Homogeneous Alloy Research Articles

Coke resistant NiCo/CeO2 catalysts for dry reforming of methane derived from core@shell Ni@Co nanoparticles

Core@shell Ni@Co and bimetallic alloyed Ni–Co nanoparticles with controlled Co/Ni compositions were prepared and supported on CeO2 to investigate their performance in catalytic dry reforming of methane (DRM) and occurrence of sintering and coking. Increasing the Co/Ni ratio significantly reduced coke deposition while maintaining catalytic activity for DRM. However, a Co/Ni ratio > 1 caused a rapid decrease in activity. The Ni@Co1/CeO2 catalyst exhibited the highest CH4 and CO2 conversions, with long-term stability during DRM at 800 °C for 100 h. The initial core@shell structure of the Ni@Co1/CeO2 catalyst transformed to a homogeneous alloy after DRM at 800 °C for 10 h, losing its Co shell. However, the bimetallic alloyed Ni–Co1/CeO2 catalyst transformed into a non-uniform alloy rich in Co on the surface after DRM for 10 h. As the elemental distribution of the NPs becomes more homogeneous, Ni–Co1/CeO2 exhibit similar catalytic activity to Ni@Co1/CeO2 after 50 h. The oxygen vacancies on the CeO2 surface provided oxygen atoms to the Ni surface, removing carbon species deposited and releasing CO. Therefore, Ni@Co1/CeO2 catalyst provides excellent catalytic activity and stability due to the rapid formation of a homogenous alloy and the synergistic effect of Co and CeO2.

Electrosynthesis and Evaluation of Homogeneous CoCrNi Medium-Entropy Alloys for Structural Applications

Electrosynthesis and Evaluation of Homogeneous CoCrNi Medium-Entropy Alloys for Structural Applications

Hydrogen permeability of 48Cu52Pd cold-rolled alloy foil and different methods of its surface pretreatment

The process of atomic hydrogen penetration into the metal phase is complicated by the phase-boundary transition from the liquid and/or gas phase. That is why the cleanliness of metal and alloy surfaces is of particular importance. The purpose of this work was to determine the effect of surface pretreatment using photon pulses, ultrasound, and potential cycling on the parameters of hydrogen permeability for 48Cu52Pd metal cold-rolled membranes. The study was focused on a foil of copper-palladium homogeneous alloy with 48 at. % Cu and 52 at. % Pd composition. The studied samples were obtained by cold rolling and their thickness were 10 and 16 μm. Surface pretreatment included rinsing in acetone, using ultrasound, pulsed photon treatment, and quadruple potential cycling over a wide range of potentials. Electrochemical studies included cyclic voltammetry and cathode-anodic chronoamperometry in a deaerated 0.1 M H2SO4 solution. Hydrogen permeability was calculated using mathematical models for samples of finite and semi-infinite thickness. It was found that the surface treatment of a 48Cu52Pd foil with photon pulses leads to both an increase in the ionisation rate of atomic hydrogen and an increase in the roughness of the foil surface. The diffusion coefficient of atomic hydrogen does not depend on the method of surface pretreatment with ultrasound and photon pulses. The extraction rate constant for the extraction of the atomic hydrogen after photon treatment increases, which facilitates the processes of both H introduction and ionisation due to the release of active centres of the surface. Electrochemical cleaning of the surface during the quadruple potential cycling contributes to the growth of the extraction rate constant for the extraction of atomic hydrogen

Investigation on the surface reactions of Mg–Al alloy/Teflon/Viton in O2

ABSTRACT A comprehensive study on the surface morphologies and element distributions of Mg–Al alloy particles and Mg–Al alloy/Teflon/Viton composite (MATV) during oxidation was conducted via scanning electronic microscopy and energy-dispersive X-ray spectroscopy. The formations of the reaction products of MATV were further analyzed via X-ray diffraction. By the temperature of 600°C, a fluorination reaction between Teflon and the alloy underwent on the alloy surface, involving the reaction of the αMg prime phase at lower temperature. A MgF2–AlFx shell surrounding the alloy particle was also generated simultaneously. This shell could elevate the oxidation temperature from the melting point of the alloy to 600~859°C. Furthermore, this shell could also severely decrease the oxidation rate of the exceeded alloy with O2. As the slower oxidation of the homogeneous liquid alloy began at higher temperature, a different mechanistic reaction from that of the pure alloy underwent. MgO particles rather than MgO ridges on the pure alloy surface were generated.

