Grain Size Boundary Research Articles

In situ observation of interaction between grain boundaries during directional solidification of Si

The interaction between grain boundaries (GBs) at a silicon crystal/melt interface was studied using in situ observations during directional solidification. Two small-angle GBs (SAGBs) interact with each other to produce a new SAGB with increased misorientaion. However, the interaction between Σ3 GBs and SAGBs reveals a different phenomenon. The extending directions and misorientations of the SAGBs and Σ3 GBs show no change before and after the convergence of these two planar defects at the crystal/melt interface.

Solute segregation in Cu: DFT vs. Experiment

A close comparison between the segregation data obtained by means of DFT-based calculations and diffusion experiments is provided using the grain boundary (GB) segregation of Ag, Au, Se, Ge, Ni, Co and Bi impurities in Cu as a case study. The density functional theory calculations along with the McLean segregation isotherm are applied to obtain the effective segregation energies of the solute atoms to a representative special Σ5 GB in Cu. GB diffusion measurements in both B- and C-type kinetic regimes in the same polycrystalline high-purity Cu are used to access the segregation behavior of solutes in the true dilute limit. In the case of Ag and Bi, a very close comparison is possible as the experiments have been conducted on ideal Cu bi-crystals with a Σ5 GB where we find an excellent agreement between theory and experiment. For the other solutes, the DFT data for the Σ5(210) GB are used to mimic behavior of general high-angle interfaces. For Bi, Se, and Ge, the experimental picture is consisted with the DFT results, while for Ni, Co and Au, serious discrepancies are found. The reasons for the discrepancies are analyzed and discussed in the paper. In addition, we investigate the influence of the selected impurities on the mechanical strength of the GB in the framework of the Rice-Thompson-Wang theory and identified the most harmful elements from the point of view of the cohesive destrengthening of the interface.

Effects of grain boundaries on irradiation-induced defects in tungsten by molecular dynamics simulations

The effects of two different symmetric tilt grain boundaries (GBs), Σ13[001](230) GB and Σ17[001](140) GB, on displacement cascade processes in tungsten were investigated using molecular dynamics simulations. By quantifying the number of interstitials and vacancies surviving after irradiation with the kinetic energy of primary knock-on atom energies of 1, 3 and 5 keV, respectively, in these simulations, it is found that the GBs have dual nature for radiation-induced defects: They absorb interstitials while leaving more vacancies to survive in the grains. The net effect is that the number of total surviving defects in the GB system is not always less than that in the single crystal. These defect behaviors are understood by quantitatively analyzing the recovery fraction of irradiation-induced defects, the time to reach steady state and the mobility of vacancies and interstitials. It is also found that the Σ17 GB is a more effective sink of radiation-induced point defects than the Σ13 GB. One of the main reasons is that the Σ17 GB has a higher GB energy.

Dynamics of defect-loaded grain boundary under shear deformation in alpha iron

Two symmetric tilt grain boundaries (GBs) (Σ3〈110〉{112} and Σ11〈110〉{332}) in alpha iron were performed to investigate the dynamics of defect-loaded GBs under shear deformation. The results show that the loaded self-interstitial atoms (SIAs) reduce the critical stress of the coupled GB motion in the Σ3 GB, but increase the critical stress in the Σ11 GB. The loaded SIAs in the Σ3 GB easily form 〈111〉 clusters and remain in the bulk when the GB moves away. However, the SIAs move along with the Σ11 GB and combine with the vacancies in the bulk, leading to the defect self-healing. The helium (He) atoms loaded into the GBs significantly affect the coupled GB motion. Once He clusters emit interstitials, the Σ11 GB carries those interstitials away but the Σ3 does not. The loaded He atoms reduce the critical stress of the Σ3 GB, but increase the critical stress of the Σ11 GB.

