Segregation Tendency Research Articles

Atomistic simulations and machine learning of solute grain boundary segregation in Mg alloys at finite temperatures

Understanding solute segregation thermodynamics is the first step in investigating grain boundary (GB) properties, such as strong yttrium (Y) effects on grain growth and texture evolution in micro-scale polycrystalline magnesium (Mg) alloys. To estimate the average GB segregation behavior in low-solute-concentration Mg alloys (e.g., 2 at.% Y), a state-of-the-art spectral approach is applied based on a per-site segregation energy spectrum for Y solute atoms at zero K obtained from molecular statistics (MS) simulations of ∼104 GB sites in Mg symmetric tilt GBs (STGBs). Although selected MS simulation results are consistent with verification by density functional theory (DFT) calculations, estimates of average segregation tendency based on the zero-K energy spectrum deviate from experimental observations. To resolve this problem, thermodynamic integration (TI) methods based on molecular dynamics (MD) simulations are used to determine the per-site segregation free energies of Y at representative GB sites, which show contributions beyond harmonic approximations can be important for certain GB sites at high temperatures. A surrogate model of per-site segregation free energy is constructed from a small set of TI data points using stacking cross-validation regressors and physics-informed descriptors. This model is applied to predict the Y segregation free energy spectra for thousands of GB sites in Mg STGBs with uncertainty quantification. The average segregation tendency predicted by the spectral approach based on the free energy spectra agrees well (within the uncertainty range) with experimental observations for micro-scale polycrystalline Mg alloys at typical thermomechanical processing temperatures (500 ∼ 800 K), where thermodynamic equilibrium states are likely to be achieved due to fast diffusion.

O- and OH-induced dopant segregation in single atom alloy surfaces: A combined density functional theory and machine learning study

In this study, we identified the significant factors affecting adsorbate-induced segregation in single-atom alloy (SAA) surfaces by performing Density Functional Theory (DFT)-based calculations and machine learning (ML) methods. We used O and OH species, which are key reactants in oxygen reduction reactions (ORR), as test adsorbates. We constructed SAA surfaces using different transition metals (Ag, Au, Co, Cu, Ir, Ni, Pd, Pt, and Rh) and calculated their segregation energies with and without the adsorbates to predict the segregation tendency of the dopant atom. We examined a total of 44 features which comprised of the elemental, energetics, and electronic properties of the SAAs. We employed a two-stage feature selection to reduce the number of features to the most important features for model training. We found that the formation energies, metallic radius difference, the d-band centers of the dopant in the surface and subsurface layer, the difference in surface energy between the host and dopant atoms, and the difference in the total number of d-electrons between the host and dopant atoms influence the segregation energy of the dopant induced by O and OH. Using these selected features, we implemented linear regression (LR), support vector machine regression (SVR), Gaussian process regression (GPR), and extra trees regression (ETR) algorithms to predict the segregation energies in the presence of adsorbates. For both O- and OH-SAA systems, SVR models exhibited the best performance for predicting adsorbate-induced segregation energies. Among the surfaces we considered, we determined Rh-Au(111) as a potential catalyst for ORR based on the calculated adsorption energies of O and OH and segregation energies in the presence of these adsorbates.

Impact of local chemical ordering on deformation mechanisms in single-crystalline CuNiCoFe high-entropy alloys: a molecular dynamics study

High-entropy alloys (HEAs), composed of multiple constituent elements with concentrations ranging from 5% to 35%, have been considered ideal solid solution of multi-principal elements. However, recent experimental and computational studies have demonstrated that complex enthalpic interactions among constituents lead to a wide variety of local chemical ordering (LCO) at lower temperatures. HEAs containing Cu typically decompose by forming of Cu-rich phases during annealing, thus affecting mechanical properties. In this study, CuNiCoFe HEA was chosen as a model with a tendency for Cu segregation at low temperatures. The formation of LCO and its impact on the deformation behaviors in the single-crystalline CuNiCoFe HEA were studied via molecular dynamics simulations. Our results demonstrate that CuNiCoFe HEA decomposes by Cu clustering, in agreement with prior experimental and computational studies, owing to insufficient configuration entropy to compete against the mixing enthalpy at lower temperatures. A softening in ultimate stress in the LCO models was observed compared to the random solid solution models. The softening is due to the lower unstable stacking fault energy, which determines the nucleation event of dislocations, thereby rationalizing the dislocation nucleation in the Cu-rich regions and the softening of the overall ultimate strength in the LCO models. Additionally, the inhomogeneous FCC–BCC transformation is closely associated with concentration inhomogeneity. CuNiCoFe HEA with LCO can be regarded as composites, consisting of clusters with different properties. Consequently, concentration inhomogeneity induced by LCO profoundly impacts the mechanical properties and deformation behaviors of the HEA. This study provides insights into the effect of LCO on the mechanical properties of CuNiCoFe HEAs, which is crucial for developing HEAs with tailored properties for specific applications.

