Ni Grain Boundaries Research Articles

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.

First-principle studies of oxidation effects on grain boundary strength in nickel

Nickel-base alloys are susceptible to stress corrosion cracking (SCC), in which the fracture mainly propagates along oxidized grain boundaries (GBs). To understand how oxidation degrades GB strength, first-principle-based density functional theory (DFT) calculations are conducted to study three types of interfaces: partially oxidized GBs, fully oxidized GBs, and metal-oxide interface, using Ni as a model system. For partially oxidized GBs, both substitutional and interstitial oxygen atoms of different coverages are inserted at three Ni GBs: Σ3(111) coherent twin, Σ3(112) incoherent twin, and Σ5(210) high-angle GB. The results show that the GB strength decreases almost monotonically with the increasing oxygen coverage at all GBs. Typically, substitutional oxygen causes a stronger embrittlement effect than interstitial oxygen, except at the Σ3(111). In addition, the oxygen-induced mechanical distortion has a smaller contribution to the embrittlement than its chemical effect except for high-coverage oxygen interstitials at the Σ3(111). For the fully oxidized GBs, three NiO GBs of the same types as in Ni are studied. Although the strengths of Σ3(112) and Σ5(210) NiO GBs are much weaker than the Ni counterparts, the Σ3(111) NiO GB has a higher strength than that in Ni. Finally, the strength of a Ni/NiO metal-oxide interface is calculated, which is found to be the weakest one among all interfaces studied in this work, suggesting that metal-oxide interfaces could be favorable crack initiation sites.

Zirconium-enhanced segregation tendency of solutes X and Zr-X co-segregation induced synergistic/antagonistic effects on Ni Σ5 [001](210) grain boundary

Manipulating grain boundary (GB) segregation of alloying elements has been established as an important pathway to optimize structural materials, due to the GB segregation induced changes in material properties, whereas solute interaction effects on GB segregation and properties remain far less developed. Here, the effects of solute interactions of Zr with X (X = Zr, Re, Ta and Cu) on the segregation behavior of solutes X at the Σ5 [001](210) GB of nickel-based superalloys, GB excess energy and fracture strength were investigated detailedly from first-principles calculations. It was found that the pre-segregation of Zr at the Ni GB not only enhances the segregation ability of subsequent solutes Zr, Ta and Cu, but also induces a GB segregation of Re that prefers grain interiors originally. Zr-Zr/Ta, Zr-Re and Zr-Cu co-segregations have synergistic potentiation, antagonistic and synergistic additive effects on improving the considered GB properties, respectively. Relative to the unsegregated GB, the Zr-Zr/Ta/Re co-segregations could improve the GB stability and fracture strength simultaneously, while the Zr-Cu co-segregation could only improve the GB stability. The co-segregation induced GB stabilizing/destabilizing and strengthening/weakening mechanisms were probed. These results would provide theoretical supports for the GB segregation engineering and quantitative composition design of high-performance nickel-based superalloys.

The spectrum of atomic excess free volume in grain boundaries

The grain boundary (GB) excess volume is an important structural factor that is strongly correlated with various thermodynamic and kinetic properties of GBs such as GB energy, GB mobility, GB diffusivity, and GB segregation energy, etc. However, the excess volume is usually reported as an average value of the entire GB. Such simplification does not consider the spectral nature of the excess volume in a GB, which cannot be used to describe the atomic mechanisms of some kinetic process, such as GB migration, that involves only a few atoms at a time. Here, we explore the spectrum of atomic excess volume in representative nanocrystalline Ni and Al samples as well as 388 Ni bicrystals based on the Olmsted dataset by using atomistic simulations. It is found that the nanocrystalline Ni and Al models show a skew-normal distribution in the spectrum of both the atomic excess volume and the atomic excess energy in the GBs, which show a weak inverse correlation between them. This is in stark contrast to the widely reported positive correlation between GB energy and excess volume based on the average value. We further show based on the statistical analysis that the correlation between the atomic excess volume and excess energy strongly depends on the GB type and a universal trend between them does not exist. While low ∑ Ni GBs generally shows a strong inverse linear correlation between these two properties, such correlation is weak for high ∑ Ni GBs. Moreover, we find that the spectrum of the excess volume shows characteristics distribution in some special Ni GBs. For example, twist GBs generally show a symmetrical unimodal distribution while most surveyed ∑3 Ni GBs with anti-thermal behavior show an apparent bimodal distribution. Nevertheless, a strong correlation is found between the atomic excess volume and the segregation energy based on the nanocrystalline Al model with Mg impurity, which implies a possible universal trend between the two properties. The current study thus shows that the excess volume provides useful insights in revealing the elemental structure–property correlations in GBs, which may be used as a structural variable in future thermodynamic modeling of GBs.

