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In this paper, molecular dynamics method was used to design the grain boundary (GB) segregation structure of solute atom Mo in nanocrystalline nickel, and the effect of the segregation structure on the migration and deformation mechanism of nanocrystalline Ni–Mo alloy boundaries was studied. The results indicate that the addition of solute atom Mo can cause segregation at GBs, and the yield strength and tensile strength of nanocrystalline Ni are significantly increased through solid solution strengthening. Mo atoms segregation result in an increase of the GB thickness and stability of the GB. In addition, by labeling GB atoms and tracking their diffusion trajectories, it was found that after adding Mo atoms, the probability of atomic diffusion at GBs decreased. This indicates that Mo atoms reduce GB energy and improve GB stability. Meanwhile, as the Mo content increases, the degree of atomic disorder increases, and the probability of GB migration decreases. This leads to the inability of grains to merge and inhibit their growth, effectively improving the mechanical properties of the material. As the strain increases, the number and length of dislocations increase, and a large amount of entanglement occurs at GBs. With the increase of Mo content, the number of dislocations decreases sharply, with Shockley dislocations having the highest number. Shockley dislocations interact with other dislocations and hinder their generation and movement, forming a more stable dislocation system structure and increasing the strength of the alloy. Our work focuses on observing the influence of GB segregation structure on the mechanical properties and deformation mechanism of nanocrystalline polycrystalline Ni–Mo alloys, establishing the mechanism of the influence of segregation structure on the stability and coarsening of nanocrystalline metal GBs, examining the influence of segregation structure on dislocation motion and GB migration process during deformation, and proposing positive research and development ideas and theoretical basis for designing nanocrystalline metal materials with excellent performance. [ABSTRACT FROM AUTHOR]

To design and construct bifunctional electrocatalyst with high activity is important to the commercial development of electrochemical water splitting (EWS) technology. The carbon clothe (CC) supported on Ni(OH) 2 nanosheet is used as template to tune the morphology of Ni 0.85 Se. To improve the activity of Ni 0.85 Se, amorphous Ni 4 Mo alloy is anchored on the Ni 0.85 Se to obtain the Ni 0.85 Se/Ni 4 Mo catalyst (NS/NM) by electrodeposition method. The electron state and conductivity of NS/NM are modulated compared to that of the NS sample. It is found that the electron transfers from Ni 4 Mo to Ni 0.85 Se. Benefiting from the tuned electron state of Ni 0.85 Se and Ni 4 Mo, the enhanced number of exposed active sites and lower R ct value, the NS/NM heterostructure electrode requires low overpotentials of 51 and 206 mV to obtain 10 mA cm−2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The optimized electrode shows fast kinetics and exhibits near to 100% Faraday efficiency for HER and OER in 1 M KOH. Importantly, it is noted that the electrocatalytic activity of NS/NM catalyst for both HER and OER is enhanced obviously in comparison with NS sample. With heterostructure catalyst, the EWS only proceeds at 1.48 V to yield 10 mA cm−2, which is reduced about 110 mV compared to NS electrode. Here, a novel kind of bifunctional heterostructure electrocatalyst is constructed. [Display omitted] • The heterostructure of crystalline Ni 0.85 Se anchored on amorphous Ni 4 Mo alloys was prepared. • The formation of heterostructure causes the redistribution of electrons. • The introduction of Ni 4 Mo enhances the activity for HER and OER obviously. • With heterostructure catalyst, the EWS only proceeds at 1.48 V to yield 10 mA cm−2. [ABSTRACT FROM AUTHOR]

Electrocatalytic water splitting provides an effective way to store electrical energy (i.e., solar and wind energy) intermittently. It is believed that developing electrocatalysts with excellent stability, high catalytic efficiency, and abundant natural reserves is the key to the industrialization of electrochemical water splitting. Herein, nickel–molybdenum electrocatalyst formed on the surface of a low‐cost etched stainless‐steel mesh via a one‐step electrodeposition method exhibits a low hydrogen evolution overpotential of 53 mV and oxygen evolution overpotential of 261 mV at a current density of 10 mA cm−2 due to the synergistic effect of nickel and molybdenum. Moreover, the as‐prepared catalyst demonstrates superior hydrogen evolution reaction (HER) catalytic performance than commercial Pt/C at current density exceed 200 mA cm−2. The electrocatalyst also shows excellent HER and oxygen evolution reaction stabilities at high current density attribute to the tightly bond between electrocatalyst and the etched stainless‐steel mesh, as well as the surface reconstruction of catalyst during the cycle test. The facile electrodeposition method has lower requirements on production equipment, using stainless steel mesh as the basement reduces the production cost, the nickel–molybdenum alloy possesses excellent electrolytic water splitting performance and stability. These results indicate that this work can provide an idea for the industrialization of electrochemical water splitting. [ABSTRACT FROM AUTHOR]

