Properties Of Grain Boundaries Research Articles

Atomic-scale investigation of interactions between Cu solute and defects in Zr alloys

The effects of dilute Cu solute on dislocations and grain boundaries in Zr alloys have been investigated at the atomic scale by atom probe tomography measurements, molecular dynamics, and molecular statics simulations. We demonstrate that Cu can segregate at dislocations and form nanoclusters in Zr alloys to retard the dislocation motion at high-stress creep. The solid solution strengthening of Cu is stronger than Sn and Nb at the same concentration and 673 K. Moreover, Cu is inclined to segregate at grain boundaries in Zr alloys, which is expected to reduce the Coble creep rate. As such, minor Cu alloying is an effective strategy to tailor the properties of dislocations and grain boundaries for enhancing the creep performance of Zr alloys.

Theoretical study of synergistic effect of P and Mg on the cohesive properties of Ni3Al grain boundaries

The addition of specific alloying elements can effectively control the microstructure of alloys, so as to improve the cohesive properties of the GBs. In this work, the first-principles plane-wave pseudopotential method is used to investigate the segregation behavior of P and Mg doping at the Ni3Al GBs and reveal the physical mechanism. Different 100%Ni Σ5 (210) [010] GB systems with and without doping elements are relaxed and larger volume expansions are obtained. The calculated segregation energies show that P or Mg atoms tend to segregate to the GBs relative to the bulk. P atoms tend to stay at the interstitial sites surrounded by 8 Ni atoms in the pure Ni holes of the GBs while Mg atoms tend to substitute Ni atoms. The segregation of P or Mg to the GBs leads to an increase of the Griffith work at the GBs, indicating that the segregation can improve the bonding properties of the GBs. Especially, the synergistic segregation of Mg can improve the detrimental effect of P in the mixed hole on the cohesive properties of the GBs. It is also found that the site occupation of Mg can be reversed from the Ni site to the Al site by the synergistic alloying of P at the 50%Ni Σ5 GBs. P and Mg tend to bond with the host atom Ni regardless of GB structure. The investigation of electronic structure shows that GB strengthening of alloying elements is attributed to the increasing P/Mg-Ni interactions across the GBs.

First-principles investigation of the Mg, Cu segregation at Al Σ5 (310) and Σ9 (221) symmetrical tilt grain boundaries

The segregation of alien solute atoms at grain boundaries (GBs) in nanocrystalline materials can dramatically affect their macroscopic properties. As important solutes in Al alloys, Mg and Cu atoms have been experimentally observed to segregate at GBs, which would render a significant impact on the material. In this work, first-principles calculations were used to conduct a systematic and comprehensive investigation to reveal the segregation behavior of Mg and Cu atoms at Al Σ5 (310) and Σ9 (221) GBs and its effect on GB properties. The negative segregation energy demonstrates that both Mg and Cu have a strong tendency to segregate to the Al GBs. Moreover, the segregation of Mg and Cu atoms can reduce the GB energy, indicating an increased thermodynamic stability of the GBs. Mechanical properties are explored by a combination of canonical Griffith fracture model with ab-initio tensile test method. Results reveal that the doped Mg will embrittle both GBs due to the elongation of interatomic bonds and depletion of charge density across the GBs. Contrarily, the segregated Cu contributes greatly to enhancing the GB strength by creating strong Cu-Al bonds with surrounding Al at GBs. Besides, the increased tensile separation displacement of GBs with Cu segregation also suggests an improved resistance to fracture, especially the Σ9 (221) GB. This work provides a theoretical understanding for the changes in microscopic properties of Al GBs caused by Mg and Cu segregation from an atomic and electronic level.

