Segregation Energy Research Articles

DFT Calculation of High-Angle Kink Boundary in 18R-LPSO Alloy

Kink boundaries formed in Mg-based long period stacking order (LPSO) alloys play a key role in strengthening of these materials. As the kink structure grows, many high-angle kink boundaries are eventually formed which has inclination angle close to 34 degrees. We show that this peculiar structure is a result of irreversible structural transformation and is energetically stable. We also calculate segregation energies of alloying elements Y and Zn to this boundary. Finally, the critical resolved shear stress for the migration of kink boundary is estimated for a pure-Mg kink and that with saturated with segregation. We show that segregated kink boundary requires very high shear stress about 700 MPa for migration.

Surface segregation machine-learned with inexpensive numerical fingerprint for the design of alloy catalysts

Metal alloy catalysts have been known to enhance many essential reactions. Surface-adsorbate interaction of an alloy catalyst depends on the composition and nanostructure of the surface, which can be changed by a phenomenon called surface segregation. Thus, thermodynamic information on the surface segregation is critical to tune the surface-adsorbate interaction, and thus the reactivity of alloy catalysts. Collected 1,366 density functional theory-calculated segregation energies (Esegr) from literature were featurized by 19-dimensional inexpensive numerical fingerprint, which represents facet-, site-, and elemental-dependencies. Deep neural network (DNN) model was constructed based on the data. Reasonable interpolative and extrapolative prediction by the DNN was clearly demonstrated using principal component (PC) analysis. On top of that, impact of each feature on the Esegr was analyzed by an explainable artificial intelligence approach, giving useful insights for alloy catalyst design based on surface segregation. As an example of the applications, the DNN model was used to explain the formation thermodynamics of site-selective nanosegregated Pt alloy catalyst, and was also used to perform elemental screening to find impurity-host metal combinations feasible to form the nanosegregated alloy catalyst. In addition, the guidance for data collection to improve the DNN model was given by analyzing PC space and performing the weight analysis of PCs.

Effects of transition metal segregation on the thermodynamic stability and strength of Ni Σ11 [110](113) symmetrical tilt grain boundary

Precisely controlling grain boundary (GB) thermodynamic stability and strength is important for processing nanograined alloys. The segregation-induced stabilization and strengthening GB mechanisms of 20 transition metal elements in Ni Σ11 [110](113) STGB with low interfacial excess energy were clarified from first-principles total-energy calculations in this work. The results indicate that the segregation energy and GB energy are inversely related to the radius difference between TM and matrix Ni atoms. The segregation of 11 elements were identified to enable the Ni Σ11 [110](113) STGB to be stable according to Schuh criterion for GB stability. The electronic mechanism of GB stabilization through TM segregation is mainly due to the less anti-bonding states of Ni-X pairs than those of Ni–Ni pairs in unsegregated GB, from the analysis of crystal orbital Hamiltonian populations. The GB embrittlement resulted from TM segregation is mainly from the mechanical contribution caused by atomic size mismatch between segregation atom and matrix atom. The segregation caused GB strengthening comes from the chemical contribution, which is due to the fact that the segregation of X leads to the formation of stronger Ni-X covalent-like bonds instead of Ni–Ni metallic bonds. This work could provide better understanding on designing high-stable Ni nanograined alloys.

Atomistic insights into the inhomogeneous nature of solute segregation to grain boundaries in magnesium

In magnesium alloys with multiple substitutional elements, solute segregation at grain boundaries (GBs) has a strong impact on many important material characteristics, such as GB energy and mobility, and therefore, texture. Although it is well established that GB segregation is inhomogeneous, the variation of GB solute composition for random boundaries is still not understood. In the current study, atomic-scale experimental and simulation techniques were used to investigate the compositional inhomogeneity of six different GBs. Three-dimensional atom probe tomography results revealed that GB solute concentration of Nd in Mg varies between 2 and 5 at.%. This variation was not only seen for different GB orientations but also within the GB plane. Correlated atomistic simulations suggest that the inhomogeneous segregation behavior observed experimentally stems from local atomic rearrangements within the GBs and introduce the notion of potential excess free volume in the context of improving the prediction of per-site segregation energies.