Alkaline water electrolysis is a versatile technology for hydrogen production in situ. In this work, we study two different electrodeposited Ni-Mo alloys, with good electrocatalytic activity to hydrogen evolution reaction (HER). We propose Ni as substrate avoiding the corrosion and electrolyte contamination problem derived from eventual coating failure during long-term operations. The stable behavior of Ni as substrate as well as its known HER catalytic activity in alkaline electrolyzers make it attractive as a good option for our purpose. We prepare and characterize several Ni-Mo coatings over Ni by direct and pulse current supply electrodepositions. Ni-Mo coating prepared by pulse plating with a peak current density of 280 mA·cm−2 and 35% duty cycle exhibited the best electrocatalytic activity for HER. The coating is an amorphous alloy whose XRD peaks correspond to an f.c.c. Ni structure, with Mo substituent atoms and a rough cauliflower-like microstructure, with Mo content of 23 at.%. After 1000 cyclic voltammetry cycles, the electrocatalytic activity of the Ni-Mo electrocatalyst remains practically unchanged, and during 200 h of H2 evolution it maintains an overvoltage reduction of ca. 200 mV compared with bare Ni.ᅟ [ABSTRACT FROM AUTHOR]

Enthalpy release of electrodeposited nano-grained Ni and Ni-Mo alloys with grain size ranging from 29.3 to 5.9 nm and their corresponding annealed samples were investigated by using differential scanning calorimetry. Grain boundary energy of the annealed samples was estimated from enthalpy release and corresponding grain size. Results indicated that the enhanced grain boundary stability in the as-annealed ng Ni-Mo alloys is ascribed to kinetically grain boundary pinning by Mo instead of grain boundary energy reduction. [ABSTRACT FROM AUTHOR]

Variations in the crystallographic texture in Ni-25 at.%Mo alloys fabricated by selective laser melting with different scanning strategies were designed for the first time. Single-crystalline-like texture with a short-range order of Mo atoms can be produced via bidirectional scanning along one axis (X-scan) and bidirectional scanning with a 90° rotation in each layer (XY-scan), while only fiber texture was formed in bidirectional scanning with a 67° rotation (Rot-scan). The aligned crystal orientation along the build direction can be varied by the scanning strategy; 〈001〉 is preferred in the XY- and Rot-scan samples, while 〈101〉 is preferred in the X-scan sample. The controlling mechanisms of the texture, focusing on the preferential growth directions of the columnar cells and the following epitaxial growth, are discussed. [ABSTRACT FROM AUTHOR]

This work details the efficiency of Ni-Mo alloy as an electrode for water splitting application through electrodeposition method. Nano-crystalline Ni-Mo alloy coatings were deposited in the current density (c.d.) range of 1.0–4.0 A dm − 2 on a copper substrate, and were investigated for their deposit characters, and their electrocatalytic behaviours in 1.0 M KOH solution. The electrocatalytic behaviour of the coatings, in terms of their hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), were evaluated by electrochemical methods, like cyclic voltammetry (CV) and chronopotentiometry (CP). Experimental results revealed that Ni-Mo alloy electrodeposited at 1.0 A dm − 2 (38.3 wt% Mo) and 4.0 A dm − 2 (33.2 wt% Mo) shows the highest electrocatalytic tendency for HER and OER, respectively. The corrosion behaviour of Ni-Mo alloy coated at 4.0 A dm − 2 is found to be the most corrosion resistant in the same alkaline medium, compared to other coatings. The highest electrocatalytic activity of Ni-Mo alloy deposit for both HER and OER, depending on deposition c.d. was attributed to their composition (in terms of Ni and Mo content), structure and surface morphology; supported by EDXA, XRD, SEM and AFM analyses. The experimental study demonstrated that Ni-Mo alloy coatings follow Volmer-Tafel type of mechanism for HER, testified by Tafel slope analyses. [ABSTRACT FROM AUTHOR]