Insights into Grain Boundary in Lithium-Rich Anti-Perovskite as Solid Electrolytes

In all-solid-state lithium batteries, anti-perovskite solid electrolyte has high ionic conductivity and high stability with lithium metal anode. However, grain boundaries (GBs) contribute to undesirable resistance limiting ionic conductivity in anti-perovskite, and there is limited knowledge about the GBs in solid electrolyte, particularly at the atomic scale. Here, using density functional theory calculations we show that four symmetric tilt (Σ3 and Σ5) GBs effects on structural characteristics and ions transport in anti-perovskite (Li3OCl) solid electrolyte. Using first-principles simulation, GBs are found to be relatively stable resulting in its high concentrations in Li3OCl. Interestingly, the presence of GBs can improve compatibility with electrode, while it decreases the ionic conductivity and band gaps in Li3OCl. Furthermore, it is noted that the Σ5 GBs structures are softer and higher ionic conductivity than Σ3 GBs, delivering a new insight that GBs types may importantly affect the softness and ionic conductivity in solid electrolyte. Significantly, we find that the easiest Li ion migration pathway is along GB direction in Li3OCl with GBs structures. The present work uncovers the GBs behaviors in anti-perovskite solid electrolyte, which can help us to guide the design of high performance anti-perovskite solid electrolyte.

Зернограничное проскальзывание и миграция специальных границ зерен в бикристаллах Al. Атомистическое моделирование

AbstractThe energy of grain boundary shears is calculated for symmetric grain boundaries (GBs) using ab initio methods and molecular-dynamic modeling in order to elucidate mechanisms that control GB shear-migration coupling in typical symmetric GBs, such as Σ3 (111), Σ5 (012), Σ5 (013) and Σ11 (113) tilt GBs, in Al bicrystal. The energy of generalized grain-boundary stacking faults (GB–SF) is determined, and the preferred directions and the energy barrier are established for grain-boundary slippage. It is shown that the relative slippage of neighboring grains at certain directions of particle shears is accompanied by conservative migration of GB in the direction perpendicular to its plain. The modeling data are comparative to known grain-boundary shear-migration coupling mechanisms in Al.

Determination of gaseous fission product behavior near the cerium dioxide [formula omitted] tilt grain boundary via first-principles study

Grain boundaries (GBs) are the most abundant structural defects in nanostructured nuclear fuels and play an important role in determining fission product behavior, which further affects the performance of nuclear fuels. In this work, cerium dioxide (CeO2) is used as a surrogate material for mixed oxide fuels to understand gaseous fission product behavior, specifically Xe. First-principles calculations are employed to comprehensively study the behavior of Xe and trap sites for Xe near the Σ3 (111)/[11¯0] grain boundary in CeO2, which will provide guidance on overall trends for Xe stability and diffusion at grain boundaries vs in the bulk. Significant segregation behavior of trap sites, regardless of charge states, is observed near the GB. This is mainly ascribed to the local atomic structure near the GB, which results in weaker bond strength and more negative segregation energies. For Xe, however, the segregation profile near the GB is different. Our calculations show that, as the size of trap sites increases, the segregation propensity of Xe is reduced. In addition, under hyper-stoichiometric conditions, the solubility of Xe trapped at the GB is significantly higher than that in the bulk, suggesting higher Xe concentration than that in the bulk. The results of this work demonstrate that the diffusion mechanism of Xe in CeO2 is comparable to that in UO2. The diffusion activation energies of Xe atoms in the Σ3 GB are lower than that in the bulk CeO2. These results suggest that the diffusivity of Xe atoms is higher along the GB than that in the bulk, which enhances the aggregation of Xe atoms near the GB.

Effects of grain boundary structure controlled by artificially designed seeds on dislocation generation

We investigated the effects of grain boundary (GB) misorientation and the change in GB energy on the generation of dislocations during the directional growth of a crystalline silicon ingot. For this purpose, two ingots were grown using artificially designed seeds to introduce plural GBs with and without misorientation from an ideal coincident site lattice boundary. We revealed that dislocations are frequently generated from Σ3 GB with misorientation. On the other hand, dislocations are hardly observed around naturally formed perfect Σ3 GBs. In addition, we found that Σ5 and Σ25 GBs with higher GB energies are not sensitive to misorientation, and few dislocations are found around Σ25 GBs.