Effects of alloying elements on the interfacial segregation of bismuth in tin-based solders

Sn-Bi solders are potential candidates for replacing traditional lead-based solders in electronic packaging due to their low melting points, superior wettability, and mechanical properties. The grain coarsening due to the interfacial Bi segregation is a vital issue that affects the reliability of Sn-Bi solder joints. Through first-principles calculations, this study systematically revealed the effects of different alloying elements on the interfacial segregation of Bi towards Sn(100) surface. Bi tends to segregate to the top-surface of Sn due to the formation of vacancies on Sn surface. However, doping alloying elements into the Sn-Bi solders could effectively suppress the interfacial Bi segregation. The presence of elements such as Pd, Pt, and Au located at the adjacent surface layer of Bi could inhibit the Bi precipitation, by apparently increasing the segregation energies of Bi to the Sn surface. The results can be interpreted by the enhanced bond orders and charge transfer between Bi, alloying elements, and their neighboring Sn atoms. This study could provide valuable insights into the design of multi-component Sn-Bi solders with minor alloying elements doping in the future.

Oxidation susceptibility of UN Σ5(210) grain boundary with Al/Cr/Ni dopant: A first-principles study

The oxidation susceptibility of UN Σ5(210) grain boundary (GB) decorated with Al, Cr or Ni is assessed by first-principles modelling. The predictions show that the low segregation energy of O at the Al-doped GB leads to increasing O enrichment, which explains the high O concentration observed in experiments. GB with Ni exhibits formidable antioxidant capability but enhances sensitivity to embrittlement. Cr dopant not only effectively reduces O segregation but also improves the cohesive capability of oxidized GB. Our work clarifies the mechanisms of (un)-doped UN GB oxidation from an atomic perspective, in which analysis so far rests on experiments only.

Identification of the Segregation Kinetics of Ultrathin GaAsSb/GaAs Films Using AlAs Markers.

For optoelectronic devices from the near to the far infrared, the advantages of using ultrathin III-Sb layers as quantum wells or in superlattices are well known. However, these alloys suffer from severe surface segregation problems, so that the actual profiles are very different from the nominal ones. Here, by inserting AlAs markers within the structure, state-of-the-art transmission electron microscopy techniques were used to precisely monitor the incorporation/segregation of Sb in ultrathin GaAsSb films (from 1 to 20 monolayers (MLs)). Our rigorous analysis allows us to apply the most successful model for describing the segregation of III-Sb alloys (three-layer kinetic model) in an unprecedented way, limiting the number of parameters to be fitted. The simulation results show that the segregation energy is not constant throughout the growth (which is not considered in any segregation model) but has an exponential decay from 0.18 eV to converge asymptotically towards 0.05 eV. This explains why the Sb profiles follow a sigmoidal growth model curve with an initial lag in Sb incorporation of 5 MLs and would be consistent with a progressive change in surface reconstruction as the floating layer is enriched.

Representative grain boundaries during anisotropic grain growth

Conventional grain growth models do not account for the variability in grain boundary (GB) mobility, GB energy, and solute segregation energy that depend in detail on the GB structure. Instead, these parameters are typically determined empirically from experimental measurements. In this study, we present a systematic quantitative analysis that accounts for anisotropic GB properties, i.e., GB mobility and solute drag pressure to rationalize the average grain size evolution using representative GB properties under normal grain growth conditions in a non–textured system. We perform two–dimensional phase field simulations to analyze the role of (1) anisotropic mobility, (2) anisotropic segregation, and (3) combined anisotropic mobility and segregation on the average grain size evolution. A corresponding “representative GB” is introduced for each case considering a phenomenological grain growth model that fits the simulated average grain size evolution. A relationship is proposed to determine the representative GB properties for an arbitrary distribution of anisotropic GB properties in the initial microstructure. Simulations with the combined variation of GB mobility and segregation energy suggest that the representative GB mobility and segregation energy determined from independent simulations with varying either GB mobility or segregation energy can be superimposed to determine the average grain size evolution in agreement with the phase field simulations. The limits of validity, i.e., the range of moderate anisotropies, are identified where the phenomenological model with representative GB properties may be applicable, i.e., as long as a mean grain size is an appropriate descriptor of the grain structure.

Designed Y3+ Surface Segregation Increases Stability of Nanocrystalline Zinc Aluminate.

