Grain Boundary Sites Research Articles

Concave Grain Boundaries Stabilized by Boron Segregation for Efficient and Durable Oxygen Reduction.

The oxygen reduction reaction (ORR) is a critical process that limits the efficiency of fuel cells and metal-air batteries due to its slow kinetics, even when catalyzed by platinum (Pt). To reduce Pt usage, enhancing both the specific activity and electrochemically active surface area (ECSA) of Pt catalysts is essential. Here, ultrafine, grain boundary (GB)-rich Pt nanoparticle assemblies are proposed as efficient ORR catalysts. These nanowires offer a large ECSA and a high density of concave GB sites, which improve specific activity. Atoms at these GB sites exhibit increased coordination and lattice distortion, leading to a favorable reduction in oxygen binding energy and enhanced ORR performance. Furthermore, boron segregation stabilizes these GBs, preserving active sites during catalysis. The resulting boron-stabilized Pt nanoassemblies demonstrate ORR specific and mass activities of 9.18 mA cm-2 and 6.40 A mg-1 Pt (at 0.9 V vs. RHE), surpassing commercial Pt/C catalysts by over 35-fold, with minimal degradation after 60000 potential cycles. This approach offers a versatile platform for optimizing the catalytic performance of a wide range of nanoparticle systems.

Segregation of P and S to frequently occurring grain boundaries of Cu: Single atoms and cooperative effects

Materials design and performance prediction relies on detailed knowledge of the distribution of solutes that can affect the materials properties. Here we report a study on the distribution and mechanisms of interaction of S and P atoms at four relevant grain boundaries (GBs) of fcc Cu: Σ3, Σ5, Σ9 and Σ11. Segregation site preference was investigated for single atoms and compared to the case where impurities segregate as pairs. Both the driving force for segregation—local minima—and the interactions between impurities at the GBs—global minima—have been investigated. An analysis of geometric and electronic structure effects in segregation was performed.Single S-atoms bind more strongly to GB sites than single P-atoms. For all GBs the driving force for segregation decays fast with distance from the planes and reaches the bulk values already at ≈ 4 Å. In the near vicinity of the GBs, with increased concentration, the interactions between S-atoms are mostly attractive, while for the same sites the interactions between P-atoms are mostly repulsive. S-atoms are capable of displacing P-atoms and the accumulation of P is not favorable at the GB planes, while the accumulation of S is favorable. We also performed geometric and electronic structure analyses using symmetry quantifying indicators and developed a descriptor of electronic structure effects called “impurity projected density of states (DOS)” for analyzing the bonding between the impurities and the Cu matrix. Overall, segregation is favored by an increase in the asymmetry of the segregation sites. S binds more asymmetrically to those sites preferring an off-center position while P-atoms bind more symmetrically adopting a central position. At GBs with the same excess volume, S-atoms bind stronger than P-atoms and fitting functions that describe these trends were obtained. The balance between bonding states, antibonding states, and the covalent contributions to the bonding between the impurities and the copper matrix are at the origin of the observed preferences.

Implication of site-specific segregation on grain boundary structural transition and deformation response in nanocrystalline Ni-Nb alloy

In this study, atomistic simulation technique is utilized to examine the impact of grain boundary (GB) character (disorientation angle), structure (boundary-free volume) and energy (boundary excess energy) on site-specific Nb solute segregation behaviour and structural transition in nanocrystalline (NC) Ni. Further, the role of site-specific solute segregation on deformation response in NC Ni is also studied. Towards this, hybrid Monte Carlo/molecular dynamics simulations is performed to obtain the equilibrium structure in a Ni-Nb binary alloy at 500 K and 1200 K. The spectral nature of GB atoms is explored in NC Ni, and found that the GBs have a skew-normal distribution for excess atomic volume and excess atomic energy. Furthermore, comprehensive segregation energy calculations are performed for each GB site, revealing the preferential affinity of Nb towards GB sites in NC Ni. The study also examined interfacial solute excess for each boundary as a function of GB energy and Voronoi volume. The findings indicate that the GB energy decreased as the solute excess increased, whereas the Voronoi volume demonstrated an increment in response to the solute excess. Moreover, simulations conducted at 1200 K reveal that not all GBs within the microstructure are transformed to amorphous complexions; rather some GBs remain ordered. The simulated tensile tests conducted at 500 K and 1200 K reveal that the strengthening in Ni-Nb enhances with the increase in dopant concentration. Additionally, the simulated shear test conducted at 1200 K shows that the deformation in Ni-5Nb occurs more uniformly than pure NC Ni due to formation of thicker amorphous GBs in the former one.