Reduced grain boundary energies in rare-earth doped MgAl2O4 spinel and consequent grain growth inhibition

Grain growth inhibition in MgAl2O4 spinel nanostructure was achieved by grain boundary (GB) segregation of rare-earth dopants. Microcalorimetric measurements showed that dense spinel compacts doped with 3 mol% of R2O3 (R=Y, Gd, and La) had decreased GB energies as compared to the undoped spinel, representing reduction in the driving force for grain growth. Segregation energies of the three dopants to the Σ3 (111) GB were calculated by atomistic simulation. The dopants with higher ionic radius tend to segregate more strongly to GBs. The GB energies were calculated from atomistic simulation and, consistent with experiments, a systematic reduction in GB energy with dopant ionic size was found. High temperature grain growth experiments revealed a significant reduction of grain growth in the doped nanostructures as compared to the undoped one, which was attributed to increased metastability or possibly also a GB dragging originated from the dopant segregation.

Fast hydrogen diffusion along Σ13 grain boundary of α-Al2O3 and its suppression by the dopant Cr: A first-principles study

To acquire knowledge of the effect of chromium dopant bringing on hydrogen diffusion along grain boundaries (GBs) of α-Al2O3, the energetics and mobility of hydrogen along Σ13 GB with and without Cr dopant have been studied via the first-principles calculations with the projector-augmented wave method. The energy barriers for hydrogen diffusion before and after Cr doped on Al-terminated Σ13 GB are determined to be 0.69 eV and 1.09 eV, respectively, which is smaller than or close to the hydrogen bulk diffusion with an activation energy of 1.10 eV. The results suggest that before doping Cr atoms, there is a rapid and easy diffusion channel for hydrogen along the Σ13 GB. However, the calculations show the segregation of Cr atoms to the GB is energetic favorable with a segregation energy of −0.39 eV/atom, and the existence of Cr can suppress hydrogen diffusion along the Al-terminated Σ13 GB.

First-principles investigation on the interaction of Boron atom with nickel part II: Absorption and diffusion at grain boundary

Abstract In this work, Boron atom absorption and diffusion behaviors at Σ3(111) [ 1 − 10 ] and Σ5(210)[001] grain boundaries (GBs) were studied with a first-principles density functional theory and the climbing image nudged elastic band method. The calculation results reveal that the two GBs show the markedly different interaction behavior with atomic Boron. The close-packed Σ3 GB plays the negative role for absorbing B atom and can accelerate the B atom diffusion, while the Σ5 GB plays the positive role for absorbing B atom and acts as a two-dimensional diffusion barrier. The ability for Ni GBs and Ni bulk to absorb B atom is controlled by the electron density ρ el at the site before inserting the B atom, and a certain range of ρ el plays the positive role for absorbing B atom, while the larger or smaller ρ el outside the certain range will both no longer have the same influence. The different energy barrier for B atom diffusing leads to the huge different diffusion rate, in which, the diffusion rate for B atom along the Σ3 GB is two orders of magnitude greater than that in the bulk Ni and nearly four orders of magnitude greater than that across the Σ5 GB.

Evolution of displacement cascades in Fe–Cr structures with different [001] tilt grain boundaries

ABSTRACTReduced-activation ferritic/martensitic steels of Cr concentration between 2.25 and 12 wt% are candidate structural materials for next-generation nuclear reactors. In this study, molecular dynamics (MD) simulation is used to generate the displacement cascades in Fe–Cr structures with different Cr concentrations by using different primary knock-on atom (PKA) energies between 2 and 10 keV. A concentration-dependent model potential has been used to describe the interactions between Fe and Cr. Single crystals (SCs) of three different coordinate bases (e.g. [310], [510], and [530]) and bi-crystal (BC) structures with three different [001] tilt grain boundaries (GBs) (e.g. Σ5, Σ13, and Σ17) have been simulated. The Wigner–Seitz cell criterion has been used to identify the produced Frenkel pairs. The results show a marked difference between collisions observed in SCs and those in BC structures. The numbers of vacancies and interstitials are found to be significantly higher in BC structures than those found in SCs. The number of point defects exhibits a power relationship with the PKA energies; however, the Cr concentration does not seem to have any influence on the number of survived point defects. In BC models, a large fraction of the total survived point defects (between 59% and 93%) tends accumulate at the GBs, which seem to trap the generated point defects. The BC structure with Σ17 GB is found to trap more defects than Σ5 and Σ13 GBs. The defect trapping is found to be dictated by the crystallographic parameters of the GBs. For all studied GBs, self-interstitial atoms (SIAs) are easily trapped within the GB region than vacancies. An analysis of defect composition reveals an enrichment of Cr in SIAs, and in BC cases, more than half of the Cr-SIAs are found to be located within the GB region.