A Comparative Study on Microstructure, Segregation, and Mechanical Properties of Al-Si-Mg Alloy Parts Processed by GISS-HPDC and SEED-HPDC.

There are multiple routes to prepare semi-solid slurries with a globular microstructure for semi-solid forming. The variations in the microstructure of semi-solid slurries prepared using different routes may lead to significant differences in the flow behavior and mechanical properties of rheo-diecasting parts. Therefore, it is crucial to have a comprehensive understanding of the microstructure evolution associated with different slurry preparation routes and their resulting effects. In this study, the gas-induced semi-solid process (GISS) and the swirl enthalpy equilibrium device (SEED) routes were employed to prepare semi-solid Al-Si-Mg slurries for their simplicity and productivity in potential industrial applications. The prepared slurries were then injected into the shoot sleeves of a high-pressure die casting (HPDC) machine to produce tensile test bars. Subsequently, the bars underwent T6 treatment to enhance their mechanical properties. The microstructure, segregation, and mechanical properties of the samples were investigated and compared with those of conventional HPDC. The results indicated that the GISS and SEED can produce semi-solid slurries containing a spherical α-Al primary phase, as opposed to the dendritic structure commonly found in conventional castings. The liquid fraction had a significant effect on the flow behavior, resulting in variations in liquid segregation and mechanical properties. It was observed that a higher solid fraction (>75%) had a suppressing effect on surface liquid segregation. In addition, the tendency for liquid segregation gradually increased along the filling direction due to the special flow behavior of the semi-solid slurry with a low solid fraction. Furthermore, under the same die-casting process parameters, the conventional HPDC samples exhibit higher yield stress (139 ± 3 MPa) compared to SEED-HPDC and GISS-HPDC samples, which may be attributed to the small grain size and the distribution of eutectic phases. After undergoing the T6 treatment, both SEED-HPDC and GISS-HPDC samples showed a significant improvement in yield and tensile strength. These improvements are a result of solution and precipitation strengthening effects as well as the spheroidization of the eutectic Si phase. Moreover, the heat-treated SEED-HPDC samples demonstrate higher ultimate strength (336 ± 5 MPa) and elongation (13.7 ± 0.3%) in comparison to the GISS-HPDC samples (307 ± 4 MPa, 8.8 ± 0.2%) after heat treatment, mainly due to their low porosity density. These findings suggest that both GISS-HPDC and SEED-HPDC processes can be utilized to produce parts with favorable mechanical properties by implementing appropriate heat treatments. However, further investigation is required to control the porosities of GISS-HPDC samples during heat treatment.

Grain boundary alloying segregation to resist hydrogen embrittlement in BCC-Fe steels: Atomistic insights into solute-hydrogen interactions

Grain boundary (GB) segregation of alloying elements is widely recognized to influence the mechanical properties of metallic materials. However, the rational utilization of GBs' chemical heterogeneity to mitigate hydrogen embrittlement (HE) remains challenging. Here, we employ comprehensive first-principles calculations to investigate solute-hydrogen interactions across five distinct GBs. The calculations provide fundamental insights into H concentration, transport, and induced decohesion at BCC-Fe Σ5(013)[100] GB with alloying segregations including W, Mo, Nb, and Mn, emphasizing the crucial importance of hydrogen adsorption energies. Through statistically identifying the effective descriptors derived from complex atomic environments of various GBs, we elucidate solute-hydrogen interaction mechanisms at GBs from broadly applicable solute valence and electronegativity factors. The analysis highlights the vital role of concurrently controlling alloying segregation tendency, GB separation work, and local H solubilities, thereby comprehending GB engineering strategies and facilitating design of high-performance alloys with GBs resistant to HE.

Predicting the Strength of Cohesive and Adhesive Interparticle Interactions for Dry Powder Inhalation Blends of Terbutaline Sulfate with α-Lactose Monohydrate.