Theoretical and experimental grain boundary energies in body-centered cubic metals

Grain boundary energy (GBE) and its temperature dependence in body-centered cubic (bcc) metals are investigated using ab initio calculations. We reveal a scaling relationship between the GBEs of the same grain boundary structure in different bcc metals and find that the scaling factor can be best estimated by the ratio of the low-index surface energy. Applying the scaling relationship, the general GBEs of bcc metals at 0 K are predicted. Furthermore, adopting the Foiles’s method which assumes that the general GBE has the same temperature dependence as the elastic modulus c44 [Scr. Mater., 62 (2010) 231–234], the predicted general GBEs at elevated temperatures are found in good agreement with available experimental data. Reviewing two experimental methods for determining the general GBEs, we conclude that the two sets of experimental GBEs for bcc metals correspond to different GB structural spaces and differ by approximately a factor of 2. The present work puts forward an efficient methodology for predicting the general GBEs of metals, which has the potential to extend its application for homogeneous alloys without strong segregation of the alloying element and facilitates GB engineering for advanced alloy design.

Electronic and Coordination Effect of PtPd Nanoflower Alloys for the Methanol Electrooxidation Reaction

Platinum-based electrocatalysts usually exhibit limited and quickly vanished activity for the methanol oxidation reaction (MOR) due to active site occupation and surface intermediate poisoning, especially CO. The attendant synergistic effect of the alloy strategy is often executed to improve these flaws. Here, PtPd nanoflower alloys uniformly anchored on the carbon paper were successfully synthesized by electrodeposition to construct well-defined active sites, over which high MOR activity and tolerance were realized. The homogeneous alloy structure of PtPd nanoflowers was confirmed by TEM and XAFS. Impressively, the Pt1Pd1-N NFs/CP exhibited an excellent mass activity of 9.25 A mgPt+Pd–1 and specific activity of 13.2 mA cmECSA–2 of MOR in the alkaline electrolyte, and 87.8% of the initial activity was still retained after 500 cycles. In situ infrared spectroscopy revealed that Pt1Pd1-N NFs/CP ensured the complete oxidation of methanol following a CO-free path. The investigation of the electronic and coordination structure demonstrated that the Pt–Pd adjacent sites with a zero valence are the highly active center for MOR, and the synergistic effect between Pt and Pd greatly improved the MOR electrocatalytic performance. This work offers an effective strategy to precisely control the number and structure of decorating species for constructing alloy nanocatalysts to enhance the electroactivity toward MOR.

Fe microalloying during the in-situ synthesis of homogeneous CuCrFe alloys by aluminothermic self-propagating

Microalloying is an effective approach to improve the performance of CuCr alloys. This paper presents a new idea for the preparation of CuCrFe alloys by self-propagation reduction in-situ microalloying. It is found that the addition of Fe alloying components reduces the Gm and ΔGm of the CuCr melt, which inhibits the occurrence of the liquid phase decomposition process. When the Fe content of CuCrFe alloy is 1.5%, compared with the Cr-rich phase of CuCr alloy, the second phase undergoes significant refinement, the average particle size refined from 21.7 μm to 19.3 μm and the standard deviation reduced from 10.59 to 8.73, and the addition of Fe is beneficial to the spheroidization and refinement of the Cr-rich phase. Fe is mainly combined with Cr, and CuCrFe alloy still exists in the form of two phases. After solution + aging treatment, the average particle size of the second phase in CuCr alloy decreases from 21.8 μm to 17.9 μm, and that in CuCrFe alloy decreases from 19.6 μm to 18.6 μm. The conductivity of CuCrFe alloy increases from 10.5 MS m−1 to 15.9 MS m−1, and the hardness increases from 95.0 HB to 117.0 HB. After heat treatment, the nano-spherical are present in the Cr-rich phase of the CuCrFe alloy, while Fe is completely solid-soluble in the Cr-rich phase; the Cu-rich matrix shows a diffuse distribution of fine second phases, which play a role in strengthening the second phase in the matrix.