Nanoscopic study of the compositions, structures, and electronic properties of grain boundaries in Cu(InGa)Se2 photovoltaic thin films

Polycrystalline Cu(InGa)Se2 (CIGS) thin film is a well-known light absorber material for photovoltaic solar cells. However, the mechanism of the benignity of its grain boundary (GB) remains in dispute. By using a combination of experimental techniques, we have systematically investigated the compositions, structures, and properties of the grain interior (GI) and GB of the polycrystalline CIGS films at nanometer scale. A definitive Cu deficient layer with thickness depending on the overall Cu content of the films was observed near the non Σ3 GBs with distinctly different characteristics from GIs. The band alignment between GB and GI was discovered to be of type II with downward offset for both conduction and valence bands at GB, well correlating to the local copper deficiency and structure. Our findings expressively support the type inversion and large hole barrier in this GB layer, and establish a comprehensive mechanism for the benignity of the CIGS grain boundaries towards carrier recombination.

Grain boundary engineering for improving conductivity of polycrystalline SrTiO3

The present paper reports the conductivity enhancement of strontium titanate (SrTiO3) using grain boundary engineering (GBE). In grain boundaries (GBs), Σ3 GBs have superior electrical properties to general GBs. Although many studies have focused on increasing the population of Σ3 GBs, the increase in the Σ3 GB population was limited mainly because the previous studies did not provide an efficient method for orienting grains (e.g., inhibition of abnormal grain growth). This study attempts to manipulate nanocrystal planes of SrTiO3 starting powders by using additives of acetic acid and citric acid. Using citric acid both populations of (111) surfaces and Σ3 GB are increased. Furthermore, our conductivity measurement indicated that the higher the Σ3 GB population the more favorable the conductivity. In summary, the findings of this study opened new possibilities for increasing the conductivity of GBE.

Fast hydrogen diffusion along the Σ7 grain boundary of α-Al2O3: A first-principles study

We have studied the energetics and mobility of hydrogen in the bulk and along the rhombohedral Σ7 grain boundary (GB) of α-Al2O3 via first-principles calculations based on the projector-augmented wave method. The temperature-dependent diffusivities D(T) in the α-Al2O3 bulk and along the Σ7 GB are derived. The ratio shows that the diffusivity along the GB is 2–5 orders of magnitude greater than in the bulk at temperatures in the range of 273–973 K, revealing that the diffusion along the GB is the underlying reason for the experimentally observed permeation reduction factor being much lower than the anticipated value. Moreover, the calculations also reveal that radiation-induced O vacancies tend to aggregate to the GB plane, thereby forming a zigzag O vacancy chain. In such circumstances, however, the calculated migration energy for hydrogen diffusion along the O vacancy chain is 2.58 eV, which is much greater than that of 1.10 eV and 0.81 eV for H diffusion in the bulk and along the GB, respectively. This finding suggests that O vacancies in the GB trap hydrogen atoms and prevent their diffusion along the GB.

Effect of transition metal impurities on the strength of grain boundaries in vanadium

Effects of 3d (Ti-Ni), 4d (Zr-Pd), and 5d (Hf-Pt) transition metal impurities on strength of two representative vanadium grain boundaries (GBs), symmetric Σ3(111) and asymmetric Σ5(210), were studied by first-principles calculations within the framework of the Rice-Wang thermodynamic model and within the computational tensile test. The desirable elements to increase the GB cohesion were predicted based on their segregation and strengthening behaviors across the different GB sites. It reveals that the elements Ti, Zr, Hf, Nb, and Ta are good choices for the GB cohesion enhancers. In addition, the GB strengthening by solutes is sensitive to the GB structures. The elements Cr, Mn, Fe, Co, and Ni decrease the GB strength of the Σ3(111) GB but they can increase the cohesion of the Σ5(210) GB. Furthermore, the origin of Ti-induced change of the GB strength was uncovered by analyzing the atomic bonds and electronic structures as well as the tensile strength. This work provides a theoretical guidance to screen promising alloying elements in V-based materials with improved resistance to GB decohesion and also helps us to understand the formation mechanism of Ti-rich precipitates in the V-Cr-Ti alloys under neutron or ion irradiation environments.