The corrosion properties of Ni-Mo alloy coatings with different molybdenum content (11–32 wt%) were investigated in the course of exposure to 0.5 M NaCl solution. Polarization resistance and impedance measurements showed that after contact with corrosive media, the formation of passive film on the coating surface, determines the corrosion resistance of Ni-Mo alloys. XPS analysis revealed that, the passive film growth was enhanced by the presence of higher amounts (26–28 wt%) of Mo in the coating. It was found, that after exposure to corrosive solution, Mo oxides occurred at the passive film/metal interface, while Ni hydroxide dominated in the bulk of the passive film. [ABSTRACT FROM AUTHOR]

The electrocatalytic activity of Ni-Mo alloy coatings for hydrogen evolution reaction (HER) was tried to increase by inducing the magnetic field ( B), perpendicular to the process of deposition. The electrocatalytic activity of Ni-Mo alloys were studied by cyclic voltammetry (CV) and chronopotentiometry (CP) measurements in 1.0 M KOH medium. Ni-Mo alloy coatings developed at c.d. = 1.0 A dm and B = 0.4 T was found to exhibit highest electrocatalytic activity for HER (with highest cathodic peak c.d. of −0.274 A cm and least onset potential of −1.24 V and highest volume of H liberated, 14.0 mL), demonstrated by CV and CP experiments. The stability factor of Ni-Mo alloy coatings were evaluated through corrosion study. The experimental results showed that Ni-Mo alloy, electrodeposited in the presence of B, is more corrosion resistant than its conventional alloy, when tested in the same alkaline medium. The increase in the electrocatalytic activity of Ni-Mo alloy coatings, developed under induced B, is attributed to the structural and morphological changes, caused by an increase of Ni content in the alloy, evident from X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. [ABSTRACT FROM AUTHOR]

In the study, the electrodeposition mechanism of Ni–Mo alloy was investigated. Potentiostatic deposition showed that the presence of MoO 4 2− could reduce the deposition overpotential of Ni, and the deposition current efficiency of Ni–Mo alloy was lower than pure Ni deposition as the potential moved negatively. Cyclic Voltammetry (CV) indicated that both the first reduction peak for Ni deposition and the deposition peak of Ni–Mo alloy moved to more positive potentials as the content of Ni 2+ increased, but the deposition peak of Ni–Mo alloy moved to more negative potentials as the content of MoO 4 2− increased. The cathodic polarization curve on Rotating Disc Electrode (RDE) presented that Ni–Mo codeposition was controlled by charge transfer. The morphology, composition and structure of Ni–Mo alloy were characterized by SEM, EDS and XRD. The results showed the Ni–Mo alloy coatings exhibited a spherical and cauliflower-like pattern, having a considerably rougher surface, with nano-crystal structure when the elements composition was Ni 80.14 Mo 19.59 . The electrochemical activity for hydrogen evolution of Ni–Mo alloy was studied in 30 wt.% KOH at 25 °C using steady-state polarization and electrochemical impedance spectroscopy (EIS) methods. The results clearly demonstrated that an increase in the electrochemical activity for hydrogen evolution of the Ni–Mo alloy coating can be attributed both higher exchange current density and larger real electrode area. [ABSTRACT FROM AUTHOR]

The paper presents several experimental results regarding the electrodeposition of Ni-Mo and Co-Mo alloy coatings from choline chloride based ionic liquids, applied onto Cu metallic substrates. Electrolytes containing Ni 2+ , Co 2+ and Mo 6+ salts in choline chloride – urea – citric acid ternary mixtures have been taken into consideration. The electrodeposited Ni-Mo and Co-Mo alloy coatings were adherent and uniform onto Cu metallic substrate. Ni-Mo alloys containing 2–35 wt. % Mo and Co-Mo alloys with 6–63 wt. % Mo have been obtained, depending on the metallic ions mass ratio and on the value of current density. As SEM analysis revealed, the operating parameters significantly influence the surface morphology of the obtained alloy coatings. According to XRD analysis, both electrochemical alloy coatings showed a nanocrystalline structure, having crystallites average sizes in the range of 3–7 nm. The alloy coatings corrosion performance has also been discussed. [ABSTRACT FROM AUTHOR]

