Grain Boundary Composition Research Articles

Computational approach to grain boundary segregation engineering of nickel-base superalloys

Grain boundary (GB) strengthening elements, such as B, C, and Zr have been added in small amounts to nickel-base superalloys. However, their strengthening effects have not been quantified and no specific design principles for GB chemistry have been reported. In this study, we propose a practical computational approach for the GB segregation engineering of nickel-base superalloys. Considering the partitioning of alloying elements into coexisting phases (strengthening phases, carbides, etc.), the equilibrium composition of a high-angle GB was computed for several nickel-base superalloys using a calculation of phase diagrams database. The computational results showed that B and Mo were enriched at the GB in most of the investigated alloys. The creep rupture strengths of the investigated alloys were predicted using the computed GB composition as a regression model feature. The regression coefficients for the features confirm that B segregation at the GB has a non-negligible strengthening effect on nickel-base superalloys.

Theoretical Prediction of Strengthening in Nanocrystalline Cu with Multi-Element Grain Boundary Segregation Decoration.

The composition of grain boundaries (GBs) determines their mechanical behavior, which in turn affects the mechanical properties of nanocrystalline materials. Inspired by GB segregation and the concept of high-entropy alloys (HEAs), we investigated, respectively, the mechanical responses of nanocrystalline Cu samples with and without multi-element GBs, as well as the grain size effects, aiming to explore the effects of GB composition decoration on mechanical properties. Our results show that introducing multi-element segregation GBs can significantly improve the mechanical properties of nanocrystalline Cu by effectively inhibiting GB migration and sliding. Additionally, we proposed an improved a theoretical model that can reasonably describe the strengths of the materials with multi-element or single-element segregation GBs. Notably, the introduction of multi-element segregation GBs inhibits both migration and sliding behavior, with migration being more effectively suppressed than sliding. These results present a novel approach for designing high-performance nanometallic materials and offer valuable insights into the role of GB composition decoration in enhancing mechanical properties.

Grain boundary segregation models for high-entropy alloys: Theoretical formulation and application to elucidate high-entropy grain boundaries

Grain boundary (GB) segregation models are derived for multi-principal element alloys (MPEAs) and high-entropy alloys (HEAs). Differing from classical models where one component is taken as a solvent and others are considered solutes, these models are referenced to the bulk composition to enable improved treatments of MPEAs and HEAs with no principal components. An ideal solution model is first formulated and solved to obtain analytical expressions that predict GB segregation and GB energy in MPEAs and HEAs. A regular solution model is further derived. The GB composition calculated using the simple analytical expression derived in this study and data from the Materials Project agrees well with a prior atomistic simulation for NbMoTaW. The simplicity of the derived analytical expressions makes them useful for not only conveniently predicting GB segregation trends in HEAs but also analyzing nascent interfacial phenomena in compositionally complex GBs. As an application example, the models are used to further derive a set of equations to elucidate an emergent concept of high-entropy grain boundaries.

Effect of cooling rate on the composition and chemical heterogeneity of quench-induced grain boundary η-phase precipitates in 7xxx aluminium alloys

The mechanical properties and stress corrosion cracking (SCC) resistance of 7xxx series aluminium alloys are significantly affected by the composition and distribution of precipitates formed during heat treatment. In particular, their quench sensitivity is related to the formation of η-phase precipitates that nucleate heterogeneously on grain boundaries at lower cooling rates after solution treatment, which has been a key factor restricting the gauge of hot rolled plates in the aerospace industry. To better understand the effects of slower cooling rates on the composition of quench-induced grain boundary precipitates (Q-GBPs) found in thick plate 7xxx alloys, plasma focused ion beam and high-resolution scanning transmission electron microscopy were used to obtain accurate composition data. The η-phase Q-GBPs have a complex- branched morphology, which develops higher aspect ratios and secondary arms as the cooling rate is reduced. Only a small change in average composition of Q-GBPs was found with cooling rate; but a large scatter was observed. This is caused by significant Zn/Cu/Al composition gradients developing along their principal growth directions in both AA7050 and AA7085 alloys. This concentration gradient did not reduce significantly after a T76 treatment. Simulations of Q-GBP growth with different cooling rates using a CALPHAD-informed phase-field model, with the η-phase represented by a two-sublattice model, gave results consistent with experimental observations. Chemical gradients were predicted to develop in the Q-GBPs due to the changing local equilibrium at the growth front during the cooling. The influence of this non-homogeneous microchemistry on the SCC behaviour of 7xxx alloys is briefly discussed.