Influence of variation in grain boundary parameters on the evolution of atomic structure and properties of [111] tilt boundaries in aluminum

Grain boundaries (GBs) are fundamental planar defects in metallic materials, exerting a profound influence on material behaviour and properties. Understanding the influence of GB structure on material performance presents challenges due to the intricate atomic arrangements involved. To overcome inconsistencies observed in previous simulations, experimental validation is necessary. However, research on [111] tilt GBs in metals other than copper, such as aluminium, remains limited. This study aims to address these gaps by investigating the characterization of [111] tilt GBs in Al, employing advanced experimental techniques and validating the findings through simulations. By exploring a range of misorientation angles and plane inclinations, two distinct misorientation groups are identified, each exhibiting unique structural configurations. However, the structural differences resulting from plane inclination are more pronounced than those stemming from misorientation angle. Additionally, it is found that deviations from precise misorientation angles and different plane inclinations significantly influence the excess properties of GBs, such as GB energy and excess volume, etc. Furthermore, comparisons between Al and Cu structures enhance our understanding of GB behaviour in different metallic materials. This study lays the foundation for robust structure-property correlations in GBs and emphasizes the importance of considering intricate GB structural details to better understand and predict their properties accurately.

Grain boundaries in polycrystalline materials for energy applications: First principles modeling and electron microscopy

Polycrystalline materials are ubiquitous in technology, and grain boundaries have long been known to affect materials properties and performance. First principles materials modeling and electron microscopy methods are powerful and highly complementary for investigating the atomic scale structure and properties of grain boundaries. In this review, we provide an introduction to key concepts and approaches for investigating grain boundaries using these methods. We also provide a number of case studies providing examples of their application to understand the impact of grain boundaries for a range of energy materials. Most of the materials presented are of interest for photovoltaic and photoelectrochemical applications and so we include a more in depth discussion of how modeling and electron microscopy can be employed to understand the impact of grain boundaries on the behavior of photoexcited electrons and holes (including carrier transport and recombination). However, we also include discussion of materials relevant to rechargeable batteries as another important class of materials for energy applications. We conclude the review with a discussion of outstanding challenges in the field and the exciting prospects for progress in the coming years.

Synergistic effect of Mn and B on the cohesive properties of grain boundaries in Ni3Al: A first-principles study

Ni-based superalloys are known as potential structural materials under extreme conditions due to their good high temperature properties. However, some alloys such as Ni3Al exhibit intrinsic brittleness which severely restricts their practical applications as structural materials. It is well known that the moderate addition of B can significantly improve the ductility of Ni3Al grain boundaries (GB) but restricted to the Ni-rich side. Adding Mn together with B can improve the ductility of Ni3Al in a wider composition range. To provide some fundamental understandings on such a cooperative effect, the segregation of Mn or/and B to the Ni3Al GBs and their effects on the cohesive properties of the GBs are systematically investigated by using a first-principles method. It is shown that B tends to segregate to the Ni-rich holes of the GBs, which means that B prefers to form stronger covalent bonds with the Ni atoms. The calculation of Griffith’s work shows its improved effect on the cohesion of the GBs. Different from site occupation tendency in bulk Ni3Al, Mn tends to segregate to the GBs and substitute the Ni sites at the GBs. It is interesting to notice that B can segregate to the Ni-deficient hole and improve the cohesive properties of the GBs in the presence of Mn substituting Ni atoms at the GBs. Whether at the Ni-rich holes or Ni-deficient holes of B segregation, Mn substituting the Al sites won’t take place at the GBs. The improvement of the ductility of composition-dependent Ni3Al extends from the Ni-rich side to the stoichiometric side with the co-doping of Mn and B. The electronic structure mechanism behind is discussed based on the calculated electronic density of states. It is suggested that the beneficial cooperative effect of Mn and B on the cohesive properties of the GBs is attributable to the increasing B-Al interaction.

Effect of irradiation on mechanical properties of symmetrical grain boundaries in BCC tungsten: an atomistic study

Effect of irradiation on mechanical properties of symmetrical grain boundaries in BCC tungsten: an atomistic study

Coupling of alloy chemistry, diffusion and structure by grain boundary engineering in Ni–Cr–Fe