Designing composition ratio of magnetic alloy multilayer for transverse thermoelectric conversion by Bayesian optimization

We demonstrated the effectiveness of the machine learning method combined with first-principles calculations for the enhancement of the anomalous Nernst effect (ANE) of multilayers. The composition ratio of CoNi homogeneous alloy superlattices was optimized by Bayesian optimization so as to maximize the transverse thermoelectric conductivity (αxy). The nonintuitive optimal composition with a large αxy of ∼10 A K−1 m−1 was identified through the two-step Bayesian optimization using rough and fine candidate pools. The Berry curvature and band dispersion analyses revealed that αxy is enhanced by the appearance of the flat band near the Fermi level due to the multilayer formation. The magnitude of the energy derivative of the anomalous Hall conductivity increases owing to the large Berry curvature near the flat band along the R-M high symmetry line, which emerges only in the optimized superlattice, leading to the αxy enhancement. The effective method verified here will broaden the choices of ANE materials to more complex systems and, therefore, lead to the development of transverse thermoelectric conversion technologies.

Effect of grain size on critical twinning stress and work hardening behavior in the equiatomic CrMnFeCoNi high-entropy alloy

While the impact of grain boundary strengthening on dislocation slip is particularly effective in the equiatomic CrMnFeCoNi high-entropy alloy (HEA), its effect on deformation twinning remains unclear. To better understand how a grain size reduction affects the onset of deformation twinning and the work hardening behavior of the CrMnFeCoNi HEA, chemically homogeneous, nearly untextured, and single-phase face-centered cubic alloys with different grain sizes were investigated. Tensile tests were performed at 293 and 77 K and interrupted at different strains followed by systematic transmission electron microscopy observations. In all cases, deformation twinning occurs above a critical stress that is independent of temperature. This uniaxial twinning stress decreases from ∼785 to ∼615 MPa when the grain size increases from 6 to 242 µm, respectively, following the Hall-Petch equation. The resistance of the grain boundaries against slip and twinning is found to be nearly identical (Hall-Petch slope: ∼500 MPa·µm1/2) but the twinning stress extrapolated to infinite grain size (592 ± 30 MPa) is larger than the uniaxial friction stress against dislocation glide at 293 and 77 K (130 and 320 MPa, respectively). Deformation twinning at 77 K is found to sustain a high work hardening rate when it is triggered in a plastic regime dominated by planar glide of dislocations. In contrast, it does not significantly contribute to the work hardening rate at 293 K when dislocation cells have already formed and the dislocation mean free path is smaller than the mean twin spacing.

Toward controllable and predictable synthesis of high-entropy alloy nanocrystals

High-entropy alloy (HEA) nanocrystals have attracted extensive attention in catalysis. However, there are no effective strategies for synthesizing them in a controllable and predictable manner. With quinary HEA nanocrystals made of platinum-group metals as an example, we demonstrate that their structures with spatial compositions can be predicted by quantitatively knowing the reduction kinetics of metal precursors and entropy of mixing in the nanocrystals under dropwise addition of the mixing five-metal precursor solution. The time to reach a steady state for each precursor plays a pivotal role in determining the structures of HEA nanocrystals with homogeneous alloy and core-shell features. Compared to the commercial platinum/carbon and phase-separated counterparts, the dendritic HEA nanocrystals with a defect-rich surface show substantial enhancement in catalytic activity and durability toward both hydrogen evolution and oxidation. This quantitative study will lead to a paradigm shift in the design of HEA nanocrystals, pushing away from the trial-and-error approach.