The thermal stability of zinc aluminate nanoparticles is critical for their use as catalyst supports. In this study, we experimentally show that doping with 0.5 mol % Y2O3 improves the stability of zinc aluminate nanoparticles. The dopant spontaneously segregates to the nanoparticle surfaces in a phenomenon correlated with excess energy reduction and the hindering of coarsening. Y3+ was selected based on atomistic simulations on a 4 nm zinc aluminate nanoparticle singularly doped with elements of different ionic radii: Sc3+, In3+, Y3+, and Nd3+. The segregation energies were generally proportional to ionic radii, with Y3+ showing the highest potential for surface segregation. Direct measurements of surface thermodynamics confirmed the decreasing trend in surface energy from 0.99 for undoped to 0.85 J/m2 for Y-doped nanoparticles. Diffusion coefficients calculated from coarsening curves for undoped and doped compositions at 850 °C were 4.8 × 10-12 cm2/s and 2.5 × 10-12 cm2/s, respectively, indicating the coarsening inhibition induced by Y3+ results from a combination of a reduced driving force (surface energy) and decreased atomic mobility.

The enhanced Zn and Ca co-segregation and mechanical properties of Mg–Zn–Ce alloy with micro Ca addition

The micro addition of Ca is demonstrated to noticeably increase the strength and ductility of Mg–Zn–Ce alloy, which is closely correlated with the enhanced alloying segregation at grain boundary. Addition of Ca is found to yield the enhanced Zn segregation and co-segregation of Zn, Ce, and Ca. The mechanism behind is that Ca decreases the segregation energy while promoting the diffusion process of Zn, ultimately accelerating the thermodynamic and kinetic process of segregation. The enhanced alloying segregation induced by Ca effectively stabilizes grain boundary, which contributes to the refined grain size, retardant mobility of dislocation and ultimately improved strength. Furthermore, the intergranular fracture energy can be noticeably increased by Ca owing to the promoted alloying element segregation at grain boundary, which contributes to the improved grain boundary cohesion and ductility. The controllable segregation at grain boundary via alloying is intended to contribute to the development of advanced magnesium alloys.

Tilt grain boundary stability in uranium dioxide and effect on xenon segregation

Grain boundaries (GBs) play a key role in the accumulation and release of xenon (Xe) in uranium dioxide (UO2). The stability and structure of tilt GBs with low sigma values in UO2 has been investigated by molecular statics/dynamics (MS/MD) simulations with three different empirical potentials. The atomistic modeling results confirm that Σ3 and Σ11 are more stable than both Σ5 and Σ9, consistent with previous work [J. Nucl. Mater. 517 (2019) 286]. In general, the higher distortion of atoms at a GB corresponds to a higher GB formation energy. Density functional theory (DFT+U) calculations indicate that a defective triangle-like pattern in the Σ5{310} is preferred rather than an oxygen-ring pattern. The energetics of Xe with or without vacancies at the most stable GBs have been studied by MS simulations. The incorporation energy profiles indicate that the Xe interstitial sites at the GBs are always energetically favorable in comparison to the bulk, while Xe incorporation in some Schottky defects (SDs) or uranium vacancy (VU) sites at the Σ5{210}, Σ5{310} and Σ9{221} boundaries are energetically unfavorable relative to the bulk. However, in general, interstitial Xe, Xe-SD, and Xe-VU complexes tend to segregate to the GBs. Notably, the symmetry degree of the distribution of excess atomic volume at a GB is correlated to the charge character of the GB. An asymmetric distribution of excess atomic volume results in the formation of electrostatic dipoles in GB systems, which significantly influences the segregation behavior of charged Xe-vacancy complexes at the GB. The segregation energy of Xe with or without vacancies is to some extent related to the GB excess volume.

DFT approach for predicting the pH-potential-dependent durabilities of Pt-skinned Pt-M (M = Ni, Co, and Ir) alloys for fuel cell cathodes

Pt-skinned Pt-M (M = Ni, Co, and Ir) alloy catalysts have been reported as cathode catalysts for polymer electrolyte membrane fuel cells (PEMFCs) that are durable and highly efficient toward the oxygen reduction reaction (ORR). While density functional theory (DFT) studies have confirmed that these catalysts are efficient, they did not clearly demonstrate the superior durability imparted by the Pt-skin layer. Herein, we introduce an atomic segregation model based on density functional theory calculated (DFT-calculated) vacancy formation energies that overcomes these limitations. Solvation-corrected segregation energies reveal that the Pt/PtNi(111), Pt/PtCo(111), and Pt/PtIr(111) surfaces, which can contain very low amounts of Pt, are highly resistant to atomic segregation from the alloy catalyst. The influence of the Pt-skin layer of the examined Pt/PtM catalysts was closely investigated by considering the PEMFC operating conditions, including solvation, the oxygen-rich environment of the cathode, the electrical double-layer, pH, potential, and temperature. As a result, we demonstrated that the Pt/PtCo catalyst exhibits superior durability with a high vacancy formation energy for the Pt-skin layer.