The diffusion–microstructure correlations for grain boundaries (GBs) in the technologically-relevant Ni-based 602CA alloy are investigated. Prolonged annealing treatments up to 744 h create distinct GB complexions with specific segregation–precipitation–structure states. Globular M23C6-type carbides at straight GBs and plate-like carbides together with NiAl-enriched (γ′-type) particles at hackly GBs are found to co-exist. Moreover, an atomic-scale GB spinodal-like decomposition, especially at straight GBs, is observed. The co-existence of the two distinct states of general high-angle GBs, indicated by tracer diffusion experiments and verified by a detailed structure examination, is explained via state-of-the-art measurements of local elastic strains. In a course of annealing at 873 K, the relatively “fast” diffusivities are found to increase by a factor of 10 or more as a result of a coupled evolution of the GB plate-like precipitates and the irregular GB structures, whereas the relatively ”slow” diffusivites remained practically unchanged representing the contributions of straight interfaces with spherical precipitates. Thus, the diffusion properties of high-angle GBs evolve together with characteristic changes of GB complexions distinguished by a growth of carbide- and γ′-type precipitates and a concomitant generation of GB dislocation networks. The obtained results provide novel insights into grain boundary tailoring by utilizing structure – kinetics correlations involving segregation, precipitation and the evolution of interface defects.

Revealing the deformation mechanisms of 〈110〉 symmetric tilt grain boundaries in CoCrNi medium-entropy alloy

The synergy of multiple deformation mechanisms responsible for the excellent mechanical properties attracts an increasing interest in CoCrNi medium entropy alloy (MEA). In this study, we employed molecular dynamics simulations to investigate the chemical properties of grain boundaries (GBs) and their influence on deformation mechanisms in CrCoNi MEA, with particular emphasis on role of lattice distortion and short-range order (SRO) in dislocation plasticity. After aging, we found Ni element segregate at GB through short-range diffusion driven by a more negative segregation enthalpy. The degree of SRO within the GB region is significantly correlated with the Ni concentration, and it markedly influences the structure and local stress distribution of the GB. Compared to the lattice distortion, SRO can effectively suppress the GB dislocations nucleation and slip, thereby increasing yield strength of the material. During the plastic stage, although the deformation microstructure is intimately linked to the active slip systems and grain orientation, the presence of SRO, as well as the resultant rugged dislocation pathways and increased slip resistance lowers the propensity for stacking faults and phase transformation. The current findings can enhance a fundamental comprehension of diverse deformation mechanisms in CoCrNi MEA and suggest that the chemical characteristics of GB could serve as a pivotal approach for modifying the mechanical properties of MEAs.

Atomic motifs control chemical properties of grain boundaries in Fe alloy

Atomic motifs control chemical properties of grain boundaries in Fe alloy

Three-dimensional phase-field simulation of intergranular bubble evolution in UO2 during irradiation

In this study, a three-dimensional phase-field model was developed to investigate the evolution of intergranular bubbles during irradiation. The model considered several grain structures and multiple fission gas bubbles, with the microstructure evolution being governed by improved dynamic equations. Verification of the phase-field model showed good agreement with the theory in describing the properties of grain boundary (GB). The study examined the dependency of bubble percolation on bubble shape, fission rate, and average grain size. The simulation results revealed that the changes in these three factors have significant effects on the shape of GB bubbles, the percolation rate, and the connectivity threshold of GB bubbles. The bubble connection threshold was obtained from the curve of triple junction coverage versus grain boundary coverage. The results indicate that the connectivity threshold increases with the higher fission rate and lager average grain size. This investigation into bubble percolation during irradiation provides valuable insights for further study on fission gas release in UO2 fuel within nuclear reactors.

Going against the Grain: Atomistic Modeling of Grain Boundaries in Solid Electrolytes for Solid-State Batteries.

Atomistic modeling techniques, including density functional theory and molecular dynamics, play a critical role in the understanding, design, discovery, and optimization of bulk solid electrolyte materials for solid-state batteries. In contrast, despite the fact that the atomistic simulation of microstructural inhomogeneities, such as grain boundaries, can reveal essential information regarding the performance of solid electrolytes, such simulations have so far only been limited to a relatively small selection of materials. In this Perspective, the fundamental properties of grain boundaries in solid electrolytes that can be determined and manipulated through state-of-the-art atomistic modeling are illustrated through recent studies in the literature. The insights and examples presented here will inspire future computational studies of grain boundaries with the aim of overcoming their often detrimental impact on ion transport and dendrite growth inhibition in solid electrolytes.

