Segregation Energy Research Articles

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.

Modified embedded-atom method interatomic potentials for Mg–Al–Ca and Mg–Al–Zn ternary systems

Al, Ca, and Zn are representative commercial alloying elements for Mg alloys. To investigate the effects of these elements on the deformation and recrystallization behaviors of Mg alloys, we develop interatomic potentials for the Al–Ca, Al–Zn, Mg–Al–Ca and Mg–Al–Zn systems based on the second nearest-neighbor modified embedded-atom method formalism. The developed potentials describe structural, elastic, and thermodynamic properties of compounds and solutions of associated alloy systems in reasonable agreement with experimental data and higher-level calculations. The applicability of these potentials to the present investigation is confirmed by calculating the generalized stacking fault energy for various slip systems and the segregation energy on twin boundaries of the Mg–Al–Ca and Mg–Al–Zn alloys, accompanied with the thermal expansion coefficient and crystal structure maintenance of stable compounds in those alloys.

Microstructure Evolution and Tempering Transformation Kinetics in a Secondary Hardened M50 Steel Subjected to Cold Ring Rolling

The microstructure evolution and tempering transformation kinetics of the M50 steel subjected to cold ring rolling (CRR) have been investigated. The results indicate that the brass R{110}<110> texture is weakened with the enhancement of the <111>//ND texture during CRR. Due to the increased low angle boundaries by CRR, the Ac1 temperature decreases while the carbon content and volume fraction of RA increase. During tempering, the activation energy of carbon atoms segregation and transition carbide precipitation decrease, while the activation energy of retained austenite (RA) decomposition increases after CRR. The kinetic analysis shows that the CRR is beneficial to the carbon atoms segregation during the beginning of tempering. Then, the CRR leads to the delay of the onset of transition carbide precipitation, but decreases the whole reaction time, which has been verified by the transmission electron microscopy (TEM) and hardness results. The lagging of transition carbide precipitation in the early stage is caused by the increased segregation trapping of carbon atoms, while the higher nucleation rate is responsible for the enhanced precipitation of transition carbide during the later stage. For the cementite formation, there are no significant changes in the predictive kinetics after the applied CRR. However, the kinetic transformation of RA decomposition is inhibited by the CRR, which is attributed to the higher carbon content and smaller grain size of RA. Additionally, the alloy carbides precipitation is also enhanced by the CRR process during secondary hardening.

Effect of Grain-Boundary Segregation on the Diffusion of Atoms in Grain Boundaries in Copper-Based Systems

The influence of the segregation energy on the diffusion of second-component atoms in copper is studied by molecular statics and dynamics methods. A number of modified potential is considered. The segregation energy of atoms in a grain boundary is calculated. The number of second-component atoms involved in a diffusion process is found to decrease because of desorption, which leads to a decrease in the grain-boundary diffusion coefficient.

First-principles study of de-twinning in a FCC alloy

First-principles calculations were used to study the effects of several alloying elements X (i.e., Al, Sc, Zn, and Ce) on the de-twinning of Cu alloys. The results show that the nanotwin (Nt) formation energy of Cu-X alloy gradually decreases with the de-twinning, but Ce improve the Nt formation energy and segregation energy of Cu alloys in T4. Furthermore, charge transfer and redistribution were investigated to explore the physical mechanisms of twinning and de-twinning. When the alloying atom is introduced, a “local effect” is formed in its neighboring regions. The anisotropic distribution of the density charges around Ce improves its bonding strength with Cu atoms, while the Sc atom and Ce atom have a smaller electronegativity and a larger atomic radius, forming strong bonds and high mechanical energy.