Ni–Mo–ZrO 2 composite coatings were produced by electrodeposition technique from citrate electrolytes containing dispersed ZrO 2 nanopowder. The influence of deposition parameters i.e. concentration of molybdate and ZrO 2 nanoparticles in the electrolyte, bath pH and deposition current density on the composition and surface morphology of the coating has been investigated. The structure, microhardness and corrosion properties of Ni–Mo–ZrO 2 composites with different molybdenum and ZrO 2 content have been also examined. It was found that ZrO 2 content in the deposit is increased by rising the nanoparticles concentration in the plating solution up to 20 g dm −3 . An increase in molybdate concentration in the electrolyte affects negatively the amount of codeposited ZrO 2 nanoparticles. The correlation between the deposition current efficiency and ZrO 2 content in the composite coating has been also observed. A decrease in deposition current efficiency leads to deposition of Ni–Mo–ZrO 2 composite with low nanoparticles content. This may be explained by formation of higher amounts of gas bubbles on the cathode surface, which prevent the adsorption of ZrO 2 nanoparticles on the growing deposit. The XRD analysis revealed that all the studied Ni–Mo–ZrO 2 coatings were composed of a single, nanocrystalline phase with FCC structure. It was found that the incorporation of ZrO 2 nanoparticles into Ni–Mo alloy matrix affects positively the microhardness and also slightly improves the corrosion properties of Ni–Mo alloy coating. [ABSTRACT FROM AUTHOR]

• Ni-38Mo alloy has been fabricated via mechanical alloying + spark plasma sintering. • A dual phase structure comprising Ni and δ-NiMo intermetallic was obtained. • δ-NiMo intermetallic was associated with high mechanical properties. • Stability of δ-NiMo intermetallic was promoted by heat treatment at 1200 °C. The present work investigated a Ni-38Mo binary alloy with a specific interest in Ni solid solution and Ni-Mo intermetallic phases. To this end, a homogeneous Ni-Mo alloy powder obtained via mechanical alloying was consolidated by spark plasma sintering (SPS). The main constitutive phases post-SPS were a Ni(Mo) solid solution and the δ-NiMo intermetallic phase. Subsequent heat treatment at 1200 °C caused an increase in microhardness to 985 HV, while an additional aging process at lower temperatures caused a softening effect associated with the presence of β-Ni 4 Mo. Microstructural observation suggested that heat treatment at 1200 °C was responsible for the promotion of δ-NiMo in favor of mechanical properties and the stability of such intermetallic phase, which prevented massive phase transformation in subsequent heating. Thus, the present work may lay the groundwork for a multipurpose Ni-Mo alloy having attractive structural and functional properties. [ABSTRACT FROM AUTHOR]

Hydrogen evolution reaction (HER) from alkaline electrolytes is one of most promising methods for producing hydrogen. The remaining obstacles include the development of high performance and earth-abundant electrocatalysts which can be cost-effectively fabricated in large-scale. Ni-Mo based materials are one of potential candidates which might meet the most needs. In this review, we summarize the latest progress in Ni-Mo based HER catalysts in alkaline electrolytes from theoretical calculation to experimental results. This work also summarizes several different strategies to enhance the HER rate. Finally, the future perspective of the next generation electrocatalysts is discussed. [Display omitted] • Latest development of Ni-Mo based electrocatalysts for hydrogen production. • Theoretical calculations and experimental evaluations of various Ni-Mo alloys. • Effective strategies to improve the hydrogen evolution reaction performance. [ABSTRACT FROM AUTHOR]

