Ni Segregation Research Articles

Unlocking high-performance hydrogen evolution: Argon-induced Ni segregation in NiO/TiO2 of core/shell catalysts

This study introduces novel core-shell structured electrocatalysts synthesized via an optimized one-pot sol-gel method, aimed at enhancing the efficiency of the Hydrogen Evolution Reaction (HER) in water splitting. The Ni–NiO/TiO2 and NiO/TiO2 composites demonstrate superior catalytic performance, with the Ni–NiO/TiO2 requiring a remarkably low potential of −0.125 V versus Ag/AgCl to achieve a current density of 10 mA.cm-2. This performance surpasses that of NiO/TiO2 and pure NiO, offering a viable alternative to Pt catalysts. Advanced characterization techniques, including X-ray diffraction, electron microscopy, and X-ray photoelectron spectroscopy, elucidate the crystalline phases, morphologies, and chemical states, enhancing our understanding of the electrocatalysts' efficiency. In particular, Rietveld refinement analysis provides detailed insights into the structural properties that underpin the observed catalytic performance. Electrochemical assessments highlighted the Ni–NiO/TiO2 composite's reduced overpotential needs, high Faradaic efficiency (∼98%), and notable long-term durability, positioning these materials as promising candidates for sustainable hydrogen production.

Wear-resistant CrCoNi nanocrystalline film via friction-driven surface segregation

Revealing the frictional behavior through the lens of structural and chemical evolution is crucial for comprehending the exceptional wear-resistance of alloys with complex composition. Here, we propose that superior wear resistance can be achieved via dynamic surface segregation during sliding at room temperature. This strategy was demonstrated in CrCoNi multi-principal element alloy (MPEA) films with nano-grain structure, which exhibit a remarkably low wear rate that is <50 % of that for their VCoNi counterpart. Such distinct wear behavior is attributed to the specific friction-driven Ni segregation on the CrCoNi surface, which facilitates the preferential oxidation and formation of a nanocomposite protective layer with equiaxed nanograins uniformly embedded in an amorphous matrix. This wear-induced unique microstructure accommodates sliding-induced plastic deformation against damage and is responsible for the superior wear-resistance. Having revealed these fundamental mechanisms by experiment and simulation, this study provides a brand-new perception for designing self-adaptive MPEA surfaces. This involves adjusting the evolution of deformation layers with specific structure and chemistry, precisely engineered for tribological applications.

Ni segregation in the oxide film of 15–15Ti austenitic steel at high-temperature CO2

A double-layer oxide film was formed on 15–15Ti exposed to high-temperature CO2, with outer Fe-rich layer and Cr/Ni-rich inner layer comprising Ni-poor and Ni-rich zones. The Ni-poor zone contained Fe-Cr spinel oxides and numerous nanopores, whereas the Ni-rich zone consisted of Fe-Cr spinel oxides and Ni-rich phases with austenite structure. The Ni segregation showed a specific evolution with exposure time and temperature: Ni-poor zones grew up and evolved into a laminar structure as the oxide film thickened. The formation and evolution mechanism of Ni-poor zones and the effect of Ni segregation on the oxidation were discussed in present work.

Facet-Dependent Ni Segregation in a Micron-Sized Single-Crystal Li1.2Ni0.2Mn0.6O2 Cathode.

Elemental surface segregation in cathode materials is critical for determining the phase and interfacial reaction between the electrode and electrolyte, which consequently affects the electrochemical properties. Single-crystal cathodes of Li1.2Ni0.2Mn0.6O2 and Li1.2Ni0.2Mn0.6O1.95F0.05 with octahedral morphologies of (102)- and (003)-dominated facets have been manifested to show enhanced electrochemical properties. However, the surface structural features of such single crystals have not been investigated. Herein, using scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy, we probe the elemental surface segregation characteristics in these single-crystal cathodes. We reveal that Ni surface segregation shows dependence on the crystal facet such that it occurs on crystal facets with a mix of cations and anions but not on the facets with only cations or anions. Furthermore, facet-dependent surface reconstructions are observed, featuring a spinel-like structure at the Ni-rich facet but a rock-salt structure at the facet without Ni segregation. The commonly known Mn reduction appears at the single-crystal surfaces and is more pronounced at the facet without Ni segregation. We further reveal that fluorination leads to stabilization of surface oxygens. This study provides detailed structural and chemical information about the facet-dependent Ni surface segregation and the resulting phase formation in the rather less explored micron-sized octahedral Li1.2Ni0.2Mn0.6O2 and Li1.2Ni0.2Mn0.6O1.95F0.05 single crystals, which is key to further exploration of the electrochemical properties of the cathodes in the form of microsized single crystals.