Analytic description of grain boundary segregation, tension, and formation energy in the copper–nickel system

In this theoretical study, a recently proposed segregation model is applied to segregation data of an exemplary Σ5 grain boundary (GB), which is investigated using a copper–nickel embedded-atom method potential. Segregation in the semi-grandcanonical ensemble is systematically studied by varying the chemical potential in order to explore the full composition range for temperatures from 500 K to 1000 K. As a major thermodynamic feature, the mentioned segregation model avoids the usage of interface compositions, for which an arbitrary volume must be defined, but rather models the thermodynamically unambiguous solute excess. It was shown that the solute excess and the interface formation energy could be described very accurately over a wide range of temperatures and over the entire composition range based on a composition-dependent, but temperature-independent effective energy of segregation. However, the model was initially derived for systems without lattice mismatch so that interface tensions were neglected. Since the copper–nickel system exhibits a moderate lattice mismatch of roughly 2.7%, the copper–nickel system is chosen in this study to further extend the segregation model by a linear-elastic theory to also account for the interface tensions. Using this extended model, it will be shown that the solute excess, GB tensions, and GB formation energies can be derived from the effective energy of segregation for all temperatures and over the whole composition range. As in principle only one single segregation isotherm is sufficient to determine the effective energy of segregation and derived thermodynamic interface quantities, the application of this model is especially interesting for experimental evaluations.

Minimized surface deficiency on wide-bandgap perovskite for efficient indoor photovoltaics

Abstract Wide-bandgap perovskite solar cells (PVSCs) possess significant potential in providing reliable power sources for applications in the Internet of Things (IoT) ecosystem under indoor light illumination. However, the wide-bandgap PVSCs usually suffer from photo-induced phase segregation and non-radiative energy loss caused by the Shockley-Read-Hall (SRH) type trap-assisted recombination at the interfaces in the devices. To address these issues, a simple strategy by applying phenethylammonium halides to reduce the energy loss and suppress the phase segregation of wide-bandgap PVSCs is developed. The devices incorporated with phenethylammonium chloride (PEACl) is revealed to achieve a high open-circuit voltage (VOC) of 1.26 V, leading to a merit power conversion efficiency (PCE) of 18.3%, which is the best performance among the inverted wide-bandgap PVSCs (~1.75 eV) under one sun illumination. Meanwhile, the photovoltaic performance of the device is also significantly enhanced, especially under a white light-emitting diode (LED) with an illumination of 1000 lux, showing a PCE of 35.6% with a high VOC of 1.08 V. Impressively, the device delivers a minimum energy loss of 670 meV, which is among the smallest value reported for perovskite-based indoor photovoltaics.

Atomistic investigation of hydrogen induced decohesion of Ni grain boundaries

The embrittlement of metallic materials by hydrogen (H) segregation is widely observed, but not understood well on an atomic scale. In the present study, an atomistic investigation of H embrittlement of various grain boundaries (GBs) has been performed by mapping H segregation energy of trapping sites and examining the effect of H segregation on the decohesion of GBs. The simulation results show that under the equilibrium concentration of H atoms typical of embrittlement in Ni, in conjunction with local H diffusion process, the maximum reduction of tensile strength and fracture energy is 6.60% and 15.75% for Σ5 (210) ⟨100⟩ and Σ17 (530) ⟨100⟩ GBs, respectively. Inspired by experimental observations of the dislocation structures beneath intergranular failure features, further calculations reveal that the embrittling effect of H atoms in metallic materials can be largely facilitated by the boundary disruption and local stress state concentrated on the GB through the plasticity process. The findings directly provide a picture of H embrittlement arising from the cooperative action of H-induced plasticity and GB decohesion.

Change in damping capacity arising from twin-boundary segregation in solid-solution magnesium alloys

ABSTRACT The change in twin-boundary mobility that is caused by twin-boundary segregation was investigated in pure magnesium and four binary solid-solution magnesium alloys (Mg-Ag, Mg-Sn, Mg-Y and Mg-Zn alloys). The damping capacity in the vicinity of {} twin boundaries was measured before and after annealing by nano-dynamic mechanical analysis. The subsequent annealing process led to a lower damping capacity in all magnesium binary alloys, which was in contrast to the results in pure magnesium. This is on account of the segregation of solute atoms in incoherent twin boundaries. The alloying elements, which have the characteristic of a low segregation energy for twin boundaries, effectively prevents the damping capacity degradation.