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.

Pathways of hydrogen atom diffusion at fcc Cu: Σ9 and Σ5 grain boundaries vs single crystal

The diffusion of H-atoms is relevant for innumerous physical–chemical processes in metals. A detailed understanding of diffusion in a polycrystalline material requires the knowledge of the activation energies (ΔEa’s) for diffusion at different defects. Here, we report a study of the diffusion of H-atoms at the Σ9 and Σ5 grain boundaries (GBs) of fcc Cu that are relevant for practical applications of the material. The complete set of possible diffusion pathways was determined for each GB and we compared the ΔEa at bulk fcc Cu with the landscape of ΔEa’s at these defects. We found that while a number of diffusion pathways at the GBs have high tortuosity, there are also many paths with very low tortuosity because of specific structural features of the interstitial GB sites. These data show that the diffusion of H-atoms at these GBs is highly directional but can be fast because at certain paths the ΔEa can be as low as 0.05 eV. The lowest energy paths for diffusion of H-atoms through the whole GB models are ΔEa = 0.05 eV for the Σ9 and ΔEa = 0.20 eV at Σ5 which compare with ΔEa = 0.42 eV for the bulk fcc crystal. This shows that H-atoms will be able to diffuse very fast at these defects. With the Laguerre–Voronoi tessellation method, we studied how the local atomic structure of the interstitial sites of the GBs leads to different ΔEa’s for diffusion of H-atoms. We found that the volume expansions and the coordination numbers alone cannot account for the magnitude of the ΔEa’s. Hence, we developed a symmetry quantifying parameter that measures the deviation of symmetry of the GB sites from that of the bulk octahedral site and hence accounts for the distortion at the GB site. Only when this parameter is introduced together with the volume expansions and the coordination numbers, it is possible to correlate the local structure with the ΔEa’s and to obtain descriptors of diffusion. The complete set of data shows that the extrapolation of diffusion data for H-atoms between different types of GBs is non-trivial and should be done with care.

Inhibition effect of segregation and chemical order on grain boundary migration in NbMoTaW multi-principal element alloy

The impacts of solute segregation and chemical order on grain boundary (GB) migration are investigated by atomistic simulations in the NbMoTaW multi-principal element alloy (MPEA). Assisted by a contrived Nb-rich model, it is found that solute segregation and chemical order synergistically inhibit GB migration. Nb segregation increases the critical stress for GB migration, and the presence of chemical order further enhances the resistance of GB to plastic deformation. The destruction of local ordering structures is responsible for the difficult GB migration. Transition pathway analyses show that GB modified with both Nb segregation and chemical order requires high migration barrier, and the prior migration of GB sites tends to avoid regions with heavier chemical order. These results provide new insight into how chemical complexity affects elementary GB motion and contribute to manipulating the stability of MPEAs.

Revisit to Grain Boundary Effect in Pt Nanocrystals toward the Oxygen Electroreduction Reaction

AbstractThe effect of catalytic surface defects in the oxygen reduction reaction (ORR) has been extensively studied to enhance the performance of proton‐exchange‐membrane fuel cells. Platinum (Pt)‐based nanocatalysts have been used to overcome the poor performance of ORR, and Sabatier‐type activity plots have been used to identify the optimal adsorption properties for ORR intermediates on the catalytic surface. Grain boundaries (GBs) within the nanostructures have been identified as the optimal active sites for ORR due to their reasonable coordination number and high oxygen residence time. However, oxidation of Pt atoms exposed at GB sites and leaching of non‐noble metals from bimetallic Pt alloys have been identified as the “Achilles Heel” of GB‐containing nanocatalysts. In this concept, we revisit the effect of GBs on nanocatalysts, summarize the mechanism of GBs towards ORR, and suggest outlooks for improving ORR for future design of nanocatalysts.

