We verified this conclusion by control experiments in which HQ-Cu and AN-Cu were pretreated to fully reduce oxides/hydroxides to Cu0, and the pretreated electrodes showed even higher C2+ selectivity compared with their unpretreated counterparts. We observed that the oxide/hydroxide crystals in HQ-Cu and AN-Cu were fragmented into nanosized irregular Cu grains under the applied negative potentials. Such a fragmentation process, which is the consequence of an oxidation-reduction cycle and does not occur in electropolished Cu, not only built an intricate network of grain boundaries but also exposed a variety of high-index facets. These two features greatly facilitated the C-C coupling, thus accounting for the enhanced C2+ selectivity. Our work demonstrates that the use of advanced characterization techniques enables investigating the structural and chemical states of electrodes in unprecedented detail to gain new insights into a widely studied system.

Micro-Texture Analyses of a Cold-Work Tool Steel for Additive Manufacturing.

Small objects of an alloy tool steel were built by selective laser melting at different scan speeds, and their microstructures were analyzed using electron backscatter diffraction (EBSD). To present an explicit correlation with the local thermal cycles in the objects, prior austenite grains were reconstructed using the EBSD mapping data. Extensive growth of austenitic grains after solidification could be detected by the disagreement between the networks of carbides and austenite grain boundaries. A rapid laser scan at 2000 mm/s led to less growth, but retained a larger amount of austenite than a slow one at 50 mm/s. The rapid scan also exhibited definite evolution of Goss-type textures in austenite, which could be attributed to the growth of austenitic grains under a steep temperature gradient. The local variations in the microstructures and the textures enabled us to speculate the locally different thermal cycles determined by the different process conditions, that is, scan speeds.

Accuracy and precision of near-field high-energy diffraction microscopy forward-model-based microstructure reconstructions

The accuracy of the near-field high-energy diffraction microscopy (nf-HEDM) technique is evaluated by directly comparing an nf-HEDM reconstructed microstructure with an electron backscatter diffraction (EBSD) characterization of the same microstructure. A high-purity gold oligocrystal was chosen for characterization in order to facilitate direct one-to-one comparison between the reconstructions given by each technique. By using the comparatively high spatial resolution of the EBSD reconstruction as the ground truth for the grain-boundary network's morphology, it is determined that nf-HEDM locates internal grain boundaries with an accuracy on average better than the resolution of the imaging detector used or within the reconstruction voxel size, whichever is larger. By taking the intragranular misorientation in well ordered grains as a proxy for orientation resolution, it is determined that standard data collection procedures determine crystallographic orientations to better than 0.1°. The effects of various modified data collection procedures are also examined.

Grain Boundary Triple Junction Dynamics: A Continuum Disconnection Model

The microstructure of polycrystalline materials consists of networks of grain boundaries (GBs) and triple junctions (TJs), along which three GBs meet. The evolution of such microstructures may be driven by surface tension (capillarity), applied stresses, or other means that lead to a jump in chemical potential across the GBs. Here, we develop a model for the concurrent evolution of the GB/TJ network based upon the microscopic mechanism of motion; the motion of line defects (disconnections) in the GB that have both dislocation and step character. The evolution involves thermally-activated disconnection formation/annihilation and migration of multiple disconnections modes/types. We propose this crystallography-respecting continuum model for the disconnection mechanism of GB/TJ dynamics derived with a variational approach based on the principle of maximum energy dissipation. The resultant TJ dynamics is reduced to an optimization problem with constraints that account for local microstructure geometry, conservation of Burgers vectors, and thermal-kinetic limitations on disconnection fluxes. We present analysis of and numerical simulations based upon our model to demonstrate the dependence of the GB and TJ mobilities and the TJ drag effect on the disconnection properties, and compare the predictions with molecular dynamics and experimental observations.

Representative and statistical volume elements for grain boundary networks: A stereological approach

Experimental and computational studies have demonstrated that the structure of grain boundary networks (GBNs), e.g. the connectivity of certain types of grain boundaries (GBs), has a significant impact on macroscopic material properties. Because minor changes in GBN structure can cause large changes in effective properties, it is common to use large microstructures or report averages over many microstructural instantiations with the assumption that results become more representative as the size or number of samples increases. For crystallographic texture, and composite materials, such ideas have been made rigorous through the definition of representative volume elements (RVEs) and statistical volume elements (SVEs). However, for GBNs, the size of an RVE and the cardinality of a set of SVEs have not yet been determined. In this study, we employ stereological methods to evaluate the convergence of triple junction fractions in both idealized and realistic 2D GBNs to quantitatively define RVEs and SVE sets for GBNs. We compare these results to those for crystallographic texture in the same polycrystals and find that the trends for GBNs and texture are remarkably different. Because the vast majority of experimental work currently relies on 2D microstructure characterization, the results obtained for 2D systems have great practical value in and of themselves. However, the stereological approach employed also allows us to make quantitative predictions of RVEs and SVEs for fully 3D microstructures. These results are expected to aid future experimental and computational work in selecting appropriately sized material volumes to achieve robust quantitative results.