Grid-based systematic search methods are used to investigate molecule-molecule, molecule-surface, and surface-surface contributions to interparticle interactions in order to identify the crystal faces that most strongly affect particle behavior during powder blend formulation and delivery processes. The model system comprises terbutaline sulfate (TBS) as an active pharmaceutical ingredient (API) and α-form lactose monohydrate (LMH). A combination of systematic molecular modeling and X-ray computed tomography (XCT) is used to determine not only the adhesive and cohesive interparticle energies but, also the agglomeration behavior during manufacturing and de-agglomeration behavior during delivery after inhalation. This is achieved through a detailed examination of the balance between the adhesive and cohesive energies with the XCT results confirming the blend segregation tendencies, through the particle-particle de-agglomeration process. The results reveal that the cohesive interaction energies of TBS-TBS are higher than the adhesive energies between TBS and LMH, but that the cohesive energies of LMH-LMH are the smallest between molecule and molecule, molecule and surface, and surface and surface. This shows how systematic grid-search molecular modeling along with XCT can guide the digital formulation design of inhalation powders in order to achieve optimum aerosolization and efficacy for inhaled medicines. This will lead to faster pharmaceutical design with less variability, higher quality, and enhanced performance.

Revealing the influence of solute segregation on the stability and strength of Cu Σ11 [110](113) symmetrical tilt grain boundary via first-principles investigation

The stability and strength of the grain boundary (GB) structure are critical for the practical application of nanocrystals. This study systematically calculates the impact of the segregation of 26 transitional alloy elements on Cu Σ11 [110](113) GB using the first-principle theoretical calculation method. The research findings demonstrate that there is a parabolic connection between atomic number and segregation energy of TM elements in the same period. In addition to Fe, Co, and Ni, all other 23 elements have the potential to segregate to Cu GB, with Zr exhibiting the strongest segregation tendency. The difference in atomic radius and electronegativity between solute atoms and copper atoms strongly affects the segregation tendency, the greater the difference, the greater the tendency for segregation. The segregation energy of solute segregation has strong linear correlation with its GB energy, and the stronger the segregation ability, the more stable the GB. Except for Zn, Ag, Cd, and Au, other 19 alloying elements can increase the strength of GB. Electronic analysis indicates that the strengthening and embrittlement of solutes are primarily related to the accumulation and consumption of electrons near GB and the bonding between atoms.

Durability and Interfacial Elemental Diffusion about BZYb based Protonic Ceramic Fuel Cell

Protonic ceramic fuel cells (PCFCs) are expected to be a highly efficient next-generation fuel cell because the fuel is not diluted by the generated water vapor during power generation. In the past research, we have found that Yb-doped BaZrO3 (BZYb) is promising in terms of the durability against CO2, proton conductivity, and less reactivity with NiO used for the fuel electrode. We are currently developing improvement of power generation efficiency and durability of PCFCs using BZYb electrolyte. In terms of the durability of PCFCs, there is concern about degradation due to diffusion of transition metals such as Co and Ni used in the air and fuel electrodes. In this study, we evaluated the durability of PCFCs using BZYb electrolyte focusing on the diffusion of transition metals derived from electrodes. We also examined the effect of a buffer layer on the air electrode side. In this study, fuel electrode supported cells with BaZr0.8Yb0.2O3-δ(BZYb20) electrolyte and NiO-BZYb20 fuel electrode was used. The fuel electrode half cells were fabricated by integrally firing process of the laminated green sheets of the electrolyte and fuel electrode to densify the electrolyte. For air electrode, BaZr0.375Yb0.125Co0.50O3-δ was screen printed on the fuel electrode half cell and fired at 900°C. For the cell with buffer layer between the air electrode and the electrolyte, BaCe0.7Zr0.1Y0.1Yb0.1O3- δ was spin-coated on the half-cell and then fired at 1300°C. The air electrode was then prepared in the same way. Durability tests were conducted at constant-current mode at 600°C, and the elemental diffusion state of the cells before/after the test was observed by cross-sectional EPMA. During 1000h power generation, the increase of ohmic resistance and polarization resistance was observed, and the increase in resistance was found to be smaller in cells with buffer layer. The results of cross-sectional EPMA analysis after the power generation test showed differences in the segregation state of Co and Ni in the electrolyte, with a tendency for segregation to be suppressed in the presence of a buffer layer.Acknowledgement: This work is partially based on results obtained from a project (JPNP20003), commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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.