Identification the Role of Grain Boundaries in Polycrystalline Photovoltaics via Advanced Atomic Force Microscope.

Atomicforce microscopy (AFM)-based scanning probing techniques, including Kelvinprobe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM), have been widely applied to investigate thelocal electromagnetic, physical, or molecular characteristics of functional materials on a microscopic scale. The microscopic inhomogeneities of the electronic properties of polycrystalline photovoltaic materials can be examined by these advanced AFM techniques, which bridge the local properties of materials to overall device performance and guide the optimization of the photovoltaic devices. In this review, the critical roles of local optoelectronic heterogeneities, especially at grain interiors (GIs) and grain boundaries (GBs) of polycrystalline photovoltaic materials, including versatile polycrystalline silicon, inorganic compound materials, and emerging halide perovskites, studied by KPFM and C-AFM, are systematically identified. How the band alignment and electrical properties of GIs and GBs affect the carrier transport behavior are discussed from the respective of photovoltaic research. Further exploiting the potential of such AFM-based techniques upon a summary of their up-to-date applications in polycrystalline photovoltaic materials is beneficial to acomprehensive understanding of the design and manipulation principles of thenovel solar cells and facilitating the development of the next-generation photovoltaics and optoelectronics.

Transport Properties of Low-Angle Grain Boundaries in High-Temperature Superconductors under the Model of Split Dislocations of the Boundary

Transport Properties of Low-Angle Grain Boundaries in High-Temperature Superconductors under the Model of Split Dislocations of the Boundary

Effect of solute atoms segregation on Al grain boundary energy and mechanical properties by first-principles study

First-principles calculations were carried out to study the segregation behavior of Mg, and Cu and their effect on the energy and mechanical properties of different Al grain boundaries (GBs). Four symmetrical tilt GBs were selected for study, namely Σ5[001](210) GB, Σ5[001](310) GB, Σ9[110](221) GB, and Σ11[110](332) GB. The results show that both Mg and Cu have a segregation tendency at the GBs, and the segregation tendency of Cu is stronger than Mg. Mg is prone to form substitutional segregation at the GBs, but Cu is more likely to segregate at the interstitial sites. The segregation of Mg and Cu can reduce GB energy, and the GB energy continues to decrease with the increase of the segregation concentration. First-principles calculation tensile test shows that the segregation of Mg has a negative effect on the strength of GBs, and the GB strength decreases with the increase of the Mg concentration, while the GB strength was gradually enhanced with the increase of the Cu concentration. The strength of Σ5(210) GB and Σ9(221) GB are more sensitive to the segregation of solute atoms than the other two GBs. By calculating the charge density and the density of states of the pristine and the segregated GBs, it was found that the segregation of Mg caused charge depletion and structure expansion at the GBs, while the segregation of Cu increases the charge density of GBs and form new bonds with the surrounding Al atoms. The results provide useful information for improving the mechanical properties of materials by using the concept of GB segregation engineering.

Ab initio calculations and empirical potential assessments of the energy and structure of symmetric tilt grain boundaries in tungsten

Molecular statics/dynamics (MS/MD) based on empirical interatomic potentials (IAPs) is an effective means for conducting grain boundary (GB) related studies. The accuracy of these MS/MD simulations, however, can be greatly affected by the IAP used. Unfortunately, the large discrepancies between the ab-initio values make it difficult to evaluate, improve and further develop IAPs. Motivated by this issue, in the present work we perform a systematic investigation of 〈100〉 and 〈110〉 symmetric tilt GBs in tungsten (W) via atomistic simulations, including the stable structure, GB energy, work of separation, excess volume, and vacancy formation energy. We here use a two-step relaxation process, initially obtaining the candidate structures from MS and then performing DFT validation of these structures to determine the ground state structure, where six empirical IAPs and four exchange–correlation (xc) functionals are used. And the step-like feature is revealed that can be used to reduce the cost of time-consuming optimization processes. Taking the results yielded by PBEsol xc-functional as a benchmark, it is shown that for the same xc-functional, the relative errors of GB energy and work of separation are almost identical, independent of the GB character. The performance of six typical empirical IAPs considered for W is also evaluated by comparing it with the DFT data. The quantities chosen for the comparison include the structure and energetic properties of GBs, as mentioned before. We demonstrate that these empirical IAPs tend to better describe only part of the properties of GBs; for example, some IAPs can predict the energetic properties well, while others can better describe the structure. This suggests that a fully reliable description of GBs remains an elusive objective, requiring further efforts in the development and optimization of the IAP.