Grain refinement induced by grain boundary segregation in FeNiCrCoCu high-entropy alloys using molecular dynamics simulation of nanoindentation

Atomistic simulation of nanoindentation is performed to investigate the effect of Cu segregation at grain boundaries (GBs) on the mechanical properties of FeNiCrCoCu high-entropy alloys. Initially, a combined Monte Carlo and Molecular dynamics simulation technique is adopted to generate the models with Cu segregation, and further the cases of random Cu distribution are considered for the purpose of comparision. The dislocation density, shear strain and GB behavior in the process of nanoindentation are analyzed in detail. The results reveal that the Cu segregation at GBs leads to superiority in hardness, strength, and stiffness. Dislocation analysis shows that the sessile dislocation density is higher at the initial state for the segregation sample, while dislocation nucleation and movements are suppressed so that dislocation-mediated plasticity is weakened. Furthermore, microrotation analysis during the nanoindentation process indicates that segregation of Cu pins the GBs, thereby inhibiting GB rotation and consequently weakening GB-mediated plastic deformation. In this case, grain refinement is observed, and in good agreement with experiment results. Overall, this study provides insights into the influence of GB segregation of Cu on the mechanical properties and deformation response of FeNiCrCoCu HEAs, and highlights the importance of GB composition for tailoring high-strength materials.

An Overview of the Principles of Low-Temperature Superplasticity in Metallic Materials Processed by Severe Plastic Deformation

Low-temperature superplasticity (LTS) is crucial to reduce manufacturing cost and to enhance the applications of superplastic forming. It is well known that grain refinement is the key point to decrease the temperature at which the superplasticity is attained. Therefore, ultrafine-grained (UFG) materials have become attractive for achieving LTS. Severe plastic deformation (SPD) techniques provide abnormal grain refinement, and thus they have been used to achieve LTS in metallic materials. This paper overviews and examines the reports of LTS in the severely-deformed metallic materials. It provides fundamentals of grain refinement via different SPD techniques in various classes of metallic materials including Al-, Mg-, and Ti-based alloys. It also gives a brief summary about the effect of microstructural requirements on LTS with an emphasis on grain size, type and chemical composition of grain boundaries and microstructural alteration during the superplastic deformation. In the last section of the manuscript, the main deformation mechanism of LTS were also explained.

Microstructure, chemical composition, and dielectric response of CaCu3Ti4O12 ceramics doped with F, Al, and Mg ions

Ceramics with nominal chemical composition CaCu3Ti4O12 (CCTO), CaCu3Ti3.96Al0.04O11.96F0.04 (CCTOAF), and Ca0.98Mg0.08Cu2.94Ti3.96Al0.04O11.96F0.04 (CCTOMAF) were prepared by the solid-state reactions technique. Using SEM, EDX, XPS, EPR, NMR, and complex impedance spectroscopy, the microstructure, elements distribution, chemical composition of grains and grain boundaries, and the dielectric response of ceramics were investigated. In the ССТО, CCTOAF, and CCTOMAF series, the average grain size increases, the degree of copper segregation at the grain boundaries is inversely related to grain size, and the dielectric loss decreases from 0.071 to 0.047 and 0.030, respectively, while dielectric permittivity ε′ at 1 kHz is 5.6 × 104, 7.1 × 104, and 4.3 × 104, respectively. Additives of Al, Mg, F and milled particles (ZrO2, Al2O3, and SiO2) can either partially introduce into the perovskite structure or form low-melting eutectics at the grain boundaries, causing abnormal grain growth. The presence of copper ions in various oxidation states, as well as evidence of exchange spin interactions between them, was confirmed in all samples.

Atomic motifs govern the decoration of grain boundaries by interstitial solutes

Grain boundaries, the two-dimensional defects between differently oriented crystals, tend to preferentially attract solutes for segregation. Solute segregation has a significant effect on the mechanical and transport properties of materials. At the atomic level, however, the interplay of structure and composition of grain boundaries remains elusive, especially with respect to light interstitial solutes like B and C. Here, we use Fe alloyed with B and C to exploit the strong interdependence of interface structure and chemistry via charge-density imaging and atom probe tomography methods. Direct imaging and quantifying of light interstitial solutes at grain boundaries provide insight into decoration tendencies governed by atomic motifs. We find that even a change in the inclination of the grain boundary plane with identical misorientation impacts grain boundary composition and atomic arrangement. Thus, it is the smallest structural hierarchical level, the atomic motifs, that controls the most important chemical properties of the grain boundaries. This insight not only closes a missing link between the structure and chemical composition of such defects but also enables the targeted design and passivation of the chemical state of grain boundaries to free them from their role as entry gates for corrosion, hydrogen embrittlement, or mechanical failure.