Rheological property and stability of nano-silica modified self-compacting concrete with manufactured sand

Self-compacting concrete (SCC) of high fluidity may lead to the high tendency of segregation, especially when manufactured sand (MS) is used. In this work, the rheological properties of SCC are optimized by adding nano-silica (NS) and limestone powder (LP) at different water-to-binder ratios (W/B). The segregation tendency of SCC is evaluated by calculating the stability index (i.e., the degree of segregation of aggregate) through image analysis method. The result shows that MS greatly decreases the stability of SCC, and the static stability index increases from 10.3% to 29.13% when the amount of MS exceeds 20%, increasing risk of segregation. NS can improve the stability of SCC during vibration significantly, however, when the dosage exceeds 0.3%, the fluidity (slump flow) is significantly reduced by 30%. LP improves the stability of SCC by increasing the plastic viscosity. Furthermore, the relationship between the dynamic yield stress/plastic viscosity and the instability index are linearly fitted, the former of which shows a higher correlation coefficient. Finally, the workability and stability of SCC are comprehensively evaluated by the rheological interval box, that is, the SCC with MS owns the best workability when yield stress is (0 ∼ 15.72 Pa) and plastic viscosity is (76.8 ∼ 250.4 Pa·s).

Optimising Spread-Layer Quality in Powder Additive Manufacturing: Assessing Packing Fraction and Segregation Tendency

Powder bed fusion (PBF), a subset of additive manufacturing methods, is well known for its promise in the production of fully functional artefacts with high densities. The quality of the powder bed, commonly referred to as powder spreading, is a crucial determinant of the final quality of the produced artefact in the PBF process. Therefore, it is critical that we examine the factors that impact the powder spreading, notably the powder bed quality. This study utilised a newly developed testing apparatus, designed specifically for examining the quality of powder beds. The objective was to analyse the influence of various factors, including the recoater shape, recoater gap size, and the different powder flow properties, on the powder bed relative packing fraction. Additionally, the study aimed to assess the variation in the particle size and shape across the build plate. The results indicated that all of the variables examined had an impact on the relative packing fraction, as well as the size and shape variations observed across the build plate.

Dislocation-assisted particle dissolution: A new hypothesis for abnormal growth of Goss grains in grain-oriented electrical steels

The physical mechanisms that lead to the abnormal growth of Goss-oriented grains in grain-oriented electrical steel (GOES) are still not well understood, despite almost a century of research. The present paper reviews the existing hypotheses on the formation of Goss-oriented grains by abnormal grain growth and provides more insights into the underlying mechanism for Goss texture formation by proposing a new hypothesis named “Dislocation-assisted particle dissolution”. Abnormally grown Goss-oriented grains in fully-processed industrial GOES samples are shown to contain a fine network of internal subgrain boundaries with very low angle (0.03° - 0.18°) each consisting of regular arrays of dislocations. These subgrain boundaries form a branched ray-like pattern from the Goss grain center towards its perimeter, i.e. they seem to have evolved with the grain during its growth. Structural and compositional analysis of these dislocations by controlled electron channelling contrast imaging (cECCI) and atom probe tomography (APT) show that these dislocations are enriched with solutes such as Sn, Cu, C, and more importantly, with Al, N and Mn, which all build the composition of the inhibitor particles that assist the abnormal growth of Goss-oriented grains. Additionally, molecular statics (MS) calculations are employed to compare the segregation tendencies of Al atoms on dislocations and on Σ9 boundaries. It is found that Al prefers to segregate to dislocations rather than to the boundaries. The origin and the role of subgrain boundaries are discussed based on the experimental and simulation results. The results indicate that, after the dissolution of inhibitors along the grain boundaries, solutes are absorbed by the subgrain dislocations. As a result, grain boundaries surrounding Goss grains become less decorated by solutes and precipitates and more mobile compared to the boundaries of matrix grains.