Solute segregation induced stabilizing and strengthening effects on Ni Σ3 [110](111) symmetrical tilt grain boundary in nickel-based superalloys

Grain boundary segregation engineering (GBSE) is a promising approach for accurately manipulating chemical composition, structure and properties of grain boundaries (GBs). With the aim of attenuating or even removing disadvantages of GBs in nickel-based superalloys, the GB segregation behaviors of 3d (Ti–Co), 4d (Zr–Rh) and 5d (Hf–Ir) transition-metal (TM) solutes X and their effects on the thermodynamic stability and fracture strength of Ni Σ3 [110](111) symmetrical tilt grain boundary were systematically investigated, using first-principles total-energy calculations based on density functional theory. We found that all TM solutes considered herein can segregate towards the Ni Σ3 [110](111) GB. For TM elements from same row of the periodic table, their segregation energies show a concave-up parabolic-like dependency on the atomic number of TM elements. The GB segregations of all considered solutes can lead to a reduction in GB energy and render the Ni GB more stable, and the GB energies of X-segregated GBs vary linearly with the GB segregation energies of solutes X. Except elements Zr, Hf and Rh, the GB segregations of the other 15 TM alloying elements increase the GB fracture strength. The Mn and Re segregations improve the GB fracture strength and thermodynamic stability to a maximum extent, respectively. The underlying mechanisms for segregation-induced GB stabilizing and strengthening effects were further revealed at the electronic level by analyzing the crystal orbital Hamiltonian populations and deformation charge densities. Our work could provide new insights into designing Ni-based superalloys with desired performances based on the concept of GBSE.

Segregation behavior of alloying elements at Ni Σ5 [001](210) symmetrical tilt grain boundary in nickel-based superalloys and their stabilization and strengthening mechanisms for the grain boundary

To reduce or even eliminate the negative effects of grain boundaries (GBs) in nickel-based superalloys, the GB-segregation behaviors of 14 main alloying elements X of superalloys and their effects on the stability and fracture strength of Ni Σ5 [001](210) GB were systematically investigated based on the concept of GB segregation engineering (GBSE), using first-principles calculations. We found that 10 alloying elements can segregate towards the GB. Their GB-segregation tendencies increase in the order of W < Mo < Al < Mn < Cu < Ti < Ru < Ta < Hf < Zr, due to the increase in atomic radius and electronegativity differences between X and host Ni atoms. The GB energies of X-segregated GBs depend linearly on the GB segregation energies of solutes X, the stronger segregation tendency of solute X could lower the GB energy and make the GB more stable. Except Cu, the segregations of the other 9 solutes increase the GB fracture strength. The segregation-induced GB stabilizing and strengthening mechanisms were further revealed on the electronic level. This work could be beneficial for manipulating the composition, structure and properties of Ni GBs and designing Ni-based superalloys with desired performances based on the GBSE.

Hydrogen-enhanced intergranular failure of sulfur-doped nickel grain boundary: In situ electrochemical micro-cantilever bending vs. DFT

Intergranular failure of nickel (Ni) single grain boundaries (GBs) owing to the segregation of sulfur (S), hydrogen (H), and their co-segregation has been investigated by employing micro-cantilever bending tests and density functional theory (DFT) calculations. A pure Ni GB shows completely plastic behavior with no fracture observed in the experiments. Electrochemical H-charging of the sample with no S present in the GB leads to a crack formed at the notch tip, which propagates by means of the mixed plastic–brittle fracture mode. Cantilever testing of the H-charged GB with S results in a clear brittle fracture of the GB. The co-segregation of S and H shifts the sudden drop in the load–displacement curves to smaller values of displacement. This is explained by the combined effect of these elements on the work of separation of the selected GB leading to severely decreased GB cohesion.