High-throughput first-principles study on the effect of Ni segregation on the formation of Cr-depleted zone and corrosion resistance of S32205 duplex stainless steels

Recent studies have found that in the absence of Cr carbide precipitation, co-segregation of Cr and other elements at the interface between α-Fe and γ-Fe can also lead to the formation of Cr-depleted zones, leading to intergranular corrosion. The study employed three-dimensional atomic probes, focused ion beams, and high-throughput first-principles calculation techniques to investigate the impact of Ni atoms on the α-Fe/γ-Fe interface of UNS S32205 duplex stainless steels. The goal was to analyze the formation of Cr-depleted zones and interface corrosion resistance. The results indicate that both Ni and Cr atoms are segregated at the interface between α-Fe and γ-Fe phases; The strong interaction between Ni and Cr atoms hinders Cr segregation at the interface and slows down the formation of Cr-depleted zones; co-segregation of Ni and Cr can improve the stability of the α-Fe/γ-Fe phase interface, reduce the generation of microcracks, and reduce the stability of Cl atom adsorption in the cracks. This establishes the theoretical basis for further suppressing the formation of Cr-depleted zones and preventing intergranular corrosion.

Deformation Behavior at Low Temperature in 9mass%Ni Steel

The strain distribution in the 9mass%Ni steel introduced by tensile deformation at cryogenic temperature was visualized using a digital image correlation method, and the relationship between the strain distribution and the microstructure of the steel was systematically investigated. Based on the obtained results, the factors that influence the strain distribution were discussed. In the present 9mass%Ni steel, regions consisting of tempered- and fresh-martensite and austenite (TFMA) were embedded in the tempered martensite matrix. The volume fraction of the retained austenite varied along the normal direction of the hot-rolled plates, indicating that Ni segregation occurred during the manufacturing process. As the tensile stress increased, the total elongation remained constant with decreasing temperature. Strain was introduced inhomogeneously via tensile deformation at 77 K. The high- and low-strain regions tended to be distributed in a unit of the block, indicating that the deformability differed among the blocks. The average strain in the block (εblock) was strongly correlated with the Schmid factor of the slip system on the habit plane (SFhabit) and area fraction of TFMA in a block (AMA). The least absolute shrinkage and selection operator regression revealed that the contributions of SFhabit and AMA to εblock were nearly equal. Therefore, the deformability of the block in the 9mass%Ni steel is dominated by SFhabit and ATFMA.

Stabilizing the Reversible Oxygen Redox Reactions in Lithium-Rich Layered Cathodes Via the Surface-to-Bulk Enhanced D-P Hybridization

The oxygen redox (OR) reaction in layered Li-rich oxide (LLO) cathode materials has been an attractive topic as it provides additional capacity. However, the suppression of the irreversible transition metal (TM) migration and oxygen release induced by the OR reaction is still a challenging issue, as it would lead to a severe capacity and voltage decay. Here, we introduce a surface-to-bulk structure modification strategy, namely surface Ni segregation and bulk Mo doping structure. Based on this strategy, the oxygen atoms near the surface are fully coordinated, and meanwhile the distortion of the metal–oxygen octahedron is alleviated in the bulk phase, thereby enhancing the degree of TM 3d and O 2p hybridization to stabilize the metal–oxygen framework for the OR reaction. It suppresses cation migration and oxygen release and maintains phase stability, allowing a reversible OR reaction. As a result, the capacity and voltage retention of the modified materials were significantly improved. This work gives an efficient method to stabilize the OR reaction through the surface-to-bulk enhanced d-p hybridization and provides new paths for the design of Li-rich cathodes.