Jahn–Teller-driven phase segregation in MnxCo3−xO4 spinel thin films

Transition metal spinel oxides comprised of earth-abundant Mn and Co have long been explored for their use in catalytic reactions and energy storage. However, understanding functional properties can be challenging due to differences in sample preparation and the ultimate structural properties of the materials. Epitaxial thin film synthesis provides a novel means of producing precisely controlled materials to explore the variations reported in the literature. In this work, MnxCo3−xO4 samples from x = 0 to x = 1.28 were synthesized through molecular beam epitaxy and characterized to develop a material properties map as a function of stoichiometry. Films were characterized via in situ x-ray photoelectron spectroscopy, x-ray diffraction, scanning transmission electron microscopy, and polarized K-edge x-ray absorption spectroscopy. Mn cations within this range were found to be octahedrally coordinated, in line with an inverse spinel structure. Samples largely show mixed Mn3+ and Mn4+ character with evidence of phase segregation tendencies with the increasing Mn content and increasing Mn3+ formal charge. Phase segregation may occur due to structural incompatibility between cubic and tetragonal crystal structures associated with Mn4+ and Jahn–Teller active Mn3+ octahedra, respectively. Our results help in explaining the reported differences across samples in these promising materials for renewable energy technologies.

Synergistic effect of multi-element co-segregation on mechanical properties of Mg twin grain boundary

Properties of grain boundaries (GBs) are pronouncedly influenced by alien elemental segregation. Synergistic effect of co-segregation behavior on mechanical properties of Mg twin GB was systematically investigated with first-principles calculations. Ten elements (Ni, Mn, Zn, Al, Ag, Ti, Li, Zr, Y, Ca) for single segregation and four solute combinations (Al–Ca, Al–Y, Zn–Ca, Zn–Y) for co-segregation were chosen respectively, the segregation energies and solubility energies with different elemental concentrations were calculated to evaluate the thermodynamic stability of corresponding segregated GBs. These solute atoms have unique effects on the stability of the system, and the interaction between solute atoms also has a synergistic effect on the stability of the system. It is found that the segregation tendency is enhanced and the GB structure tends to be stable with the increase of the concentration of solute atoms at the appropriate sites, regardless of single-element segregation or co-segregation. The redistribution of GB charges due to co-segregation significantly affects the binding and mechanical properties of GBs. With the increase of the co-segregation concentration, the lamellar charge is transferred between the matrix and the twins, which further enhances the electron hybridization and leads to a notable enhancement of interface binding. This shows that the appropriate combination of solute elements can effectively improve the mechanical properties of the interface. The manuscript reveals the theoretical understanding of the effect of elemental segregation on the mechanical properties of GB, and provides design ideas for the tuning of mechanical properties of magnesium alloy by GB engineering.

Effect of Intensification Pressure on Eutectic Si Segregation of Al-12Si-1Cu Alloy in Squeeze Casting

Squeeze casting is an advanced technology to produce aluminum alloy components with excellent mechanical property. Due to good fluidity and castability, the Al-Si alloy is the most common aluminum material in squeeze casting. However, the segregation in solid α-Al phase and liquid eutectic Si phase happens during solidification, which leads to microstructural inhomogeneous in the final microstructure. In this study, the eutectic Si segregation of Al-12Si-1Cu alloy under different intensification pressure in squeeze casting was investigated by Optical Microscopy (OM). The results show that increasing intensification pressure promotes the eutectic Si segregation at center zone, while the eutectic Si phase at skin zone is not much influenced. Increasing the intensification pressure from 65 MPa to 140 MPa, the segregation degree of eutectic Si at center zone increases from 0.05 to 0.29, while the segregation degree at skin zone levels at 0.2. Under high intensification pressure, the feeding effect of liquid phase at center zone is prominent, which results to high tendency of eutectic Si segregation in the final microstructure.

Nonmetallic trace elements induced rhenium co-segregation in nickel Σ5 [001](210) symmetrical tilt grain boundary

The thermally/mechanically-driven structural instability of nano- and coarse-grained polycrystalline metals originates from the unstable grain boundaries (GBs). GB segregation of solute elements could not only stabilize but also embrittle GBs. To improve GB stability and cohesion in parallel via solute segregations, in this work the segregation behavior of six non-metallic impurities X at the Σ5 [001](210) GB of nickel-based alloys, their co-segregations with Re element that has no GB segregation tendency, and segregation-induced changes in GB properties were investigated comprehensively from first-principles calculations. We discovered that these nonmetallic elements can segregate to the Ni GB and thus stabilize it, but they are GB embrittlers except B. Annealing temperature and bulk concentration have no effect on the S segregation. With the X pre-segregations, induced GB segregations of Re were observed, which lowers the excess energy and enhances the cohesive strength of all X-segregated GBs. The attractive interactions between C/N/B and Re atoms promote the Re segregation. The stabilizing and strengthening mechanisms of Re segregation were mainly attributed to the changes in Fermi level location and the increased interplanar bonding, respectively. Our findings may gain more insights into the atomic-scale GB co-segregations and stabilization of polycrystalline Ni-based alloys.