Effects of Alloying on the Interface Energy of the gamma ''-Phase in Nickel-Based Superalloys

Nickel-based superalloys of the 718 type have been extensively used in the production of engine parts which operate at high temperatures. The alloy is strengthened by the precipitation of small gamma ''-phase particles with a large lattice distortion in the gamma matrix. These gamma ''-phase particles, however, may transform to the more thermodynamically stable delta -phase. Such transformation might be mitigated by adding other alloying elements. In this work, we used density functional theory calculations at 0 K to study the influence of relevant alloying elements on the interface energy of the gamma ''/gamma coherent interfaces. By replacing an atom either in the gamma ''-phase or at the interface and calculating the corresponding interface segregation energy, we assessed the favorable replacement position of each alloying element. Implications of our results on the gamma ''-phase growth are also discussed.

Segregation of Gallium in Diluted Al–Ga Solid Solutions, According to Data from Auger Spectroscopy

Electron Auger spectroscopy is used to study the surface segregation of gallium in diluted solid solutions of Al–Ga with volume concentrations of 1.1, 2.4, 4.2, 7.4 wt % Ga at temperatures of room temperature to 573 K. Considerable segregation of gallium is observed for all of the studied alloys. Values obtained for the surface concentration of Ga are used to construct the dependence of the energy of segregation on the surface concentration of Ga atoms at different temperatures

Single Metal Atoms Embedded in the Surface of Pt Nanocatalysts: The Effect of Temperature and Hydrogen Pressure

Embedding energetically stable single metal atoms in the surface of Pt nanocatalysts exposed to varied temperature (T) and hydrogen pressure (P) could open up new possibilities in selective and dynamical engineering of alloyed Pt catalysts, particularly interesting for hydrogenation reactions. In this work, an environmental segregation energy model is developed to predict the stability and the surface composition evolution of 24 Metal M-promoted Pt surfaces (with M: Cu, Ag, Au, Ni, Pd, Co, Rh and Ir) under varied T and P. Counterintuitive to expectations, the results show that the more reactive alloy component (i.e., the one forming the strongest chemical bond with the hydrogen) is not the one that segregates to the surface. Moreover, using DFT-based Multi-Scaled Reconstruction (MSR) method and by extrapolation of M-promoted Pt nanoparticles (NPs), the shape dynamics of M-Pt are investigated under the same ranges of T and P. The results show that under low hydrogen pressure and high temperature ranges, Ag and Au—single atoms (and Cu to a less extent) are energetically stable on the surface of truncated octahedral and/or cuboctahedral shaped NPs. This indicated that coinage single-atoms might be used to tune the catalytic properties of Pt surface under hydrogen media. In contrast, bulk stability within wide range of temperature and pressure is predicted for all other M-single atoms, which might act as bulk promoters. This work provides insightful guides and understandings of M-promoted Pt NPs by predicting both the evolution of the shape and the surface compositions under reaction gas condition.

The vibrational entropy spectra of grain boundary segregation in polycrystals

The vibrational contributions to the energetics of grain boundary segregation are calculated for binary Ni polycrystals with solutes of Pd, Pt, Ag, Au and Cu. The computational challenge of the problem is to assess each different grain boundary site and produce full spectra of both site energies and entropies, which is addressed through a multiscale approach in which a full harmonic calculation is completed only in the vicinity of the solute atom, and a buffer region based on a local harmonic calculation effects connectivity to the bulk polycrystal. The resulting segregation free energy spectra illustrate the artificial nature of segregation temperature dependencies when the vibrational entropy is not considered, and highlight the importance of including the vibrational entropy spectrum for rigorous calculations. For a 5% solute loading level, for example, the magnitude of the artificial temperature dependence calculated ranges from 31 to 43% of the total effective segregation entropy for the various systems explored. Finally, a strong linear correlation between site segregation energy (which is not much affected by vibrational effects within the harmonic approximation) and vibrational entropy is observed in polycrystals, in agreement with prior works of coincident site lattice boundaries; this correlation promises significant simplification of segregation calculations in polycrystals generally.