Accelerating Industrial-Level NO3 - Electroreduction to Ammonia on Cu Grain Boundary Sites via Heteroatom Doping Strategy.

Although the electrocatalytic nitrate reduction reaction (NO3 - RR) is an attractive NH3 synthesis route, it suffers from low yield due to the lack of efficient catalysts. Here, this work reports a novel grain boundary (GB)-rich Sn-Cu catalyst, derived from in situ electroreduction of Sn-doped CuO nanoflower, for effectively electrochemical converting NO3 - to NH3 . The optimized Sn1% -Cu electrode achieves a high NH3 yield rate of 1.98mmol h-1 cm-2 with an industrial-level current density of -425mA cm-2 at -0.55V versus a reversible hydrogen electrode (RHE) and a maximum Faradaic efficiency of 98.2% at -0.51V versus RHE, outperforming the pure Cu electrode. In situ Raman and attenuated total reflection Fourier transform infrared spectroscopies reveal the reaction pathway of NO3 - RR to NH3 by monitoring the adsorption property of reaction intermediates. Density functional theory calculations clarify that the high-density GB active sites and the competitive hydrogen evolution reaction (HER) suppression induced by Sn doping synergistically promote highly active and selective NH3 synthesis from NO3 - RR. This work paves an avenue for efficient NH3 synthesis over Cu catalyst by in situ reconstruction of GB sites with heteroatom doping.

Nucleation Sites in the Static Recrystallization of a Hot-Deformed Ni-30 Pct Fe Austenite Model Alloy

In the present study, the nucleation of static recrystallization (SRX) in austenite after hot deformation is experimentally analyzed using a Ni-30 pct Fe model alloy. In agreement with the predictions by current models, nucleation rate exhibits a strong peak, early during SRX. Whereas such an early peak is explained by current models by the saturation of nucleation sites, this condition is far from reached, even after the peak declines. In addition, triple-junction and grain-boundary sites are shown to make a quantitatively similar contribution to nucleation. However, for a given boundary between deformed grains, nucleation predominantly starts at one of the triple junctions. Triple-junction nucleation initiates by strain-induced boundary migration of the nucleus (bulging) along one of the boundaries at the junction. Annealing twin boundaries contribute negligibly to nucleation through their grain-boundary sites. By contrast, their junctions with the boundaries of the parent grains do play a relevant role. The earlier nucleation at the triple junctions is attributed to the higher dislocation density observed around them, and the energy of the boundary consumed by the bulge. Both the maximum and average number of nuclei formed per boundary between deformed grains increase with increasing boundary length.

Corrosion inhibition at emergent grain boundaries studied by DFT for 2-mercaptobenzothiazole on bi-crystalline copper

Inhibition of the initiation of intergranular corrosion was modeled at the atomic scale for 2-mercaptobenzothiazole (MBT) adsorbed on a (110)-oriented copper bi-crystal exposing an emergent Σ9 coincident site lattice (CSL) grain boundary (GB) using dispersion-corrected density functional theory (DFT-D). At both isolated molecule and full, dense monolayer coverages, the molecule adsorbed on the grain and GB sites stands perpendicular or tilted with no parallel orientation to the surface being favored. Chemical bonding of the thione and thiolate conformers involves both S atoms or the exocyclic S and N atoms, respectively. The full, dense monolayer is formed with a net gain in energy per surface area, but at the cost of a significant molecule deformation. It significantly enhances the Cu vacancy formation energy at the grain and GB sites, revealing that MBT also inhibits Cu dissolution for the more susceptible GBs with efficiency depending on atomic density of GB emergence.