Energetics of point defect interacting with bi-crystal Σ3 copper grain boundaries

Point defects interacting with grain boundary (GB) may shed light on point defect sink strength of GB. In this work, vacancy and self-interstitial atom (SIA) formation energies within 25 Σ3 〈110〉 copper grain boundaries (GBs) were calculated using embedded atom method (EAM) potential. The energetics of point defects interacting with these GBs was analyzed. It is found that the lowest vacancy formation energies in 25 Σ3 GBs in general decrease with increasing their inclination angles with respect to Σ3 coherent twin GB (CTB). Lowest SIA formation energies in 25 Σ3 GBs except CTB, however, shows an increasing trend. The lowest vacancy within Σ3 GBs correlates well with average GB energy, but not so for SIA formation energies. Moreover, overall trend of the correlation between lowest vacancy formation energies and GB energies for Σ3 GBs and other ten GBs with Σ ranging from 5 to 13 shows a remarkable decreasing trend, but not obvious for SIA. Void denuded zone (VDZ) widths of GBs that can be used to evaluate the vacancy sink strength of them, correlates with lowest vacancy formation energies as well as average energies of GBs at their equilibrated states.

Twinning structure of M23C6 carbide precipitated at twin-related grain boundaries in Alloy 600

The microstructure features of carbides precipitated at twin boundary and Σ9 grain boundary (GB) in nickel-based Alloy 600 after aging at 715 °C for different time were investigated by high-resolution transmission electron microscopy. The carbides precipitated at incoherent twin boundary and Σ9 GB were identified as M23C6. The bar-like carbides grow parallel to each other and into each side of grain nearby incoherent twin boundary, while only growth into one side nearby Σ9 GB is observed. The carbides precipitated at incoherent twin and Σ9 GBs contain twins. The carbides have (1 1 1) matrix // (1 1 1) M23C6 orientation relationship with the nearby matrix. Based on the experimental results, the formation of twinning microstructure features in the M23C6 is discussed.

Quantitative measurements of grain boundary excess volume from HAADF-STEM micrographs

A novel approach for quantitative measurements of grain boundary (GB) excess volume has been developed using correlative analytical transmission electron microscopy (TEM) and successfully demonstrated for several simulated symmetrical [100] tilt GB configurations as well as for the experimental case of an Al bicrystal containing a near Σ13 GB with an additional twist component. The reliability and precision of this new approach is analyzed and the limitations are discussed.

Roles of grain boundary and oxygen vacancies in Ba0.6Sr0.4TiO3 films for resistive switching device application

Oxygen vacancies are widely thought to be responsible for resistive switching (RS) effects based on polycrystalline oxides films. It is also well known that grain boundaries (GB) serve as reservoirs for accumulating oxygen vacancies. Here, Ar gas was introduced to enlarge the size of GB and increase the quantity of oxygen vacancies when the Ba0.6Sr0.4TiO3 (BST) films were deposited by pulse laser deposition technique. The experimental results indicate that the RS properties of the device exhibits better in the Ar-introduced BST films than in the O2-grown BST films. High resolution transmission electron microscopy images show that an amorphous region GB with large size appears between two lattice planes corresponding to oxygen vacancies defects in the Ar-introduced BST. Fourier-transform infrared reflectivity spectroscopy results also reveal highly accumulated oxygen vacancies in the Ar-introduced BST films. And we propose that the conduction transport of the cell was dominantly contributed from not ions migration of oxygen vacancies but the electrons in our case according to the value of activation energies of two kinds of films.