The design of Pd-containing high-entropy alloys and their hardening behavior under He ion irradiation

Imparting additional chemical heterogeneities of high-entropy alloys (HEAs) is considered as a novel strategy to improve their irradiation resistance on dislocation loop growth; however, there is still a lack of knowledge about the effects of this strategy on bubble evolution and irradiation-induced hardening. In this study, a series of Pd-containing FeCrNiCo HEAs with proven potential for chemical short-range order were designed and irradiated by 400 keV He ions to the fluences of 5 × 1016 cm−2, 1 × 1017 cm−2, and 5 × 1017 cm−2 at 723 K. Microscopic characterization demonstrates a remarkably inhibited growth of dislocation loops and promoted bubble coarsening in the Pd-containing HEAs compared to the HEA without Pd alloying. The underlying mechanism is that although the alloying of Pd can suppress the evolution of dislocation loops through severe local lattice distortion, it also significantly reduces the vacancy formation energy of the entire HEA system, leading to the encouraged bubble evolution. Pd-containing HEAs exhibit improved resistance to irradiation-induced hardening at low doses, but this advantage is unexpectedly absent with increasing dose. The hardening mechanism is explained by identifying the loop-dominated hardening at low dose and the bubble-induced hardening which makes an increased contribution at high dose. In addition, the Ni segregation around bubbles and loop punching mechanism are also discussed to provide essential evidence. These insights can help to shed light on the effects of Pd alloying on bubble evolution and irradiation-induced hardening behavior in HEAs, and thus contribute to the design of irradiation-resistant HEAs.

Integration of hardness and toughness in (CuNiTiNbCr)Nx high entropy films through nitrogen-induced nanocomposite structure

The high-entropy alloy films show excellent comprehensive properties and have great application prospects in space, polar, deep-sea exploration. Compared with traditional ceramic films, the application of high-entropy alloy films is limited by the relative low hardness. In this paper, based on the nanocomposite structure formation principle in thermodynamics, we design the nanocomposite (CuNiTiNbCr)Nx films (HEFs). The introduced N atoms react with Ti, Nb, Cr to form (TiNbCr)N phase (accompanied by Cu and Ni segregation), and the structure of the HEFs changes from amorphous to a nanocomposite structure of CuNi-rich amorphous matrix-(TiNbCr)N nanocrystalline phase. The nanocomposite HEFs exhibit high hardness, high toughness, and excellent wear performance. These results indicate that nanocomposite HEFs have potential applications in the field of extreme environmental wear protection.

A Comparison of Solidification Structures and Submicroscale Cellular Segregation in Rapidly Solidified Stainless Steels Produced via Two-Piston Splat Quenching and Laser Powder Bed Fusion.

Fusion-based additive manufacturing techniques leverage rapid solidification (RS) conditions to create parts with complex geometries, unique microscale/nanoscale morphological features, and elemental segregation. Three custom composition stainless steel alloys with varying chromium equivalence to nickel equivalence ratio (Creq/Nieq) between 1.53 and 1.95 were processed using laser powder bed fusion (LPBF) and/or two-piston splat quenching (SQ) to produce solidification rates estimated between 0.4 and 0.8 m/s. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to collect high-resolution images, electron backscatter diffraction (EBSD) phase identification, and measure cellular segregation. Similar features were observed in both LPBF and SQ samples including phase and microstructure, nanoscale oxide particles, cell size, and segregation behavior. However, dislocation pileup was observed along the cell boundaries only in the LPBF austenite solidified microstructure. Targeted adjustment of the SQ feedstock Cr and Ni concentrations, within the ASTM A240 specification for 316L resulted in no observable impact on the cell size, oxide particle size, or magnitude of segregation. Also, the amount of Ni segregation in the ferrite solidified microstructures did not significantly differ, regardless of Cr/Nieq or processing technique. SQ is demonstrated as capable of simulating RS rates and microstructures similar to LPBF for use as an alternative screening tool for new RS alloy compositions.