Effects of alloy compositions on hydrogen behaviors at a nickel grain boundary and a coherent twin boundary

The effects of several common metallic and nonmetal alloy compositions (i.e., Cr, Mo, B, and P) on the energetics and kinetics of hydrogen behaviors at a nickel grain boundary (GB) and a coherent twin boundary (CTB) were systematically investigated by first-principles calculations. H, Cr, Mo, B, and P have a stronger segregation into Ni GB than Ni CTB due to the presence of a cavity in GB. Cr, Mo, B, and P all act as obstacles for H segregation and diffusion in both GB and CTB, but the physical mechanisms are different: In Ni GB, Cr and Mo result in the shrinkage of isosurfaces of optimal charge densities for H, and B and P provide a strong competitive tendency to accumulate into the GB; in Ni CTB, Cr and Mo induce charge accumulation, and B and P result in a repulsive interaction to H. The present study provides the microscopic images of H compositions in Ni GB and CTB under the effects of alloy compositions; this is essential for understanding the mechanism of hydrogen embrittlement (HE) and improving the ability of alloys against HE.

Effect of Nb on He segregation behavior in NiΣ5 grain boundary: First-principles study

Abstract The segregation of He at the grain boundary (GB) significantly affects the application of Ni-based alloys in the nuclear industry. However, the influence mechanism of other elements on the segregation of He at Ni GB is still unclear. Therefore, first-principles calculation was used to investigate the segregation behavior of He at NiΣ5 GB with various Nb segregation concentration. We found that Nb atoms tend to replace the site “1” at NiΣ5 GB, and Nb atoms can weaken the segregation tendency of He atoms at the NiΣ5 GB gap. The charge density results show that the segregation of He atoms causes the NiΣ5 GB to expand, and the Ni(2)-Ni(-2) bonds weaken so that the tensile strength decreases significantly. When Nb atoms are doped at Ni GB, strong Nb-Ni(4) bonds and Nb-Ni(-4) bonds are formed. Therefore, the GB remains strong enough when the Ni(2)-Ni(-2) bonds break. The results are helpful to the optimum design of Ni-based alloy.

Zr segregation in Ni–Zr alloy: implication on deformation mechanism during shear loading and bending creep

Grain boundary (GB) migration and grain growth in nanocrystalline metals are major obstructions to exploit these materials to their fullest potential, thus limiting their utility as advanced structural materials. Here, we investigate the effect of Zr segregation to a Ni GB on migration and subsequent strengthening during shear deformation and high-temperature bending creep. Specifically, using molecular dynamics simulation, we simulate deformation of a Ni bicrystal specimen having a symmetric ∑5 grain boundary with and without segregated Zr. It is found that GB segregation up to 0.4 at.% pins the CSL boundary resulting in increased shear strength. Similar behavior is also observed in the case of bending creep deformation where the creep resistance increases only up to 0.4 at.%. This study indicates that larger amounts of GB segregation cause destabilization of the local atomic structure and consequently affect the GB stiffness. Moreover, we investigate the effect of Zr segregation on the interaction between the GB and a dislocation. We find that the preexisting dislocation is absorbed in the GB region for both clean and segregated specimens; however, the absorption path of the dislocation is different due to the change in GB stiffness caused by the Zr segregation.

An embedded-atom method potential parameterized for sulfur-induced embrittlement of nickel

The embrittling or strengthening effect of solute atoms at grain boundaries (GBs), commonly known as the embrittling potency, is an essential thermodynamic property for characterizing the effects of solute segregation on GB fracture. One of the more technologically relevant material systems related to embrittlement is the Ni–S system where S has a deleterious effect on fracture behavior in polycrystalline Ni. In this work, we develop a Ni–S embedded-atom method (EAM) interatomic potential that accounts for the embrittling behavior of S at Ni GBs. Results using this new interatomic potential are then compared to previous density functional theory studies and a reactive force-field potential via a layer-by-layer segregation analysis. Our potential shows strong agreement with existing literature and performs well in predicting properties that are not included in the fitting database. Finally, we calculate embrittling potencies and segregation energies for six [100] symmetric-tilt GBs using the new EAM potential. We observe that embrittling potency is dependent on GB structure, indicating that specific GBs are more susceptible to sulfur-induced embrittlement.

Influence of hydrogen environment on dislocation nucleation and fracture response of 〈1 1 0〉 grain boundaries in nickel

The segregation of solute H has a profound effect on mechanical properties of grain boundaries (GBs). In this work, systematic molecular dynamics (MD) simulations have been performed to elucidate the influence of H environment on dislocation nucleation process and fracture response of different Ni GBs. The results show that the effect of H on dislocation nucleation depends strongly on GB structures. The presence of H decreases the yield stress required to nucleate dislocations from GBs with C and D structural units (SUs), while increases the yield stress of GBs with E SUs. This difference is attributed to different deformation mechanisms related to the dislocation nucleation from various GBs with H. By analysing the decohesion results, it is found that H has stronger embrittling effect on GBs with E SUs than C and D SUs, originating from the fact that H significantly elongates the Ni-Ni bonds around the open E SUs while slightly expands the compact C and D SUs. These findings present fundamental understanding on H embrittlement mechanism and provide mechanistic insights for engineering microstructure against H embrittlement in metallic materials.