Hydrogen energy is the most promising clean energy at present and hydrogen production by electrocatalytic water splitting is the frontier technology of alternative energy in the future. Herein, the Ni–Mo alloy supported with nickel foam (NF) was prepared by hydrogen reduction strategy at high temperature. The catalytic activity and stability of NiMo@NF-x catalyst (x stands for temperature) with multistage (sheet/particle) array structure were studied under different temperatures. The performance of the catalyst was evaluated by contact angle and related electrochemical characterization. It was found that the surface of NiMo@NF-550 catalyst prepared at 550 °C had superhydrophilic and superaerophobic properties, which contributed to the deactivation of bubbles from the surface of the electrode and improved the efficiency of the catalytic reaction. Moreover, the multi-stage array structure of sheet/particle will form a greater specific surface area than the plain sheet structure, offering more reaction sites. These results indicate that the self-supported superhydrophilic and superaerophobic Ni–Mo catalysts have excellent electrocatalytic hydrogen evolution ability. Hydrogen energy is the most promising clean energy at present and hydrogen production by electrocatalytic water splitting is the frontier technology of alternative energy in the future. Herein, the Ni–Mo alloy supported with nickel foam (NF) was prepared by hydrogen reduction strategy at high temperature. The catalytic activity and stability of NiMo@NF-x catalyst (x stands for temperature) with multistage (sheet/particle) array structure were studied under different temperatures. The performance of the catalyst was evaluated by contact angle and related electrochemical characterization. It was found that the surface of NiMo@NF-550 catalyst prepared at 550 °C had superhydrophilic and superaerophobic properties, which contributed to the deactivation of bubbles from the surface of the electrode and improved the efficiency of the catalytic reaction. Moreover, multistage array structure of sheet/particle facilitates the formation of a larger number of catalytic active centers. These results indicate that the self-supported superhydrophilic and superaerophobic Ni–Mo catalysts have excellent electrocatalytic hydrogen evolution ability. [Display omitted] • Efficient HER self-supporting electrocatalysts are prepared via a simple two-step method. • The prepared NiMo@NF catalytic electrodes exhibit excellent superhydrophilic and supergasophobic properties. [ABSTRACT FROM AUTHOR]

In this paper, NiMo coatings were electrochemically deposited on a copper electrode (Cu/NiMo) and on an electrodeposited nickel onto copper plate (Cu/Ni/NiMo) in citrate solutions. Effects of electrolyte composition, pH value, and temperature on hydrogen-evolution reaction (HER) as well as the electrochemical stability in alkaline solution were investigated, and the electrochemical activation energy was determined for the NiMo alloys. This was evaluated by the determination of kinetic and mechanism of HER in alkali medium by the polarization measurements, cyclic voltammetry, and electrochemical impedance spectroscopy techniques. The morphology and chemical composition of the electrodeposited Ni-Mo were investigated using SEM and EDS analyses. The results showed that the corresponding HER overpotential of the Ni-Mo film depends on alloy composition and surface morphology. As the wt% of Mo content in the alloy is increased, the onset potential of electrode for HER shifted in the positive direction favoring hydrogen generation with lower overpotential. The overall experimental data indicated that the porous Ni-Mo coating on electrodeposited nickel plate was obtained when the molybdenum content was ca. 41 wt%. This electrodes exhibited high catalytic activity in the HER ( η = −48 mV at 100 mA cm and 80 °C), and their stability was tested by polarization measurements after different anodic and cathodic treatments in 1 M NaOH solution. Moreover, the corrosion behaviors of Ni and Cu/Ni/NiMo electrodes at open-circuit potential were also investigated, and their corrosion resistances were compared. Graphical abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

In this study, Ni-Mo alloy coatings were electrochemically deposited on a copper plate in citrate solutions. The effects of Ni/Mo mole ratio in the electrolyte and pH value on hydrogen evolution reaction (HER) as well as the electrochemical stability were investigated in the alkaline solution for electrodeposited NiMo. The electrocatalytic activity of the fabricated NiMo alloys for HER in alkaline solutions was investigated by the polarization measurements and electrochemical impedance spectroscopy techniques. The morphology and chemical composition of the electrodeposited Ni-Mo were investigated using SEM and EDS analyses. It was found that NiMo electrode with the highest molybdenum content (ca. 38 wt.%) and high surface area show high electrocatalytic activity in the HER. This was produced from a bath with a pH of 9.5, Ni/Mo ratio of 1/10 and 0.5 M sodium citrate concentration. The stability of this coating was tested by polarization measurements after different anodic and cathodic treatment in 1 M NaOH solution. The open circuit potential ( E) of the electrode as a function of immersion time was also measured. [ABSTRACT FROM AUTHOR]