Metastable Al-Fe intermetallic stabilised by epitaxial relationship

We report the adsorption of Fe for several dosing conditions on Al(1 0 0) surface held at different temperatures. For 2 monolayer equivalent of Fe deposited on the substrate held at 593 K, we observe two rotational domains of a (5×5)R26.6° structure by low energy electron diffraction and scanning tunneling microscopy. We identify this structure as the formation of the metastable Al9Fe2 intermetallic phase. The segregation energy and the structural stability of this intermetallic has been analysed using density functional theory based calculations. We propose that the formation of the Al9Fe2(0 0 1) metastable phase at the expense of other thermodynamically stable Al-Fe compounds, relies on a specific structural similarity between pure Al (0 0 1) planes of the intermetallic and Al(1 0 0) layers. These ‘common’ planes lead to the epitaxial relationship (1 0 0)Al [0 0 1]Al∥(001)Al9Fe2[13¯0]Al9Fe2. The Al9Fe2-Al matrix interface is sharp as revealed by transmission electron microscopy measurements.

Dynamic Restructuring Induced Oxygen Activation on AgCu Near-Surface Alloys.

Recent studies have shown that the addition of Cu to Ag catalysts improves their epoxidation performance by increasing the overall selectivity of the bimetallic catalyst. We have prepared AgCu near-surface alloys and used scanning tunneling microscopy to gain an atomistic picture of O2 dissociation on the bimetallic system. These data reveal a higher dissociative sticking probability for O2 on AgCu than on Ag(111), and density functional theory (DFT) confirms that the O2 dissociation barrier is 0.17 eV lower on the alloy. Surprisingly, we find that, after a slow initial uptake of O2, the sticking probability increases exponentially. Further DFT calculations indicate that surface oxygen reverses the segregation energy for AgCu, stabilizing Cu atoms in the Ag layer. These single Cu atoms in the Ag surface are found to significantly lower the O2 dissociation barrier. Together, these results explain nonlinear effects in the activation of O2 on this catalytically relevant surface alloy.

Segregation of interstitial light elements at grain boundaries in molybdenum

We investigated the segregation of H, O, N and C at the grain boundaries in molybdenum using ab initio calculations. These light elements have the short-distance interactions with GBs and show the preferential to segregate around GBs. The most energetically favorable trapping sites were found to locate right at the open space of boundaries. Such segregation is associated with the low electron density at the GBs, where the segregation energy is the lowest. Our studies would help understand the possible strengthening or weakening caused by impurities in molybdenum.

Structural, electronic properties and boron stability of (001) surface of Nb5Si3 intermetallic by first-principles calculations

The structural, electronic and surface properties of (001) surface with four different terminations of Nb5Si3 are discussed by using the first-principles calculations in this research. Firstly, we discuss the formation enthalpy and density of states of bulk Nb5Si3, finding that the calculated results are consistent with the previous experimental and calculated data. It can be interpreted as that the calculated method used is credible. The relaxed degree is explicitly described by the relaxed parameters (Δz, δz, di,i+1 and ri) of (001) surface. Density of states shows that the outermost layer states are the major ingredient for surfaces and the surface states arise after the formation of surfaces. The lowest surface energy reflects that (001)-Si surface is the most stable surface. The segregation energy, interlayer spacing and density of states are calculated to discuss the boron stability in Nb2-and NbSi-terminations surfaces. According to the segregation energy, we know that boron is more stable at surface than Sub2 and the segregation energy of inner layer is positive, indicating that we can adopt some methods like heating to drive boron segregating to surface, which can form the protective oxide layers to ameliorate the oxidation resistance of surface. The intensive bonding interactions between B and its contiguous atoms origin from the investigation of interlayer relaxations. After B substitutions, the density of states has the changes that appear at its substitutional layer and the metallic properties are decreased in the two terminations surfaces. Oxygen adsorption further verify that alloying B can promote the oxidation resistance of Nb5Si3.