A way to improve the uniformity of nanometer-thickness graphite film synthesized on polycrystalline Ni substrate: From large grain to small grain

A pellicle is an important part of mask protection in extreme ultraviolet (EUV) lithography, and nanometer-thickness graphite film (NGF) is an ideal material for pellicle fabrication. In our previous work, we synthesized large-scale NGF on polycrystalline Ni foil by a “two-stage” chemical vapor deposition process with an EUV transmittance of 79 ± 2%. The nonuniformity of EUV transmittance is mainly caused by the thicker NGF formed on the Ni grain boundaries (GBs). Here, we present a simple and cost-effective method to weaken the effects of the Ni GBs by sputtering thin Ni films onto Ni foils. The Ni film provides smaller grains with high GB density so that the non-uniformity of NGFs caused by Ni GBs was averaged out by the large EUV beam size. We investigated the grain growth of Ni films with different thicknesses, and 1-μm Ni film/Ni foil had the minimum average grain size of ∼6 μm. The NGF synthesized on it showed a higher EUV transmittance with better uniformity of 86.3 ± 0.9%. Fullerenic CC and sp3 CC bonding were detected by the X-ray photoelectron spectroscopy, and the NGF on the Ni GBs showed a higher defect density.

First-principles investigation on the interaction of Boron atom with nickel part II: Absorption and diffusion at grain boundary

Abstract In this work, Boron atom absorption and diffusion behaviors at Σ3(111) [ 1 − 10 ] and Σ5(210)[001] grain boundaries (GBs) were studied with a first-principles density functional theory and the climbing image nudged elastic band method. The calculation results reveal that the two GBs show the markedly different interaction behavior with atomic Boron. The close-packed Σ3 GB plays the negative role for absorbing B atom and can accelerate the B atom diffusion, while the Σ5 GB plays the positive role for absorbing B atom and acts as a two-dimensional diffusion barrier. The ability for Ni GBs and Ni bulk to absorb B atom is controlled by the electron density ρ el at the site before inserting the B atom, and a certain range of ρ el plays the positive role for absorbing B atom, while the larger or smaller ρ el outside the certain range will both no longer have the same influence. The different energy barrier for B atom diffusing leads to the huge different diffusion rate, in which, the diffusion rate for B atom along the Σ3 GB is two orders of magnitude greater than that in the bulk Ni and nearly four orders of magnitude greater than that across the Σ5 GB.

Aligned Growth of Millimeter‐Size Hexagonal Boron Nitride Single‐Crystal Domains on Epitaxial Nickel Thin Film

Atomically thin hexagonal boron nitride (h-BN) is gaining significant attention for many applications such as a dielectric layer or substrate for graphene-based devices. For these applications, synthesis of high-quality and large-area h-BN layers with few defects is strongly desirable. In this work, the aligned growth of millimeter-size single-crystal h-BN domains on epitaxial Ni (111)/sapphire substrates by ion beam sputtering deposition is demonstrated. Under the optimized growth conditions, single-crystal h-BN domains up to 0.6 mm in edge length are obtained, the largest reported to date. The formation of large-size h-BN domains results mainly from the reduced Ni-grain boundaries and the improved crystallinity of Ni film. Furthermore, the h-BN domains show well-aligned orientation and excellent dielectric properties. In addition, the sapphire substrates can be repeatedly used with almost no limit. This work provides an effective approach for synthesizing large-scale high-quality h-BN layers for electronic applications.

Effects of rare-earth on the cohesion of Ni Σ5 (0 1 2) grain boundary from first-principles calculations

A first-principles calculation is performed to investigate the effect of segregated rare-earth (RE) on a Σ5 (012) nickel grain boundary (GB). Energetic studies show that the GB substitutional site is the preferred segregation site for all the REs. According to the Rice-Wang model, the strengthening/embrittling energy is calculated to find that all the REs can impair the GB cohesion. Furthermore, the strengthening/embrittling energy is decomposed into the mechanical and chemical components. The radius and charge of the RE in GB, along with the charge density distribution, are analyzed to uncover the physical origins of these two components and their change tendencies. Our work presents a fundamental understanding of the effects of all the REs on the Ni GB cohesion systematically for the first time.