Tuning Catalytic Activity of Ni–Co Nanoparticles Synthesized by Gamma‐Radiolytic Reduction of Acetate Aqueous Solutions

AbstractTransition metal‐based catalysts show great potential to replace Pt‐based material in energy conversion devices thanks to their low cost, reasonable intrinsic activity, thermodynamic stability, and corrosion resistance. The electrochemical performance of such catalysts is sensitive to their composition and structure. Here, it is demonstrated that homogeneous alloy nanoparticles with varying Ni‐to‐Co ratio and controlled structure can be synthesized from aqueous Ni(Co) acetate solutions using a facile γ‐radiolytic reduction method. The obtained samples are found to possess defects that are ordered to form polytypes structures. The concentration of these defects depends on the Ni‐to‐Co ratio, as supported by the results of ab initio calculations. It is found that structural defects may influence the activity of catalysts toward the oxygen evolution reaction, while this effect is less pronounced with respect to the oxygen reduction reaction. At the same time, the activity of Ni–Co catalysts in the hydrogen evolution reaction is affected by formation of NiOH bonds on the surface rather than by the presence of structural defects. This study demonstrates that the composition of NiCo nanoparticles is an essential factor affecting their structure, and both composition and structure can be tuned to optimize electrochemical performance with respect to various catalytic reactions.

Tuning surficial atomic configuration of Pt–Ir catalysts for efficient ammonia oxidation and low-temperature direct ammonia fuel cells

Low-temperature direct ammonia fuel cell via ammonia oxidation reaction (AOR) is one of the most attractive ways for ammonia utilization. The Pt–Ir alloyed catalysts have been proven greatly efficient in AOR to overcome the sluggish kinetics and complex multi-electron processes; however, it still remains challenging for further improvement because of the complexity of alloyed types and unclear fundamental understanding. Herein, we systematically fabricated a series of PtxIry/XC-72 catalysts via tuning the Pt/Ir compositions and these catalysts demonstrate significant composition-dependent behaviours in AOR, whereas the Pt favours increasing current densities and the Ir facilitates reducing onset potentials. Importantly, the actual surface Pt/Ir atomic configuration is totally different with the classic view for homogeneous alloys by combining catalytic and characteristic analyses. The Pt prefers to be segregated on the topmost surface. Then the quantified correlations between current density/onset potential and Pt composition in Pt − Ir nanoparticles (NPs) were further established and discussed. The onset potential of AOR is significantly reduced by Ir incorporation while it is insensitive to Ir contents. Accordingly, the well-studied Pt5Ir5/XC-72 catalyst was further employed in an alkaline exchange membrane fuel cell, exhibiting a peak power density (230.0 mW cm−2). Our work provides insights into the effect of Pt/Ir compositions on AOR, with the goal of catalysts engineering for AOR and ammonia fuel cells.

Microstructure evolution and property strengthening of CuCr50 prepared by thermite reduction-electromagnetic casting during the heat treatment process

A homogeneous CuCr50 alloy ingot prepared in situ by thermite reduction-electromagnetic casting is a type of nonequilibrium solidification structure with a Cu matrix supersaturated with Cr. In this study, the effects of heat treatment on the microstructure of the CuCr50 alloy were studied using scanning electron microscopy (SEM), metallography microscopy and transmission electron microscopy (TEM), and the conductivity, macrohardness and tensile strength of the CuCr50 alloy were measured. The obtained results show that the optimal heat treatment conditions are solution treatment (975 °C for 1 h) and aging treatment (500 °C for 2 h), for which the hardness, conductivity, and tensile strength of the CuCr50 alloy are the best; the macrohardness is 103.33 HB, the conductivity is 18.60 MS/m, and the tensile strength is 199.72 MPa. TEM characterization showed that nanosized Cr particles were precipitated and dispersed in the Cu matrix after the aging treatment; the relationship between the precipitates and the Cu matrix was incoherent, and Cu dissolved into the Cr phase in the form of a solid solution to strengthen the second Cr phase.