Potassium clusters in tungsten grain boundaries: Formation mechanism and strengthening effect

We systematically studied the solution and aggregation behavior of Potassium (K) in the symmetrical tilt Tungsten (W) grain boundary (GB) Ʃ5(310)/[001] through first-principles simulations. The lowest-energy substitutional sites are the nearest neighbor sites of GB (V1) in the W-GB, instead of the GB sites themselves, which can be understood by the charge redistribution between the K and W atoms. Interestingly, our simulations show that segregation of multiple K atoms in W GB will form a cluster structure around the GB, which is well consistent with previous experimental reports. Electronic analysis reveals that previously trapped K atoms adjust the electronic densities around them to be more suitable for trapping more K atoms. Due to the greater binding energy of K-K over W-W at the GB, incorporation of K atoms leads to a slight increase in the fracture energy of the GB structure.

Statistical perspective on embrittling potency for intergranular fracture

Embrittling potency is a thermodynamic metric that assesses the influence of solute segregation to a grain boundary (GB) on intergranular fracture. Historically, authors of studies have reported embrittling potency as a single scalar value, assuming a single segregation site of importance at a GB and a particular cleavage plane. However, the topography of intergranular fracture surfaces is not generally known a priori. Accordingly, in this paper, we present a statistical ensemble approach to compute embrittling potency, where many free surface (FS) permutations are systematically considered to model fracture of a GB. The result is a statistical description of the thermodynamics of GB embrittlement. As a specific example, embrittling potency distributions are presented for Cr segregation to sites at two Ni $\ensuremath{\langle}111\ensuremath{\rangle}$ symmetric tilt GBs using atomistic simulations. We show that the average embrittling potency for a particular GB site, considering an ensemble of FS permutations, is not equal to the embrittling potency computed using the lowest energy pair of FSs. A mean GB embrittlement is proposed, considering both the likelihood of formation of a particular FS and the probability of solute occupancy at each GB site, to compare the relative embrittling behavior of two distinct GBs.

Grain‐Boundary‐Rich Noble Metal Nanoparticle Assemblies: Synthesis, Characterization, and Reactivity

AbstractHere, a comprehensive study on the synthesis, characterization, and reactivity of grain‐boundary (GB)‐rich noble metal nanoparticle (NP) assemblies is presented. A facile and scalable synthesis of Pt, Pd, Au, Ag, and Rh NP assemblies is developed, in which NPs are predominantly connected via Σ3 (111) twin GBs, forming a network. Driven by water electrolysis, the random collisions and oriented attachment of colloidal NPs in solution lead to the formation of Σ3 (111) twin boundaries and some highly mismatched GBs. This synthetic method also provides convenient control over the GB density without altering the crystallite size or GB type by varying the NP collision frequency. The structural characterization reveals the presence of localized tensile strain at the GB sites. The ultrahigh activity of GB‐rich Pt NP assembly toward catalytic hydrogen oxidation in air is demonstrated, enabling room‐temperature catalytic hydrogen sensing for the first time. Finally, density functional theory calculations reveal that the strained Σ3(111) twin boundary facilitates oxygen dissociation, drastically enhancing the hydrogen oxidation rate via the dissociative pathway. This reported large‐scale synthesis of the Σ3 (111) twin GB‐rich structures enables the development of a broad range of high‐performance GB‐rich catalysts.

Tandem catalysis on adjacent active motifs of copper grain boundary for efficient CO2 electroreduction toward C2 products