Here, we show for the first time a catalytic chemical vapor deposition (CCVD) synthesis of carbon nanotubes (CNTs) using polyoxomolybdate clusters Mo 12 O 28 (μ 2 -OH) 12 {Ni(H 2 O) 3 } 4 and Mo 12 O 28 (μ 2 -OH) 12 {Co(H 2 O) 3 } 4 as a source of catalyst nanoparticles. X-ray diffraction analyses indicated that the products of thermal decomposition of the clusters contain NiMoO 4 and CoMoO 4 phases, which are converted into Ni–Mo and Co–Mo alloys at 900 °C in hydrogen environment. High-resolution transmission electron microscopy in combination with energy-dispersive X-ray spectroscopy confirmed the CNT growth from bimetallic nanoparticles. Synergism between two metals in an alloy resulted in large-scale production of non-bundled few-walled CNTs with narrow diameter distribution and high quality. [ABSTRACT FROM AUTHOR]

Ni–Mo coatings with good mechanical properties were obtained by pulse plating. This study is focused on the current efficiency, molybdenum content and microhardness of the Ni–Mo coatings, which were affected by the pulse electroforming parameters. In addition, the wear resistance of the coatings was examined. The experimental results indicate that the Mo content in the PC-plated coatings increases upon increasing the frequency in the range 10–1000 Hz. The maximum Mo content (30.2 wt.%) in the coating occurred at a current density of 10 A/dm 2 , a duty cycle of 0.7, and a frequency of 1000 Hz. The XRD results indicate that the NiMo coatings prepared by pulse current are nanocrystalline and amorphous. The wear test results indicate that the wear resistance of the Ni–Mo coatings produced by pulse electroforming was higher than that produced by direct current plating. [ABSTRACT FROM AUTHOR]

Abstract: Enhancement of catalytic activity of Ni–Mo alloy for hydrogen evolution reaction (HER) has been observed when the alloy is electrodeposited under magnetic field, B control. Using cyclic voltammetry at low scan rate and electrochemical impedance spectroscopy (EIS) measurements, the significant modification of the activity of the alloy has been characterized in acidic and in basic water at 25 °C and in 33% NaOH solution at 85 °C. For the deposit obtained with B, the polarization curves show high current densities values (about 50% higher then without B), low ohmic drop and overpotential shift to noble way more then 100 mV. EIS measurements show a low polarization resistance and a high double layer capacitance for the alloy deposited under magnetic field control. In basic media, with or without B, the EIS measurements exhibit a supplementary inductive type relaxation. [Copyright &y& Elsevier]

Abstract: The amorphous Ni–Mo film with high HER (hydrogen evolution reaction) activity was obtained by pulse plating. The optimum electrodeposition conditions with respect to HER overpotential were determined, e.g. Na2MoO4·2H2O concentration, current density and duty cycle. Correspondingly, the compositions and components of the Ni–Mo coatings with various molybdenum contents were investigated systematically. The results showed that when the ratio of nickel and molybdenum concentrations in the electrodeposition bath is lower than 1 (mol%), the molybdenum content in the coating decreases with the increasing Na2MoO4·2H2O concentration, while the corresponding HER overpotential of the Ni–Mo film increases. The amorphous Ni–Mo coating was obtained when the molybdenum content was c.a. 30 mass%, which shows high HER activity (η 200 =62mV at 200mAcm−2 and 80°C) and excellent corrosion resistance. After galvano-static electrolysis for 100h in 33 mass% NaOH solution, the amorphous structure was destroyed due to the dissolution of molybdenum. [Copyright &y& Elsevier]

Abstract: The hydrogen evolution reaction on the electrodeposited Ni–Mo alloy coatings, as well as their electrochemical properties in the NaOH solutions, have been investigated by the polarization measurements, cyclic voltammetry and EIS technique. It was shown that the Ni–Mo alloy coatings electrodeposited from the pyrophosphate-sodium bicarbonate bath possess high catalytic activity for hydrogen evolution in the NaOH solutions. Their stability in the 1M NaOH at 25°C under the condition of the reverse polarization was shown to be very good, while in the 33% NaOH at 85°C (conditions of the industrial electrolysis) the electrodeposited Ni–Mo alloy coatings exhibited also high catalytic activity, but low stability, as a consequence of a deterioration of the alloy coatings. [Copyright &y& Elsevier]