Grain boundary design based on fractal theory to improve intergranular corrosion resistance of TWIP steels

The grain size in TWIP steel had a double fractal distribution, with a uniform diameter threshold of approximately 36 μm. Based on this threshold value, the grains were divided into fine and coarse grain sets. A good linear relationship was found between the fractal dimension of the fine grain set (Dg1) and the fraction of Σ3 gain boundaries (GBs) (FΣ3). The distribution characteristics of the three types of GBs satisfy multifractal and generalized fractal characteristics. Importantly, the fractal structural parameters of the Σ3 GBs control the distribution characteristics and complexity of the total grain boundary network (GBN). Combined corrosion test, a quantitative relationship between the corrosion thickness loss and the Σ3 GBs fractal parameter as well as corrosion time was established. In the end, the optimal microstructure characteristics for improving the intergranular corrosion resistance of TWIP steel are discussed from the GB perspective. GB design based on fractal theory provides a promising solution to optimize material corrosion properties through analyzing complex GBN.

Spectrum of grain boundary segregation energies in a polycrystal

Solute segregation at grain boundaries (GBs) is emerging as an alloy design tool, uses of which include the stabilization of nanocrystalline alloys. To predict the equilibrium segregation state in a given alloy, most thermodynamic models treat the full network of GBs as a single “entity”, and thus use an “effective” segregation energy to describe it. This simplification ignores the spectral nature of available GB segregation energies in a polycrystal, which we elucidate here computationally for a Mg solute in an Al polycrystal; the distribution is found to be captured accurately with a skew-normal function. A thermodynamic segregation isotherm that incorporates this spectrum is outlined and employed to study the effect of such a spectrum on predictions of the equilibrium GB segregation state. The ramifications for experimentally-extracted GB segregation energies are shown to be potentially significant, and nanocrystalline stability criteria are extended to account for this spectral nature of GB segregation.

Universal Spin Diffusion Length in Polycrystalline Graphene.

Graphene grown by chemical vapor deposition (CVD) is the most promising material for industrial-scale applications based on graphene monolayers. It also holds promise for spintronics; despite being polycrystalline, spin transport in CVD graphene has been measured over lengths up to 30 μm, which is on par with the best measurements made in single-crystal graphene. These results suggest that grain boundaries (GBs) in CVD graphene, while impeding charge transport, may have little effect on spin transport. However, to date very little is known about the true impact of disordered networks of GBs on spin relaxation. Here, by using first-principles simulations, we derive an effective tight-binding model of graphene GBs in the presence of spin-orbit coupling (SOC), which we then use to evaluate spin transport in realistic morphologies of polycrystalline graphene. The spin diffusion length is found to be independent of the grain size, and it is determined only by the strength of the substrate-induced SOC. This result is consistent with the D'yakonov-Perel' mechanism of spin relaxation in the diffusive regime, but we find that it also holds in the presence of quantum interference. These results clarify the role played by GBs and demonstrate that the average grain size does not dictate the upper limit for spin transport in CVD-grown graphene, a result of fundamental importance for optimizing large-scale graphene-based spintronic devices.

Theoretical evaluation of electronic density-of-states and transport effects on field emission from n-type ultrananocrystalline diamond films

In the nitrogen-incorporated ultrananocrystalline diamond [(N)UNCD] films, representing an n-type highly conductive two-phase material comprised of sp3 diamond grains and sp2-rich graphitic grain boundaries, current is carried by a high concentration of mobile electrons within large-volume grain-boundary networks. Fabricated in a simple thin-film planar form, (N)UNCD was found to be an efficient field emitter capable of emitting a significant amount of charge starting at the applied electric field as low as a few volts per micrometer, which makes it a promising material for designing electron sources. Despite semimetallic conduction, field emission (FE) characteristics of this material demonstrate a strong deviation from the Fowler–Nordheim law in a high-current-density regime when (N)UNCD field emitters switch from a diodelike to a resistorlike behavior. Such a phenomenon resembles the current-density saturation effect in conventional semiconductors. In the present paper, we adapt the formalism developed for conventional semiconductors to study current-density saturation in (N)UNCD field emitters. We provide a comprehensive theoretical investigation of (i) partial penetration of the electric field into the material, (ii) transport effects (such as electric-field-dependent mobility), and (iii) features of a complex density-of-states structure (position and shape of π−π∗ bands, controlling the concentration of charge carriers) on the FE characteristics of (N)UNCD. We show that the formation of the current-density saturation plateau can be explained by the limited supply of electrons within the impurity π−π∗ bands and decreasing electron mobility in a high electric field. Theoretical calculations are consistent with the experiment.

Quantitative theory of the grain boundary impact on the open-circuit voltage of polycrystalline solar cells.