Heterojunction Mo-based binary and ternary nitride catalysts with Pt-like activity for the hydrogen evolution reaction

The development of earth-abundant electrocatalysts with Pt-like catalytic activity for the hydrogen evolution reaction (HER) is of great significance to green hydrogen production. Herein, a novel strategy is described to construct dual-phase nitride nanobelts composed of Mo2N and Ni0.2Mo0.8N by nitridation of Ni2+ intercalated layered MoO3 nanobelts (NBs). The Mo2N/Ni0.2Mo0.8N catalyst exhibits superior stability and a low overpotential of 26 mV at 10 mA cm−2 and Tafel slope of 31 mV dec−1 in both the alkaline electrolyte and simulated seawater, which is comparable to or better than that of the benchmark Pt/C catalyst. The excellent alkaline HER characteristics is attributed to the Mo2N/Ni0.2Mo0.8N heterostructure with adjustable content and robust interfaces and without metal Ni segregation and structure collapse during nitridation. Density-functional theory (DFT) calculations and experiments reveal that the Mo2N/Ni0.2Mo0.8N interface with strong electronic interactions optimizes H adsorption/desorption yielding moderately weak bonding metal sites with positive ΔGH* as the catalytic centers, thereby accelerating the HER kinetics and boosting the HER activity. The results reveal a simple strategy for the preparation of heterostructured nitride-based catalysts with Pt-like activity for hydrogen evolution.

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

The durability of protonic ceramic fuel cells (PCFC) using Yb-doped BaZrO3 (BZYb) as electrolyte was investigated by 1000 h power generation tests and EPMA analysis of Co from cathode and Ni from anode. In addition, the effect of buffer layer of BaCeZrYYbO3-δ (BCZYYb) between the cathode and the electrolyte on durability and elemental diffusion was also investigated. In the cell with the buffer layer, the degradation of polarization resistance was suppressed in 1000 h power generation test. From EPMA analysis, Ni and Co segregation was observed in the electrolyte and the cathode after 1000 h power generation test in the cell without the buffer layer, which was suppressed in the cell with the buffer layer. These results suggested that the suppression of the segregation by use of BCZYYb buffer layer might have reduced the degradation of polarization resistance.

Effect of a direct aging heat treatment on the microstructure and the tensile properties of a 18Ni-300 maraging steel produced by Laser Powder Bed Fusion

The microstructure and the tensile properties of a 18Ni-300 maraging steel manufactured by Laser Powder Bed Fusion (L-PBF) were investigated after a Direct Aging Treatment (DAT) and compared to those obtained after a conventional Solution and Aging Treatment (SAT). DAT results in a 5% lower strength and a 70–80% higher ductility than SAT. The crystallite size of martensite laths and the characteristics of the submicrometric compounds precipitated during aging are very similar between the two variants. The different properties are attributed to the higher content of austenite (both residual and reverted) in DAT material, resulting from the Ni, Ti and Mo segregation during L-PBF. Such an austenite increases tensile ductility through a Transformation Induced Plasticity (TRIP) effect that was demonstrated by investigating the microstructure with Transmission Electron Microscopy (TEM) close to the fracture surface where deformation is localized.

Investigation on hybrid bonding of tungsten/steel with in-situ formed composites interlayer

Hybrid bonding between steel and W using 88(97W-2.7Cu-0.3Ni)−12Ni powder mixtures sandwiched between Ni and Cu foils was carried out in vacuum for 60 min at 1120 ℃ with a pressure of 5 MPa. The assembly sequence of bonding specimens was steel/Ni/Cu/88(97W-2.7Cu-0.3Ni)−12Ni powder mixtures/Cu/Ni/Cu/W. The microstructures and composition distribution of the joint were studied by SEM and EDS. Joint properties were evaluated by shear experiment tests, and the fracture characteristics of the shear samples for the joints were also determined by SEM. The results showed that the joints comprised steel/Ni/composites/Cu-Ni/W, wherein the composites interlayer consisting of tungsten heavy alloy and irregular Ni agglomerate in-situ formed by the combined sintering of 88(97W-2.7Cu-0.3Ni)−12Ni powder mixtures. The hybrid bonding mechanisms of the interfaces in the joint include solid-phase diffusion bonding and transient liquid phase diffusion bonding. An additional hot-dip experiment found that obvious solid-liquid interface diffusion and Ni segregation occurred under the bonding conditions, and a large number of W atoms diffused into the Cu-Ni liquid phase. In the Cu-Ni alloy layer, the content of element W is more than 4.22% (at.%). The average shear strength of W/steel joints was 263 ± 7 MPa and all the shear samples were fractured in the W near the bonding interface.