Copper (Cu) is a special electrocatalyst for CO2 reduction reaction (CO2RR) to multi-carbon products. Experimentally introducing grain boundaries (GBs) into Cu-based catalysts is an efficient strategy to improve the selectivity of C2+ products. However, it is still elusive for the C2+ product generation on Cu GBs due to the complex active sites. In this work, we found that the tandem catalysis pathway on adjacent active motifs of Cu GB is responsible for the enhanced activity for C2+ production by first principles calculations. By electronic structure analysis shows, the d-band center of GB site is close to the Fermi level than Cu(100) facet, the Cu atomic sites at grain boundary have shorter bond length and stronger bonding with *CO, which can enhance the adsorption of *CO at GB sites. Moreover, CO2 protonation is more favorable on the region III motif (0.84 eV) than at Cu(100) site (1.35 eV). Meanwhile, the region II motif also facilitate the C–C coupling (0.72 eV) compared to the Cu(100) motif (1.09 eV). Therefore, the region III and II motifs form a tandem catalysis pathway, which promotes the C2+ selectivity on Cu GBs. This work provides new insights into CO2RR process.

Basic Structural Units of Tilt Grain Boundaries. II. Misorientation Axes [110] and [111

The computer simulation methods have been applied to calculate structure and energy of symmetric tilt grain boundaries (GB) with the misorientation axes [110] and [111]. The calculations have been carried out with the use of the structural-vacancy model. The study of the atomic structure has been carried out within the entire range of misorientation angles. The reverse density of coincidence sites in special grain boundaries has amounted Σ ≤ 57. The calculations have been carried out with the use of the Morse pair potential and the Cleri-Rosato many-body potential. When calculated with different potentials, the dependence of GB energy on the misorientation angle has a similar form, and the atomic structure completely coincides. It has been shown that the structure of any GB with the misorientation axes [110] and [111] may be represented by a limited number of basic structural units. All found basic structural units defined as units of A, B, C and D types are based on the structures of special grain boundaries. Such special GBs shall be Σ3(111), Σ3(112), Σ11(113) and Σ9(114) for GBs with the misorientation axis [110], and as regarding GBs with the misorientation axis [111], such special GBs shall be Σ3(112), Σ7(123) and Σ13(134). Ranges of angles within which certain basic structural units are found have been defined.

Learning grain boundary segregation energy spectra in polycrystals

The segregation of solute atoms at grain boundaries (GBs) can profoundly impact the structural properties of metallic alloys, and induce effects that range from strengthening to embrittlement. And, though known to be anisotropic, there is a limited understanding of the variation of solute segregation tendencies across the full, multidimensional GB space, which is critically important in polycrystals where much of that space is represented. Here we develop a machine learning framework that can accurately predict the segregation tendency—quantified by the segregation enthalpy spectrum—of solute atoms at GB sites in polycrystals, based solely on the undecorated (pre-segregation) local atomic environment of such sites. We proceed to use the learning framework to scan across the alloy space, and build an extensive database of segregation energy spectra for more than 250 metal-based binary alloys. The resulting machine learning models and segregation database are key to unlocking the full potential of GB segregation as an alloy design tool, and enable the design of microstructures that maximize the useful impacts of segregation.

Electronic origin of grain boundary segregation of Al, Si, P, and S in bcc-Fe: combined analysis of ab initio local energy and crystal orbital Hamilton population

In steel, P and S cause serious grain boundary (GB) embrittlement, which is associated with high segregation energies. To investigate the origins of such high segregation energies of P and S, we applied the combination of ab initio local energy analysis and crystal orbital Hamiltonian population (COHP) analysis for the GB segregation of Al, Si, P, and S in bcc-Fe, which can provide local energetic and bonding views of segregation behavior of each solute, associated with the replacement between solute–Fe and Fe–Fe bonding at GB and bulk sites. The local energy analysis revealed that GB segregation of such solutes is mainly caused by the difference between local energy changes of Fe atoms adjacent to a solute atom in the GB and bulk sites, and that the local energy change of each Fe atom depends on the solute–Fe interatomic distance with a unique functional form for each solute species. The COHP analysis showed that such distance dependency of the Fe-atom local energy change is caused by that of solute–Fe bonding interactions, relative to the Fe–Fe ones, governed by the valence atomic-orbital characters of each solute species. P and S have smaller extents of atomic orbitals and larger numbers of valence electrons; thus, they greatly lower the local energies of Fe atoms at interatomic distances shorter than the bulk first-neighbor one, and they greatly increase those of Fe atoms at longer interatomic distances around the bulk second-neighbor one. Thus, high segregation energies of P and S occur at GB sites with short first-neighbor distances and reduced coordination numbers within the bulk second-neighbor distance. The GB embrittlement by P and S was also discussed by this local-bonding viewpoint. The combination of local energy and COHP analyses can provide novel insights into the behavior of solute elements in various materials.