Mechanical Properties of Ti–5Al–5V–5Mo–3Cr Alloy Sintered From Blended Elemental Powders

The field-assisted sintering technique (FAST) was successfully used to sinter, homogenize, and age the Ti–5Al–5V–5Mo–3Cr (Ti-5553) aerospace alloy from blended elemental powders in a single processing run. Sintering at a comparatively high temperature of 1500 °C assured chemical homogeneity of the material. Subsequently, without cooling to the room temperature, the temperature was lowered to 600 °C for 30 min to provide annealing treatment and to produce a desired lamellar α + β microstructure. Using an in-house designed assembly, a fully dense rod of 15 mm in diameter and 70 mm in length was successfully sintered. Microstructural and mechanical properties of the alloy were investigated and compared with the conventionally processed Ti-5553 alloy. A good combination of strength (1183 MPa) and ductility (6 pct) was achieved; these values are fully comparable to the conventional cast, forged, and aged Ti-5553 alloy. It was shown that homogeneous Ti-5553 alloy can be produced by FAST, FAST can instantly provide necessary annealing steps, and the FAST process can be upscaled to produce homogeneous rods.

Near‐Threshold Fatigue Crack Propagation in Sintered 304L Stainless Steel Compact with Network Structure Composed of High‐Entropy CrMnFeCoNi Alloy

High‐entropy alloys (HEAs) have several superior mechanical properties. However, for these alloys, there is a trade‐off between tensile strength and fatigue‐crack propagation resistance, as is the case for materials with a homogeneous microstructure. The purpose of the present study is to achieve both high‐strength and fatigue‐crack propagation resistance by using a bimodal structure. CrMnFeCoNi alloy and AISI304L powders with different particles diameters are mixed and then consolidated using spark plasma sintering to fabricate sintered compacts with a network structure composed of coarse‐grained 304L surrounded by fine‐grained HEA microstructure. Stress intensity factor K‐decreasing tests are conducted at various force ratios in the ambient laboratory atmosphere to examine near‐threshold fatigue‐crack propagation in the sintered compacts. The ductility and threshold stress intensity range ΔKth tested at a high force ratio for the sintered compacts are higher than that for a homogeneous fine‐grained CrMnFeCoNi alloy. Fatigue cracks in the sintered compacts mainly propagate through the coarse‐grained 304L microstructure, which has high‐fatigue‐crack propagation resistance near the threshold. Therefore, ΔKth for the sintered compacts increases because the ΔKeff,th, which is derived from the 304L without degrading the tensile strength, is higher than that for homogeneous fine‐grained CrMnFeCoNi alloy.

Pilling–Bedworth Ratio for Homogeneous Alloys: A Physically Sound Practical Generalization

Pilling–Bedworth Ratio for Homogeneous Alloys: A Physically Sound Practical Generalization

Microstructure and mechanical properties of as casted Al-Si-Sc-Ce alloys prepared by molten salt electrolysis with varied cooling rate