Ni Ingress and Egress in SrTiO3 Single Crystals of Different Facets

Metal-ion surface interactions and/or doping in host perovskite-oxides are techniques that are widely employed for electronic structure tuning purposes and in developing novel heterogeneous catalysts; however, an in-depth understanding of the different elementary steps and factors involved in these processes is lacking. Herein, we use Atomic Force Microscopy (AFM), Scanning Transmission Electron Microscopy (STEM), and ab initio thermodynamics through density functional theory (DFT) to specifically investigate Ni surface adsorption, ingress, migration, segregation, and egress processes across different SrTiO3 (STO) single-crystal facets and terminations, specifically the (001), (110), and the (111). Under oxidizing and reducing conditions at different temperatures, Ni egress is observed on (110) STO samples, but not the (001). DFT results demonstrate Ni to have a higher thermodynamic egress propensity, specifically through an oxygen-terminated (110) facet in comparison to other (001) terminations, whereas for the (111)-Ti terminated facet, Ni is likely to remain in the bulk post ingress. We suggest that the observed uniqueness of the (110) surface facet toward Ni egress is possibly a consequence of a surface phase transition. These results can help guide design interests with regard to Ni surface stabilization, ingress/egress suppression, or facilitation in STO by elucidating the nuances involved across different facets.

Density functional theory study of Ni segregation in CuNi(111) alloy with chemisorbed CO, O, or H

Adsorption-induced surface segregation can affect the electrochemical and catalytic properties of metal alloys. We have used density functional theory to study the Ni segregation in CuNi(111) alloy in presence of adsorbed CO, O, or H. The calculated results show that adsorbates can significantly affect the Ni segregation behavior on the CuNi(111) surface. In presence of adsorbed H, the Ni atom still prefers to stay in the bulk of the alloy as under vacuum conditions, while Ni atom prefers to segregate towards the alloy surface with adsorbed CO or O. Moreover, the analysis of the adsorption behavior of these three adsorbates and the electronic structure of the alloy surface indicates that the surface-adsorbate binding strength directly affects the Ni segregation behavior on the CuNi(111) surface in a reactive environment. This study provides a theoretical basis for the practical application of Cu–Ni alloy as CO2 reduction catalysts.

Effect of intercritical heat treatment on microstructure, nano-size precipitation and mechanical properties of Fe–Ni–Cu–Al low carbon steel

In recent years, the co-precipitation of nanoscale Cu and Ni–Al particles in Fe–Ni–Cu–Al low-carbon steel provide an extremely high strengthening increment while minimizing the addition of carbon and alloy elements, however, the toughness and plasticity dramatically decreased. This research focuses on the improvement of toughness and plasticity utilizing an intercritical (Quenching-Lamellarization-Aging (QLA)-type) heat treatment for Fe–Ni–Cu–Al steel. The microstructure and nanoparticles precipitation of Fe–Ni–Cu–Al steel were characterized by TEM, HRTEM and phase analysis after a QLA treatment. The results indicate that the multi-phase microstructure consisting of maraging martensite, intercritical ferrite and austenite reversion were obtained by QLA treatment. With decreasing intercritical quenching temperature between 820 °C and 660 °C, −80 °C V-type impact energy and plasticity of Fe–Ni–Cu–Al steel was obviously improved due to the formation of intercritical ferrite and austenite reversion. Meanwhile, the coarsening of Ni3Al particles were markedly accelerated because of Ni segregation in matrix and abundant Fe atoms for the substitution of Al atoms within Ni3Al particles. The strengthening mechanisms of Ni3Al particles in Fe–Ni–Cu–Al steel was mainly Orowan bypassing strengthening after QLA treatment, and the corresponding strengthening increment was calculated as 89% (214 MPa) at the intercritical quenching temperature of 720 °C, which was 17% (259 MPa) lower than the value of order strengthening obtained by 820 °C primary quenching. The combination of multi-phase microstructure and co-precipitation strengthening contributed to excellent mechanical properties (yield strength>1050 MPa at 20 °C, the elongation>13%, a Charpy impact toughness of >80 J at −80 °C) during the intercritical quenching temperature of 720 °C–780 °C.