Heterogeneity of grain boundary properties in Cu2ZnSnS4: A first-principles study

Using first-principles density functional calculations, we investigate the structure and properties of previously unstudied grain boundaries (GBs) in the solar absorber material copper-zinc-tin-sulfide (CZTS). We identify four stable low-Σ value symmetric tilt GBs with low formation energies: Σ3 (111) and Σ5 (201), each with two different GB terminations. Compared to CdTe and CuInSe, GBs in the quaternary semiconductor CZTS exhibit a wider variety of electronic states due to the more complex chemical environment near the GB, including under-coordinated atoms and dangling bonds. Further analysis confirms that strong dangling bonds introduce deep gap states in all GBs studied. We also investigate segregation and electronic properties of intrinsic point defects to GBs and find that one of the Σ3 (111) GBs exhibits an abnormal defect segregation behavior that favors Cu-poor (Zn-rich) GB composition, which is beneficial for its overall performance.

Decoupling the electrical resistivity contribution of grain boundaries in dilute Fe-alloyed Cu thin films

To study the role of chemical composition on the resistivities of grains and grain boundaries (GB) for dilute Fe-alloyed Cu thin films, Cu films with grain sizes varying over three orders of magnitude and compositions of 0.025 and 0.25 at.% Fe were prepared by magnetron co-sputtering. Character, morphology and compositions of bulk and GBs were studied using electron backscatter diffraction, transmission electron microscopy and atom probe tomography, respectively. The specific resistivities of both individual GBs and within grains were obtained through local electrical measurements assisted by micromanipulation in situ within a scanning electron microscope. In addition, global resistivity characterisation of the thin films allowed for calculation of the GB reflection coefficient. A decoupling of GB and grain interior resistivities is found with alloying, where the GB resistivity increases by an order of magnitude while the grain interior is affected only to a minor extent in comparison.

Metavalent bonding impacts charge carrier transport across grain boundaries

Understanding the mechanisms underpinning the charge carrier scattering at grain boundaries is crucial to design thermoelectrics and other electronic materials. Yet, this is a very challenging task due to the complex characteristics of grain boundaries and the resulting difficulties in correlating grain boundary structures to local properties. Recent advances in characterizing charge transport across grain boundaries are reviewed, demonstrating how the microstructure, composition, chemical bonding and electrical properties of the same individual grain boundary can be correlated. A much higher potential barrier height is observed in high-angle grain boundaries. This can be ascribed to the larger number density of deep trapping states caused by the local collapse of metavalent bonding. A novel approach to study the influence of the local chemical bonding mechanism around defects on the resulting local properties is thus developed. The results provide insights into the tailoring of electronic properties of metavalently bonded compounds by engineering the characteristics of grain boundaries.

48.2: Formation of potential barriers at grain boundaries in multicomponent ZnO‐based transparent thin films

The article discusses the reasons for the deterioration of the electrical characteristics of ZnO‐based thin films in conditions of both high humidity and high ambient temperatures. Based on the standard damp heat test, in which thin samples subjected to temperature 85�C and relative humidity 85 % for 1000 h, it is shown that the stability of electrical performances of the transparent ZnO‐based films may be increased with an increase in the temperature of their deposition, which, in turn, leads to the formation of denser compact films. It is shown that the porous structure formed in ZnO‐based films, which leads to poor thermal stability, is due to the relatively low mobility of adatoms on the growing surface during film deposition at relatively low substrate temperatures. The analysis of the obtained data shows that the key factor determining both the composition and state of grain boundaries (GBs) in nanocrystalline ZnO‐based films, as well as the formation of potential barriers at the GBs is the possible segregation of impurity materials at the GBs, which depends on the arrangement of impurity atoms with respect to Zn in the series of metal activity. The conditions for the amorphization of complex composite oxide structures are considered.