Atomic-Scale Insight into Exsolution of CoFe Alloy Nanoparticles in La0.4 Sr0.6 Co0.2 Fe0.7 Mo0.1 O3-δ with Efficient CO2 Electrolysis.

In situ exsolution of metal nanoparticles in perovskite under reducing atmosphere is employed to generate a highly active metal-oxide interface for CO2 electrolysis in a solid oxide electrolysis cell. Atomic-scale insight is provided into the exsolution of CoFe alloy nanoparticles in La0.4 Sr0.6 Co0.2 Fe0.7 Mo0.1 O3-δ (LSCFM) by in situ scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy and DFT calculations. The doped Mo atoms occupy B sites of LSCFM, which increases the segregation energy of Co and Fe ions at B sites and improves the structural stability of LSCFM under a reducing atmosphere. In situ STEM measurements visualized sequential exsolution of Co and Fe ions, formation of CoFe alloy nanoparticles, and reversible exsolution and dissolution of CoFe alloy nanoparticles in LSCFM. The metal-oxide interface improves CO2 adsorption and activation, showing a higher CO2 electrolysis performance than the LSCFM counterparts.

Atomic‐Scale Insight into Exsolution of CoFe Alloy Nanoparticles in La0.4Sr0.6Co0.2Fe0.7Mo0.1O3−δ with Efficient CO2 Electrolysis

AbstractIn situ exsolution of metal nanoparticles in perovskite under reducing atmosphere is employed to generate a highly active metal–oxide interface for CO2 electrolysis in a solid oxide electrolysis cell. Atomic‐scale insight is provided into the exsolution of CoFe alloy nanoparticles in La0.4Sr0.6Co0.2Fe0.7Mo0.1O3−δ (LSCFM) by in situ scanning transmission electron microscopy (STEM) with energy‐dispersive X‐ray spectroscopy and DFT calculations. The doped Mo atoms occupy B sites of LSCFM, which increases the segregation energy of Co and Fe ions at B sites and improves the structural stability of LSCFM under a reducing atmosphere. In situ STEM measurements visualized sequential exsolution of Co and Fe ions, formation of CoFe alloy nanoparticles, and reversible exsolution and dissolution of CoFe alloy nanoparticles in LSCFM. The metal–oxide interface improves CO2 adsorption and activation, showing a higher CO2 electrolysis performance than the LSCFM counterparts.

Brain Network Segregation and Glucose Energy Utilization: Relevance for Age-Related Differences in Cognitive Function.

The human brain is organized into segregated networks with strong within-network connections and relatively weaker between-network connections. This "small-world" organization may be essential for maintaining an energetically efficient system, crucial to the brain which consumes 20% of the body's energy. Brain network segregation and glucose energy utilization both change throughout the lifespan. However, it remains unclear whether these processes interact to contribute to differences in cognitive performance with age. To address this, we examined fluorodeoxyglucose-positron emission tomography and resting-state functional magnetic resonance imaging from 88 participants aged 18-73 years old. Consistent with prior work, brain network segregation showed a negative association with age across both sensorimotor and association networks. However, relative glucose metabolism demonstrated an interaction with age, showing a negative slope in association networks but a positive slope in sensorimotor networks. Overall, brain networks with lower segregation showed significantly steeper age-related differences in glucose metabolism, compared with highly segregated networks. Sensorimotor network segregation mediated the association between age and poorer spatial cognition performance, and sensorimotor network metabolism mediated the association between age and slower response time. These data provide evidence that sensorimotor segregation and glucose metabolism underlie some age-related changes in cognition. Interventions that stimulate somatosensory networks could be important for treatment of age-related cognitive decline.