Ab-initio calculations of substitutional co-segregation interactions at coherent bcc Fe-Cu interfaces

Cu rich precipitates (CRPs) are commonly observed to form core-shell structures in neutron irradiated pressure vessel steels. The core region is typically composed of pure Cu whilst the shell is formed of other solute elements including Ni and Si. In this work we calculate the segregation energies for Ni and Si substitutional defects segregating to coherent Fe-Cu interfaces decorated with different types of point defect (vacancies and other solute substitutional defects). By comparing these values to those found for Ni and Si segregation to clean Fe-Cu interfaces we can establish how the presence of point defects on the interface influences solute segregation behaviours. Interestingly we find that the segregation of Ni to Si decorated interfaces and the segregation of Si to Ni decorated interfaces demonstrate a relatively strong co-segregation interaction. We additionally observe that both solute species experience more attractive segregation to vacancy decorated Fe-Cu interfaces. These findings suggest that mixed solute interactions and the presence of vacancies may both play an important role in assisting the formation of large solute shell regions in CRPs. We further observe that the presence of Ni and vacancies on coherent Fe-Cu interfaces both result in substantial reductions in interfacial energy density. These findings support experimental predictions and indicate both features may significantly contribute to CRP formation.

Microstructural understanding of general and localized corrosion of Fe13Cr4Al2Mo1.2Nb alloy in high temperature aerated water and superheated steam

The corrosion behaviors of a Fe13Cr4Al2Mo1.2Nb alloy in 360 °C aerated water and 400 °C superheated steam were investigated. A similar dual-layered oxide scale was formed under both conditions. However, the inner oxide layer was much thicker in 400 °C steam than in 360 °C water, indicating the occurrence of steam-accelerated corrosion. Nodular corrosion was observed following exposure to 360 °C water, while localized corrosion and Ni segregation along the grain boundary occurred after 120-d exposure to 400 °C steam. The Fe2Nb phase experienced delayed oxidation with respect to matrix, which involved transformation from amorphous Nb oxides to crystalline Nb2O5.

He-enhanced heterogeneity of radiation-induced segregation in FeNiCoCr high-entropy alloy

• Atom probe tomography reveals radiation-induced segregation around He bubbles • Electron energy-loss spectroscopy measures He density and pressure inside bubbles • The inverse Kirkendall effect dominates RIS around over-pressurized He bubbles • He atoms retard Co diffusion via the vacancy mechanism and enhance Co segregation • Local chemical order of medium-/high-entropy alloys can be tailored via He bubbles Radiation-induced segregation (RIS) is a typical non-equilibrium process that can dramatically alter the behavior of defect sinks and material properties under irradiation. However, RIS mechanisms have been rarely studied around small He bubbles owing to the technical challenges involved in direct measurements of local chemistry. Here, using state-of-the-art atom probe tomography, we report the RIS behavior near He bubbles in the FeNiCoCr high-entropy alloy that indicates Co segregates most strongly, followed by weaker Ni segregation, whereas Fe and Cr are depleted almost to the same degree. Exceptionally, the magnitude of Co segregation around He bubbles is higher than previously measured values at voids and dislocation loops. Electron energy-loss spectroscopy was used to measure the He density and pressure inside individual bubbles. We demonstrate that He bubbles are over-pressurized at the irradiation temperature that could result in the vacancy bias and the subsequent vacancy-dominated RIS mechanism. First-principles calculations further reveal that there are repulsive interactions between He and Co atoms that may reduce the frequency of Co-vacancy exchange. As a result, He atoms likely retard Co diffusion via the vacancy mechanism and enhance the heterogeneity of RIS in Co-containing multicomponent alloys. These insights could provide the basis for understanding He effects in nuclear materials and open an avenue for tailoring the local chemical order of medium-and high-entropy alloys.