Feasibility of High-Density and Non-Segregable Niobium Ore Tailings

Tailings disposal in the form of diluted slurries has a tendency for particle size segregation, where coarse particles settle near the discharge point, and finer particles are carried by the water flux to more distant regions. This causes a loss of reservoir capacity due to voids between the coarser particles and increased water content in the deposit. This work aimed to evaluate the feasibility of reaching non-segregable high-density slurries with fine tailings from the niobium oreflotation process and measure its disposal parameters. The innovation is to achieve increased solids percentage in the settled deposit and to avoid particle size segregation along the slurry path with niobium tailings. The study involved physical, chemical, and mineralogical characterization and semi-pilot thickening tests to produce enough volume of underflow with different bed heights and solids flux rates. Slump, rheology, and flume tests were performed to evaluate underflow disposal characteristics. The results indicated that the thickener bed height did not significantly influence the underflow solids content, yield stress, or slump. The solids flux rate, on the other side, had a greater influence—the higher it was, the lower the solids content, yield stress, and disposal angle, along with a higher slump. In flume tests, a high density of non-segregable tailings slurry was achieved with 1.96 t/m3, corresponding to an underflow with 66.8% solids, 43.9 Pa of yield stress with 0.5 (t/h)/m2, and 0.5 m of bed height.

The Evolution of Structural Defects under Irradiation in W by Molecular Dynamics Simulation.

Tungsten (W) can be used in plasma-facing components in a fusion reactor because of its excellent radiation resistance. Some studies have found that nanocrystalline metals with a high density of grain boundary show a higher ability to resist radiation damage compared to conventional coarse-grained materials. However, the interaction mechanism between grain boundary and defect is still unclear. In the present study, molecular dynamics simulations were carried out to explore the difference of defect evolution in single-crystal and bicrystal W, while the effects of temperature and the energy of the primary knocked atom (PKA) were taken into account. The irradiation process was simulated at the temperature range of 300 to 1500 K, and the PKA energy varied from 1 to 15 keV. The results show that the generation of defects is more sensitive to the energy of PKA than temperature; the number of defects increases at the thermal spike stage with the increase of the PKA energy, but the correlation with temperature is not strong. The presence of the grain boundary prevented the recombination of interstitial atoms and vacancies during the collision cascades, and the vacancies were more likely to form large clusters than interstitial atoms in the bicrystal models. This can be ascribed to the strong segregation tendency of the interstitial atoms to grain boundaries. The simulations provide useful information for understanding the role of grain boundaries in the evolution of irradiated structural defects.

Grain boundary segregation of nickel vacancies and space charge zone formation in NiO through interactions among Ni2+, O2−, and Ni3+

Defects like nickel vacancies (VNi″), holes (Ni3+) with different segregation tendencies to grain boundary (GB) can govern the properties of nickel oxide (NiO) – a promising material for energy applications. An atomistic model of non-stoichiometric NiO bicrystal with θ°001100 GB is developed to understand the segregation of VNi″, Ni3+ to GB from various ionic interactions. According to the model lattice positions available for VNi″ in GB possess lower attractive (from Ni2+-O2− interactions) and higher repulsive (from Ni2+-Ni2+ interactions) energies when compared with those in grain interior. This is due to the differences in these inter-ionic distances in GB and grain interior. Hence VNi″ segregate to GB. Contrarily, grain interior is more favorable for formation of Ni3+ in GB. These Ni3+ form a space charge zone adjacent to the GB. The model helps in understanding VNi″ segregation to GB and space charge zone formation.

Hydrostatic pressure-induced transition in grain boundary segregation tendency in nanocrystalline metals

Impurity segregation at grain boundaries (GBs) can have a profound impact on the mechanical behavior of nanocrystalline metals. In this study, we employ atomistic simulations to investigate the effects of hydrostatic pressure on the GB segregation energy spectra in a series of binary alloy systems. Our results demonstrate that, depending on the alloy system, hydrostatic pressure can either enhance or hinder GB segregation tendencies. However, in specific alloys, such as AlMg and NiNb, hydrostatic pressure can also result in transitions in the preferred segregation sites, characterized by an initial increase followed by a decrease at certain pressures. The underlying mechanisms for these transitions can be attributed to the changes in the elastic component brought about by hydrostatic pressure, which dominate the other contributions to the total segregation energy.