Uncovering the Zr segregation behavior and its effect on the fracture strength of Al Σ5 (210) [100] symmetrical tilt grain boundary: Insight from first principles calculation

Segregation behavior of Zr solute and its effect on the Al Σ5 (210) [100] symmetrical tilt grain boundary (GB) were investigated through first-principles calculations. Preferential sites and sequence for the replacement of Al by Zr in the GB were determined, so were the fracture energy and tensile strength of the GBs with the varied number of segregated Zr atoms. Segregation energies indicates the GB can accommodate no more than three Zr atoms. The calculation results about grain boundary and strengthening/embrittling energies suggest the strengthening effect of Zr solute on the GB, confirmed by higher fracture energy and peak theoretical tensile stress of the Zr-doped GBs than that of the clean GB, which is the result of the shorter length of the bond that holds the GB together, the contraction of the GB region and the shrink of charge depletion after the segregation of Zr. The conclusions of this investigation shed light on the design of aluminum alloys with high performance GB.

Surface energetics of [formula omitted] alloys

The (100), (110) and (111) surface energies of random AlxTi1-xN alloys with homogeneous concentration profile are determined in first-principles calculations. The (100) surface has the lowest energy of 1.25 J/m2 in the case of TiN and 1.32 J/m2 for cubic AlN and exhibit very little concentration dependence. The (110) and (111) surfaces have much higher energy for all the compositions. The segregation energies of Ti and Al were obtained for the (100) surface of pure TiN and cubic AlN as well as Al0.5Ti0.5N and Al0.9Ti0.1N random alloys. In the latter case, we have used two different methods: direct averaging of the substitution energies with respect to the local environment of the substitution site and the cluster expansion technique (CLE). We find that the segregation of Ti is favorable in the whole concentration range of random AlxTi1-xN alloys. However, it is weak in almost the whole concentration range except in the Al-rich alloys: Al0.9Ti0.1N and cubic AlN. The strengthening of the surface segregation of Ti in the latter case is related to the sharp increase in phase separation tendency in AlxTi1-xN alloys at compositions close to pure AlN. The increased tendency for Ti segregation close to pure AlN helps explain the formation of a lamella structure in industrially important AlxTi1-xN coatings. As for nitrogen vacancies, their segregation energies are relatively small in the whole concentration range, but for pure AlN where they exhibit strong preference for the surface. The latter is obviously connected with the fact that cubic AlN has a high degree of ionic bonding and a vacancy creation on the nitrogen sublattice is highly energetically unfavorable, especially in the bulk due to a larger number of Al-N ionic bonds.

First-Principles study on the nucleation of precipitates in ternary Al alloys doped with Sc, Li, Zr, and Ti elements

The nucleation phenomenon of the precipitates with L12-type of ternary Al-rich alloys with Sc, Li, Zr, and Ti elements was studied using first-principles density functional calculations. We calculated certain properties that are closely related to nucleation behavior: surface energy, adhesion work, interfacial energy, segregation energy and interfacial charge transfer. Research indicates that Al (or Al3Z) (0 0 1)/Al3Z (0 0 1) (Z = Sc, Li, Zr, and Ti) with minimum interfacial energies nucleate and grow effectively. The critical Al3Z radii were calculated via the Gibbs free energy for homogeneous nucleation. The heterogeneous nucleation mechanism of Al3Z (0 0 1) substrates were characterized via wetting angles. A core–shell structure can form between any two of the four precipitates, which can verify the observed nucleation results in this experiment.

Prediction of interface and vacancy segregation energies at silver interfaces without determining interface structures

Interfaces play a crucial role in determining the functional and mechanical properties of materials. However, predicting interface properties is not straightforward because the atomic arrangements at the interface are different from those in the bulk. Hence, in this study, we discovered a descriptor from the bulk that helps predict the interface properties without the need to determine the interface structure. The descriptors related to the angle of elevation effectively described the structure units on both the bulk surface and optimized interface. Our model successfully predicted the interface and vacancy segregation energies at silver interfaces without using the interface structure.