Inhibiting the inverse Hall-Petch behavior in CoCuFeNiPd high-entropy alloys with short-range ordering and grain boundary segregation

The mechanical implications of short-range ordering (SRO) and grain boundary (GB) segregation in the inverse Hall-Petch behavior of nanocrystalline CoCuFeNiPd high-entropy alloys (HEAs) were studied using hybrid Monte Carlo (MC)/Molecular Dynamics (MD) simulations. Results show that the presences of SRO and GB segregation inhibit the grain size softening (inverse Hall-Petch relation), leading to grain-size independence of the high flow stress. We find that the dislocation nucleation at triple junctions is suppressed by the SRO due to the increased stacking fault energy, which attenuates the dislocation activities. Furthermore, the strain localization at GBs is intensified by the GB segregation due to the increased GB energy, which facilitates the GB activities and glass-like deformation. The GB-governed plasticity associated with glass-like deformation leads to suppression of the inverse Hall-Petch effect. These findings may provide useful strategies for the design of high-performance HEAs by tuning the GB composition and SRO structure.

Grain boundary segregation and its implications regarding the formation of the grain boundary α phase in the metastable β-Titanium Ti–5Al–5Mo–5V–3Cr alloy

We investigated the origin of the deleterious grain boundary α phase formed during the aging of the β-titanium alloy, Ti–5Al–5Mo–5V–3Cr–0.5Fe (wt.%). We probed the composition of low and high-angle grain boundaries from the as-quenched β condition correlating electron microscopy and atom probe tomography. Our analysis reveals strong segregation of some α-stabilizing elements at both types of boundaries, especially oxygen, along with a depletion of β-stabilizing elements. The grain boundary maintains the body-centered-cubic structure despite the presence of this local composition. Our thermodynamic calculations, based on the measured grain boundary compositions, indicate that the segregation significantly increases the chemical driving force for α nucleation upon aging. Our work provides essential insight into the formation of the undesired grain boundary α layers along prior-β grain boundaries in metastable β-Ti alloys and paves the way for microstructural engineering of these alloys with enhanced mechanical properties.

Computing grain boundary diagrams of thermodynamic and mechanical properties

Computing the grain boundary (GB) counterparts to bulk phase diagrams represents an emerging research direction. Using a classical embrittlement model system Ga-doped Al alloy, this study demonstrates the feasibility of computing temperature- and composition-dependent GB diagrams to represent not only equilibrium thermodynamic and structural characters, but also mechanical properties. Specifically, hybrid Monte Carlo and molecular dynamics (MC/MD) simulations are used to obtain the equilibrium GB structure as a function of temperature and composition. Simulated GB structures are validated by aberration-corrected scanning transmission electron microscopy. Subsequently, MD tensile tests are performed on the simulated equilibrium GB structures. GB diagrams are computed for not only GB adsorption and structural disorder, but also interfacial structural and chemical widths, MD ultimate tensile strength, and MD tensile toughness. This study suggests a research direction to investigate GB composition–structure–property relationships via computing GB diagrams of thermodynamic, structural, and mechanical (or potentially other) properties.

Structure and chemical composition of grain boundaries in the magnetic semiconductor GaSb<Mn>

The structure and chemical composition of grain boundaries in GaSb<Mn> magnetic semiconductors have been investigated. We determined that quenching of the GaSb melt with 2% Mn results in the formation of a textured polycrystal (111). The grain boundaries of the texture are formed by split 60 degree dislocations with <110> dislocation lines. Microinclusions based on the ferromagnetic compound MnSb are located on the stacking faults of split dislocations. The chemical compositions of microinclusions differ, but their average composition is close to Mn1.1Sb. The synthesized GaSb<Mn> is a soft ferromagnet with a coercive force of 10 Oe and a magnetic state approaching superparamagnetic

Grain Boundaries in Cu(In, Ga)Se2: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy

AbstractCu(In,Ga)Se2 thin‐film solar cells have attracted significant research interest in recent decades due to their high efficiency in converting solar energy into electricity for enabling a sustainable future. Although the Cu(In,Ga)Se2 absorber can be grown as a single crystal, its polycrystalline form is dominating the market not only due to its lower costs, but also due to its unexpectedly higher cell efficiency. However, this absorber contains a high fraction of grain boundaries. These are structural defects where deep‐trap states can be localized leading to an increase in recombination activity. This controversy is mirrored in the existing literature studies where two main contradictory believes exist: 1) to be crucial grain boundaries in Cu(In,Ga)Se2 absorber are anomalous, being benign in terms of cell performance, and 2) grain boundaries are regions characterized by an increased recombination activity leading to deteriorated cell performance. Therefore, the present review tackles this issue from a novel perspective unraveling correlations between chemical composition of grain boundaries and their corresponding electronic properties. It is shown that features such as Cu depletion/In enrichment, segregation of 1‐2at.% of alkali dopants, and passivation by a wide‐bandgap or type inversion at grain boundaries are crucial ingredients for low open‐circuit voltage loss and, hence, for superior cell performance.