Segregation of P and S Impurities to A Σ9 Grain Boundary in Cu

The segregation of P and S to grain boundaries (GBs) in fcc Cu has implications in diverse physical-chemical properties of the material and this can be of particular high relevance when the material is employed in high performance applications. Here, we studied the segregation of P and S to the symmetric tilt Σ9 (22¯1¯) [110], 38.9° GB of fcc Cu. This GB is characterized by a variety of segregation sites within and near the GB plane, with considerable differences in both atomic site volume and coordination number and geometry. We found that the segregation energies of P and S vary considerably both with distance from the GB plane and sites within the GB plane. The segregation energy is significantly large at the GB plane but drops to almost zero at a distance of only ≈3.5 Å from this. Additionally, for each impurity there are considerable variations in energy (up to 0.6 eV) between segregation sites in the GB plane. These variations have origins both in differences in coordination number and atomic site volume with the effect of coordination number dominating. For sites with the same coordination number, up to a certain atomic site volume, a larger atomic site volume leads to a stronger segregation. After that limit in volume has been reached, a larger volume leads to weaker segregation. The fact that the segregation energy varies with such magnitude within the Σ9 GB plane may have implications in the accumulation of these impurities at these GBs in the material. Because of this, atomic-scale variations of concentration of P and S are expected to occur at the Σ9 GB center and in other GBs with similar features.

Базовые элементы структуры границ зерен наклона. Часть 2. Оси разориентации [110] и [111]-=SUP=-*-=/SUP=-

The computer simulation methods have been applied to calculate structure and energy of symmetric tilt grain boundaries (GB) with the misorientation axes [110] and [111]. The calculations have been carried out with the use of the structural-vacancy model. The study of the atomic structure has been carried out within the entire range of misorientation angles. The reverse density of coincidence sites in special grain boundaries has amounted Σ≤57. The calculations have been carried out with the use of the Morse pair potential and the Cleri-Rosato many-body potential. When calculated with different potentials, the dependence of GB energy on the misorientation angle has a similar form, and the atomic structure completely coincides. It has been shown that the structure of any GB with the misorientation axes [110] and [111] may be represented by a limited number of basic structural units. All found basic structural units defined as units of A, B, C and D types are based on the structures of special grain boundaries. Such special GBs shall be Σ3(111), Σ3(112), Σ11(113) and Σ9(114) for GBs with the misorientation axis [110], and as regarding GBs with the misorientation axis [111], such special GBs shall be Σ3(112), Σ7(123) and Σ13(134). Ranges of angles within which certain basic structural units are found have been defined.

Hydrogen sorption capacity of crystal lattice defects and low Miller index surfaces of copper

The effect of hydrogen on the physical–chemical properties of copper is directly dependent on the types of chemical bonding between H and lattice defects in Cu. In this work, we performed a systematic study of the bonding of H-atoms with crystal lattice defects of copper. This included three types of symmetric tilt grain boundaries (GBs), Σ3, Σ5 and Σ11, and the low Miller index surfaces, (111), (110) and (100). A comparison with literature data for the bonding of H-atoms with point defects such as vacancies was done. From the defects investigated and analyzed, we conclude that the bond strength with H-atoms varies in the decreasing order: surfaces [(111), (110) and (100)] > vacancy > Σ5 GB > Σ11 GB > bulk ≈ Σ3 GB. A study on the effects of the fcc lattice expansion on the binding energies of H-atoms shows that the main driving force behind the segregation of H-atoms at some GBs is the larger volume at those interstitial GB sites when compared to the interstitial bulk sites.