Abstract: Electrochemical Ni–Mo alloying of the surface of a nickel substrate was investigated using alternating pulsed electrolysis in an aqueous solution containing only molybdate ions (MoO4 2−) as a metal ion component. In this electrochemical process, the nickel substrate was slightly dissolved during the anodic pulses, providing nickel ions into the solution in the vicinity of the substrate, while Ni and Mo were both electrodeposited on the substrate surface during the subsequent cathodic pulses. Through the optimization of anodic and cathodic conditions independently based on a set of direct-current electrolysis data, amorphous Ni–Mo alloy layers were found to be formed at the surface of the nickel substrate by the alternating pulsed electrolysis using the MoO4 2− solution of pH 3.0–5.0. The conditions for Ni–Mo alloy formation were discussed in terms of the dissolving regime of ionic species in the electrolytes determined by an equilibrium calculation. [Copyright &y& Elsevier]

The ordering behaviour of a Ni-Mo alloy in the presence of a ternary additive, viz. Cr, has been studied. The sequence of ordering transformations in binary Ni–Mo alloys has been shown earlier to be controlled by a competition between several f.c.c.-based superlattices, viz. Ni2Mo (Pt2Mo type), Ni3Mo (DO22), Ni4Mo (D1a) and the so-called short range ordered (SRO) structure characterized by the presence of diffraction intensity maxima at the 11/20f.c.c. positions in reciprocal space. The effect of ternary addition of chromium in the selection of the superlattice structure has been examined in this paper in an alloy of composition Ni–24 at.% Mo–6 at.% Cr. The presence of Cr has been experimentally found to favour the formation of Ni2(Mo,Cr) (Pt2Mo-type) phase in preference to the Ni3Mo (DO22) and Ni4Mo (D1a) superlattices. This leads to a sequence of transformation different from that obtained in binary alloys. The effect of Cr on the ground state phase stability is determined, in this alloy, using the first-principles local-density based full-potential augmented plane wave (FP-LAPW) method. From the differences in the electronic structures, densities of states and total energies of binary Ni2Cr, Ni2Mo (Pt2Mo type), Ni3Mo (DO22) and Ni4Mo (D1a) phases and the corresponding ternary superlattice structures, an attempt has been made to predict the hierarchy of relative phase stabilities. Theoretical predictions based on these electronic structure calculations have been found to be consistent with the experimental microstructural observations of the evolutionary stages of ordering in the ternary Ni–24 at.% Mo–6 at.% Cr alloy. [ABSTRACT FROM AUTHOR]

• Thermo-mechanical-related failure of Ni-Mo alloy is examined. • The failure is correlated with improper cooling practice of hot-rolled slabs. • Embrittlement by disorder-to-order transition is determined to be the cause of failure. • The embrittlement mechanism is correlated with grain boundary ordering. • The embrittlement problem can be mitigated by microalloying with B orY. During thermo-mechanical processing of a Ni-Mo alloy, three hot-rolled slabs at 1250 °C have been stacked on top of each other during air-cooling. Upon reaching ambient temperature, the middle slab is found to develop a mid-length transverse crack running through the entire thickness. However, no cracks have been detected in the other slabs. It is shown that the middle slab has been embrittled by ordering to Ni 4 Mo due to comparative slower cooling rate. An investigation of the ordering behavior of stoichiometric Ni 4 Mo alloy has shown that ordering occurs homogeneously during the earlier stages of thermal aging at elevated temperatures which results in nanosized structure of ordered Ni 4 Mo domains capable of deforming by twinning and characterized by high strength and high ductility However, due to self-induced recrystallization by grain boundary migration during extended thermal exposure, the homogeneous nanosized structure is replaced by coarse platelets in the matrix and coarse lamellar structure at grain boundaries resulting in severe embrittlement. It is demonstrated that this problem can be combated by doping with either boron or yttrium which tend to segregate at grain boundaries and reduce their tendency toward migration and therefore self-induced recrystallization. Therefore, the desirable nanosized morphology of Ni 4 Mo is restored in the matrix phase and the lamellar grain boundary morphology is replaced by nanosized layers of Ni-Mo solid-solutions with compositions not susceptible to Ni 4 Mo ordering. As a result, the doped alloys become able to maintain high strength and high ductility after extended thermal aging at elevated temperatures. [ABSTRACT FROM AUTHOR]