Highly porous Sr-doped TiO2 ceramics maintain compressive strength after grain boundary corrosion

Maintaining sufficient mechanical support during bone healing is an essential property for ceramic bone scaffolds. However, grain boundary (GB) dissolution may compromise the mechanical strength of polycrystalline ceramics in the physiological environment. Therefore, we investigated the GB formation and its impact on the compressive strength and corrosion behavior in TiO2 scaffolds doped with calcium and strontium. To alter the GB composition and densification process, sintering conditions were altered. Prolonged sintering times and increased sintering temperature led to improved densification and increased strength in Ca-doped scaffolds. However, dissolution of the resulting amorphous GBs caused a significant loss of compressive strength when exposed to an acidic environment. In contrast, a crystalline SrTiO3 GB phase present in Sr-doped scaffolds, for which increased sintering temperature combined with rapid cooling led to a significantly improved compressive strength. Formation of SrTiO3 crystals in the GBs maintained the strength for over 4 weeks in an acidic environment.

Universal prediction of strain footprints via simulation, statistics, and machine learning: low-Σ grain boundaries

Interface engineering usually imposes a trade-off between catalytic activity, product selectivity, and property tunability, facilitating multiple product yields. Due to their structural disorder, metal grain boundaries (GBs) can be constructed to satisfy all of these conditions. Though experiments show GB catalytic footprints are determined by dislocation-induced elastic strain, the atomistic structural and energetic origins of GB catalytic performance are not currently known. In former models solely reviewing metastable GBs defined by macroscopic degrees of freedom (DOFs), exceptional low-Σ GBs anticipated by the coincident site lattice model cannot be consistently discerned. Herein, a structure-energy correlation linking changes in atomistic properties to physical models is developed, distinguishing exceptional GBs from unexceptional high-angle GBs over structural DOFs. This work constructs particular GB elastic responses by varying three microscopic DOFs, comparing low-Σ distinguished and other GB responses via a directed statistical approach. Such elastic properties successfully distinguish low-Σ GBs from high-angle grain boundaries (HAGBs) by their strain-related footprints without failure of a single unique GB structure and composition combination. A priori application of machine learning classification is completed using metastable GB and elastic response related properties. This classification not only validates distinguished strain footprints, but also identifies GB subgroups with footprints unexpected by former models.

Structure‐composition-property correlations of symmetrical tilt grain boundaries in copper-based binary alloys

The thermodynamic stability and mechanical strength of grain boundaries (GBs) govern many key properties of nanocrystalline or polycrystalline metals. In order to design novel Cu-based alloys with extraordinary properties and broaden the application scopes of Cu alloys, it is essential to clarify the effects of GB structure and composition on the stability and cohesion of Cu GBs. Firstly, the GB energies and works of separation of 12 Cu symmetrical tilt GBs with various GB orientations were figured out by using first-principles calculations, to link the GB properties with structures. Our results shown that the GB energies and works of separation of 12 Cu GBs are notably anisotropic. The GB energies for these GBs have an inverted and linear relationship with their works of separation. As to GB composition, the segregation tendencies of 8 alloying elements (Mg, Ca, Cr, Ni, Zn, Zr, Ag and Sn) in the Cu Σ5 [001](210) symmetrical tilt GB and their cohesive and embrittling effects on the GB were studied. It was found that all considered solutes excluding the element Ni can segregate to the Cu GB and thus play stabilizing roles into the Cu GB. Among 8 solutes, only the Zr and Cr segregations can improve the thermodynamic stability and fracture strength of Cu GB simultaneously. The GB strengthening effects of Zr and Cr segregations was attributed to their significant chemical contributions to the embrittling potencies, i.e., the presence of covalent-like bonding features in newly-formed Cu–Zr/Cr bonds. The reduction/increase of anti-bonding/bonding states below the Fermi level in Cu–Zr/Cr atomic pairs should be responsible for the GB stabilizing effects of Zr and Cr segregations.