Grain boundary microstructure-property relationships in the cast & wrought Ni-based superalloy René 41 with boron and carbon additions

Cast & wrought Ni-based superalloys are required for manufacturing high-performance components in gas turbine engines. Their excellent high-temperature properties and corrosion resistance are brought about by their complex microstructure of a γ-matrix, intra- and intergranular γ' precipitates, carbides, and borides. The most highly-alloyed superalloy grades provide the high-temperature strength required for the design of next generation gas turbine engines but their processability remains challenging due to grain boundary (GB) cracking. A common mitigation approach is the addition of B and C to improve GB cohesion and strength. However, their detailed roles in GB segregation and precipitation and the link to GB properties during processing remains largely unexplored.We provide a systematic study on decorated GBs in the cast & wrought Ni-based superalloy René 41 with B and C additions. Tensile testing reveals that B additions reduce its ductility due to GB cracking but slow down grain growth during heat treatments more effectively, than C additions. GB-M2B, M6C, M23C6, and GB-γ' precipitates decorating GBs are sequentially precipitated. Their structures and compositions are shown to be unaffected by B and C additions. However, significant differences in the interfacial segregation of solutes are revealed. B is captured by GB-M2B and therefore unavailable to improve GB cohesion. C depletion and enrichment of solutes at interphase boundaries are identified as origin for the reduced ductility and thus, GB cracking. The findings are summarized as microstructural model and advance the detailed understanding of the processing – microstructure – properties relationships of cast & wrought Ni-based superalloys.

Structure and Physical Properties of Ceramic Materials Based on ZrO2-Sc2O3 for SOFC Electrolytic Membranes Obtained from Powders of Melted Solid Solutions with a Similar Composition.

This paper presents the results of studying the phase composition, luminescent characteristics, and ionic conductivity of ceramic scandium-stabilized solid solutions of zirconium dioxide containing 9 and 10 mol% Sc2O3. Ceramic samples were prepared by sintering powders obtained by grinding melted solid solutions of the same composition. A comparative analysis of the obtained data with similar characteristics of single crystals has been carried out. Differences in the phase composition of ceramics and initial single crystals were found. The effect of the structure and properties of grain boundaries on the ionic conductivity of ceramic samples is discussed. It is shown that the differences in the ionic conductivity of ceramic samples and crystals are mainly due to changes in the structure and phase composition.

Weight-averaged solute segregations at grain boundaries in advanced ferritic alloys

Solute segregation can have profound effects on the atomic structures and properties of grain boundaries (GBs), and hence the overall mechanical properties of polycrystalline alloys. In this work, a two-step site- and GB-weighted average method was proposed to predict the temperature-dependent segregation behaviors and effects from first-principles. Validated by the experimental S enrichments at GBs, this method was extended to a series of alloying elements in ferrite alloys. The calculation results suggested that the equilibrium weight-averaged GB concentrations follow the general ordering of Y>La>Zr>Hf>Al>Si>Cr>Ti>W, and decrease slowly with temperature. The segregation of Y, La, Ce, Zr, Hf, Al and Si, evidently reduces the GB adhesion strength with La being the most detrimental, while Ti, Cr and W have only minimal impacts on GB adhesion. These segregation effects do not change much with temperature. The new method is not limited to ferritic alloys but can be extended to any other alloy systems.

Theoretical derivation and MD simulation of size dependent mechanical properties of grains and grain boundaries in nanocrystalline nickel Bi-crystal

Theoretical derivation and MD simulation of size dependent mechanical properties of grains and grain boundaries in nanocrystalline nickel Bi-crystal