Al-7Si-0.8Sc-0.6Ce alloys were prepared with combined addition of low-cost Ce and high-priced Sc through molten salt electrolysis with controlled cooling process. The phases of as-casted alloys consisted of α-Al, eutectic silicon, ternary AlSi2Sc2 and CeAl2Si2. Rapid cooling transformed the huge hollow ternary AlSi2Sc2 blocks (300–500 µm) into fine Al3Sc particles (dozens of μm), and simultaneously refined the AlSi2Sc2 with reducing its interlayer thickness from 15.26 µm down to 1.94 µm (the cooling rate from 0.05 K s−1 up to 30 K s−1). The eutectic Si became fine fibrous-like clusters and the thickness of CeAl2Si2 phase reduced by 62.6 % when the cooling rate reached 30 K s−1. Micro-hardness measurement and first principles-based calculation were applied for evaluating the individual mechanical performance of each phase in the alloy. The calculated Young’s modulus (E) suggests that Al3Sc (E = 168 GPa), AlSi2Sc2 (E = 178 GPa) and CeAl2Si2 (E = 39.2 GPa) can toughen Al-matrix (E = 30.6 GPa). The rapid cooling (30 K s−1) strongly enhances all phase components but diminish the huge hollow AlSi2Sc2 blocks (ultrahigh micro-Vickers hardness ∼ 780 HV), making a homogenous Al-RE alloy with improved mechanical property (68 ∼ 84 HV balanced among phase components). The refining mechanisms on the improved mechanical performance are also discussed.

Thermally stable membranes based on PdNiAu systems with high nickel content for hydrogen separation

PdNiAu-based composite membranes were obtained through sequential electroless deposition, varying their composition by modifying the Ni deposition time. After annealing at high temperature, a single phase with FCC structure was obtained in all the membranes, showing homogeneous alloy composition. A decrease in the lattice parameter was evident as the Ni content increased in the alloy. The permselective properties of the membranes were evaluated at different temperatures and pressures. The results showed that H2 permeance decreased as the Ni composition increased in the film and increased as the lattice parameter increased. The best performance was obtained with the membrane containing 29 at.% Ni (11.40 × 10−4 mol m−2 s−1 Pa−0.5), even higher than that reported for pure Pd. The membrane with higher Ni content (54 at.%) and lower permeance (0.20 × 10−4 mol m−2 s−1 Pa−0.5 at 723 K) overcame the performance of some Ni-containing membranes previously reported. The membranes showed high stability for several days at high temperatures.

The microstructure and thermal stability of the two-phase amorphous melt-spun alloys ejected from a double chamber crucible.

This work presents the microstructure and properties of two-phase amorphous melt-spun alloys ejected from the crucible with partition between liquids. The microstructure was studied by scanning electron microscopy and transmission electron microscopy and the phase composition was studied by X-ray diffraction. The thermal stability of the alloys was determined using differential scanning calorimetry. The microstructure study proves that the composite alloys are heterogeneous because of the existence of the two amorphous phases obtained due to the use of a partition between the liquids. This microstructure correlates with complex thermal characteristics not found in homogeneous alloys of the same nominal composition. The layered structure of these composites influences the formation of fractures during tensile tests. This article is protected by copyright. All rights reserved.

Ultrafast magnetization and energy flow in the laser-induced dynamics of transition metal compounds

An electronic theory of the laser-induced ultrafast magnetization dynamics in transition-metal alloys is presented. A many-body model Hamiltonian is considered that incorporates the fundamental electronic hopping, local Coulomb interaction, and spin-orbit coupling (SOC) on the same footing. Exact time propagations are performed on a tetrahedral cluster, from which the time dependencies of the local spin moments, orbital occupations, and single-particle energies of homogeneous systems and binary alloys are obtained. The consequences of inhomogeneities in the laser absorption and in the SOC strengths are investigated giving emphasis to the nature of spin-density transfers between the alloy components. A local perspective on the optically induced spin transfer is proposed in terms of which two main steps emerge: a dominantly local optical excitation followed by hopping-driven spin-density transfers among the different alloy components. The conjoint action of the spin-orbit interactions at the origin of local spin flips and spin-to-orbital angular-momentum transfer and the interatomic hoppings responsible for charge, spin, and energy flows between different sublattices is demonstrated. Transient and steady-state dynamical regimes are identified that result in delays in the onset of the local demagnetizations and in ultrafast redistributions of the spin. The central role of electron delocalization and electronic hopping in the spin-density redistribution and demagnetization of the different alloy components is demonstrated.