Thin film polycrystalline photovoltaics are a mature, commercially-relevant technology. However, basic questions persist about the role of grain boundaries in the performance of these materials, and the extent to which these defects may limit further progress. In this work, we first extend previous analysis of columnar grain boundaries to develop a model of the recombination current of "tilted" grain boundaries. We then consider systems with multiple, intersecting grain boundaries and numerically determine the parameter space for which our analytical model accurately describes the recombination current. We find that for material parameters relevant for thin film photovoltaics, our model can be applied to compute the open-circuit voltage of materials with networks of inhomogeneous grain boundaries. This model bridges the gap between the distribution of grain boundary properties observed with nanoscale characterization and their influence on the macroscale device open-circuit voltage.

Fe-based superconducting thin films on metallic substrates: Growth, characteristics, and relevant properties

The discovery of Fe-based superconductors (FBS) as the second class of high-temperature superconducting transition (high-Tc) materials after the cuprates generated a significant impact on the community of fundamental and applied superconductivity research. Whenever a new class of high-Tc superconducting materials is discovered, a lot of effort is devoted to growing single crystals and epitaxial thin films for exploring basic physical quantities. Although several properties of FBS are similar to the cuprates (large upper critical fields, resulting from a short coherence lengths, and small carrier density), others are distinctly different. For instance, in FBS the symmetry of the superconducting order parameter is most likely not a d-wave but an s+-wave, depending on the stoichiometry, crystallographic system, and doping level. Additionally, the critical current densities of FBS are less sensitive to the presence of grain boundaries (GBs) than those of the high-Tc cuprates. These features are highly beneficial for the realization of cheaper conductors for high-field magnets at low temperatures. Indeed, several groups have demonstrated FBS thin films on technical metallic substrates and powder-in-tube processed FBS wires as proof-of-principle studies for conductor applications. FBS on technical substrates also give many opportunities for studying how GB networks affect the critical current and how uniaxial strain impacts the superconducting properties. In this article, we review FBS thin films, especially on technical metallic substrates, and focus on application-relevant properties like pinning improvement by natural and artificial defects as well as the transparency of GBs and GB networks. The recent development of FBS thin films on technical substrates and their superconducting properties are presented and the performance gap with respect to films on single crystals is discussed.

Self-organized multigrain patterning with special grain boundaries produced by phase transformation cycling

In crystalline solids, grain boundaries (GBs) play a significant role in determining a large number of material properties. The design and synthesis of special GBs has been a long-standing challenge in materials science and engineering. Here we demonstrate a mechanism to produce special GBs. Unique multigrain structures can be obtained through cyclic, diffusionless phase transformations under external fields, with all GBs being coherent special GBs. The crystallographic character of the GBs produced in this way is dictated by the broken symmetry during the phase transformations, while the topology of the GB network is determined by the geometric compatibility and self-organization of the multigrain structures. Such a mechanism not only suggests an alternative method of GB engineering, but also reveals a fundamental relationship between special GBs and phase transformations from a crystallographic point of view.

<i>In Situ</i> Observation of the Phase Transformations in Ti15Mo Alloy Deformed by High Pressure Torsion

Metastable β-Ti alloys including Ti15Mo alloy are perspective candidates for use in medical applications. During thermal treatment Ti15Mo alloy undergoes various phase transformations. After solution treatment it contains metastable β-phase and ω-phase. During annealing the ω-phase partially dissolves as well as stable α-phase particles are formed. The solution treated Ti15Mo alloy was deformed by high pressure torsion (HPT) at room temperature. Significant grain refinement with grain size of ~100 nm was achieved even after 1/4 of HPT rotation. The effect of the ultra-fine grained (UFG) structure achieved by HPT on the phase transformations was studied by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) during in-situ heating. High density of lattice defects, dense network of grain boundaries as well as ongoing recovery and recrystallization upon heating significantly affected the phase transitions. Observation of the microstructure during in-situ heating in TEM revealed no representative changes in transparent part of the sample due to the “thin foil effect”.

Correlated Disorder in YBCO and Composite YBCO Films Revealed by Means of Synchrotron X-Ray Diffraction

The presence of lattice correlated disorder in real materials places further demands on the understanding of the structure-function relationship. The study of this disorder is quite challenging, since it can be due to various and interconnected reasons. This requires advanced probes able to measure weak structural features. Thanks to synchrotron radiation features, nowadays, it is possible to visualize structural disorder alongside its spatial distribution and time dynamics. In this context, we aim to characterize thin-film superconducting YBa 2 Cu 3 O 7-x , whose structure is particularly rich. Indeed, here, the nanogranularity related to superlattices, distribution of dopants, and strain-induced stoichiometry inhomogeneity are coupled with the intrinsic granularity arising from the material island growth mode, which leads to small grains (~200 nm) and a large network of grain boundaries. X-ray Grazing incidence diffraction has been used to visualize low-dimensional lattice disorder in optimally doped YBa 2 Cu 3 O 7-x with inclusion of BaZrO 3 nanodefects grown on SrTiO 3 single crystals. The results are expected to shade light on interplay of lattice disorder and the electronic quantum coherence in the superconducting state.