Origin of the Bismuth-Induced Decohesion of Nickel and Copper Grain Boundaries

Ductile metals such as Ni and Cu can become brittle when certain impurities (e.g., Bi) diffuse and segregate into their grain boundaries (GBs). Using first-principles calculations, we investigate the microscopic origin of the Bi-induced loss of cohesion of Ni and Cu GBs. We find that the Bi bilayer interfacial phase is the most stable impurity phase under the Bi-rich condition, while the Bi monolayer phase is a metastable phase regardless of the value of the Bi chemical potential. Our finding is consistent with the recent experimental observation for Ni GBs [Luo et al. Science 333, 1730 (2011)]. The electric polarization effect of the Bi bilayer substantially enhances the strength of the Bi-metal interfacial bonds, stabilizing the bilayer phase over other phases. The Bi-Bi interlayer bonding is significantly weakened in the GBs, leading to a factor of 20 to 50 decrease in the GB cohesion, which has strong implications for the understanding of Bi-induced intergranular fracture of Ni and Cu polycrystals.

The effect of vanadium impurity on Nickel Σ5(012) grain boundary

We report first-principle density functional theory investigation on the effect of vanadium impurity in the nickel Σ5(012) symmetrical tilt grain boundary (GB) using Norm-conserving pseudopotentials. We first calculate segregation energies for interstitial and different substitutional sites in order to determine site preference and the segregation properties of V in the Ni GB. It is found that vanadium atoms prefer to segregate in the substantial sites of the GB. Furthermore, the calculations of tensile strength for different V positions in Ni Σ5(012)GB show an enhancement of the maximum tensile strength σMax up to 17%, indicating that vanadium acts as a strengthening element in Ni GB, which is in agreement with experimental observations.

Tracer Diffusion of 63 Ni in Nano-γ-FeNi Produced by Powder Metallurgical Method: Systematic Investigations in the C , B , and A Diffusion Regimes

63Ni radiotracer diffusion in a well-compacted nanocrystalline (grain size d ∼ 80 to 100 nm) γ-Fe–40wt%Ni alloy was measured by the serial sectioning technique in an extended temperature range from about 610 to 1010 K. Since the material microstructure reveals two different length scales with nano-size grains forming micrometer-size clusters (or agglomerates), three main diffusion paths determine the diffusion behavior: the nanocrystalline grain interior, the nanocrystalline grain boundaries (GB), and the inter-agglomerate interfaces. The systematics of diffusion in a compacted nanostructured material with such a bimodal distribution of interface characteristics was elaborated and the experimental data were analyzed in dependence on the diffusion regime. The absolute values and the Arrhenius parameters of Ni GB diffusion in the nano-γ-Fe–40wt%Ni alloy (D 0 = 9.3 × 10−4 m2 s−1 and Q = 177 kJ/mol) are similar to the Ni GB diffusivity in coarse-grained poly-crystalline γ-Fe. Accordingly, the nanocrystalline GBs are concluded to have quasi-equilibrium structures, particularly because of a pronounced grain growth (from about 30 to about 100 nm) during the production stage of the nanomaterial. In contrast, the inter-agglomerate interfaces, which present the fastest diffusion path in the present investigation (D 0 = 1.9 × 10−3 m2 s−1 and Q = 134 kJ/mol), are likely to be in a non-equilibrium state due to specific features of the applied powder metallurgical process.

Influence of nickel substrate grain structure on YBa2Cu3O7−x supercurrent connectivity in deformation-textured coated conductors

Coupled magneto-optical imaging and local misorientation angle mapping have been used to demonstrate the percolative nature of supercurrent flow in YBa2Cu3O7−x(YBCO) coated conductors grown on deformation-textured Ni substrates. Barriers to current flow occur at many YBCO grain boundaries (GBs) which have propagated through the buffer layers from the underlying Ni substrate, and all Ni GBs with misorientation angles &amp;gt;4° initiate percolative current flow. This type of current barrier is characteristic of the conductor form and has been found to exist in samples with Jc(0 T,77 K) values &amp;gt;2 MA/cm2. Sharpening of the local substrate texture or improving in low-angle GB properties should lead to higher Jc values.