Effect of Mo addition on hydrogen segregation at α-Fe grain boundaries: A first-principles investigation of the mechanism by which Mo addition improves hydrogen embrittlement resistance in high-strength steels

The development of high-strength steels is essential for realizing a decarbonized society; however, to achieve this, the problem of hydrogen embrittlement must be solved. The effectiveness of adding Mo has been demonstrated to address this issue, but its underlying mechanism has not yet been clarified. To investigate this mechanism, we developed a calculation method to evaluate the effect of adding alloying elements on the grain boundary segregation of H using parameters calculated via first-principles calculations. Accordingly, the segregation energies of H with and without Mo segregation near the grain boundary were calculated. Based on the obtained segregation energies, the concentration of H at the grain boundary was calculated for a given amount of Mo under the heat treatment conditions normally assumed. The results showed that a small amount of Mo segregates to α-Fe grain boundaries and significantly reduces the grain boundary concentration of H owing to repulsive interactions between Mo and H. This indicates that Mo addition improves the resistance to hydrogen embrittlement by reducing the grain boundary H concentration. The physical origin of these repulsive interactions was further investigated by decomposing them into mechanical and chemical contributions; we found that these repulsive interactions are mainly caused by the chemical contribution. Furthermore, the crystal orbital Hamiltonian population indicated that the chemical contribution can be attributed to the large core repulsion between Mo–H at α-Fe grain boundaries (in comparison to that between Fe–H) owing to the larger atomic orbital of Mo. The proposed calculation method as well as the information obtained from our evaluations can help in the development of high-strength steels with excellent resistance to hydrogen embrittlement and in the improvement the resistance to hydrogen embrittlement of various metallic materials.

Computational assessment of solute segregation at twin boundaries in magnesium: A two-factor model and solute effect on strengthening

This work presents a comprehensive first-principles density functional theory (DFT) study of solute segregation at {101¯1} and {101¯2} twin boundaries (TBs) in Mg. A total of 56 solute elements were investigated. For each solute element, the preferential segregation sites at two TBs were identified and the associated segregation energies were computed. A two-factor model that considers both lattice strain and electronegativity, representing the mechanical and chemical effects respectively, has been proposed to predict the solute segregation energy. The model prediction shows good agreement with the DFT calculation. It was found that the mechanical effect dominates the solute segregation energy. However, depending on the site of segregation, the chemical effect can become sizable to warrant consideration. The degree of solute segregation at TBs at different temperatures was then quantified by calculating the solute concentration at TBs at different temperatures. The effect of solutes in either strengthening or weakening the TB was also evaluated. The results provide a basis for selecting promising solutes in the development of new high-performance Mg alloys.

First-principles study of the effect of N on the ∑5 (210) [001] grain boundary of γ-Fe

High-nitrogen austenitic steels possess excellent properties owing to the solid solution strengthening of nitrogen, which is widely used in many fields. So it’s significant to investigate the effect of nitrogen on the properties of high austenitic nitrogen steels. In order to get atomic scale understanding of the influence of nitrogen, the first principles method was used to investigate the energetics, site preference, and segregation of nitrogen at γ-Fe ∑5 (210) [001] grain boundary (GB) and different atomic layers near GB plane. It is found that the nitrogen atom prefers to occupy the octahedral site. As nitrogen locates at different atomic layers away from the GB plane, the segregation energy presents increase first and then decrease. The mechanical properties were analyzed by Rice-Wang theory and theoretical tensile stress test, which shows that nitrogen at ∑5 (210) [001] GB has an embrittling effect on GB cohesion, while nitrogen located away from the GB plane presents a strengthening effect. The underlying mechanism was analyzed by bond length, charge density, and density of states (DOS). The segregation of nitrogen at ∑5(210) [001] grain boundary has negative effect on grain boundary cohesion, while nitrogen locating away from grain boundary plane presents strengthening effect on grain boundary.

Fully coupled segregation and precipitation kinetics model with ab initio input for the Fe-Au system

For designing new and improved materials, predictive modeling of precipitation and segregation kinetics is required. So far, no modeling approach is available that couples precipitation with ab initio segregation data. We derive the mathematical basis for a model to describe segregation and precipitation kinetics with grain boundary segregation energies obtained from ab initio simulations. The model is implemented rigorously and validated with experimental data on a Fe-Au system from literature.