59Co NMR study in Co–Fe alloys/Co magnetite composites

Iron–cobalt compounds containing 20–30 at.% Fe with unusual crystallographic structure have been observed in metal-oxide composite synthesized in an aqueous media at 120–140°C. The oxide is a cobalt-containing magnetite. The metallic component is found in two crystallographic structures depending on the preparation conditions: a b.c.c. (α-Fe) structure and an α-Mn, also called Re₂₄Ti₅, structure (space group I-43m). The α-Mn-like CoFe alloy is encountered for the first time and transforms into the b.c.c. structure during annealing at temperatures as low as 160°C. In the b.c.c. structure itself, the cobalt and iron atoms present a chemical short-range order totally different from the regular B2 phase of the ordered bulk CoFe compound. This particular order is similar to the one encountered in high vacuum co-deposited thin films. For annealing temperatures larger than 400°C, the metallic component loses some Fe and transforms into nearly pure f.c.c. cobalt. This study shows that chemical as well as physical low temperature preparation techniques favor original CoFe ordered phases not achievable by conventional metallurgy.     

Effect of electrolyte and agitation on the anomalous behavior and morphology of electrodeposited Co–Ni alloys

This study continues previous work which showed that the anomalous behavior of Co–Ni deposition could be alleviated or eliminated through use of cyclic voltammetry (CV) or pulse reverse (PR) plating. The research focuses on aspects not considered in this previous work: the effects of the anion and agitation in the plating bath. A comparison is made of Co–Ni electrodeposition using the CV and PR techniques in sulfate and chloride baths at pH 3 containing equimolar Co(II) and Ni(II) concentrations under both stirred and unstirred conditions. The anomalous behavior can be significantly suppressed and even eliminated with current efficiencies above 90 % through use of PR plating, in particular, but only if carried out in a chloride solution under quiescent conditions. Both metal ion reduction during the cathodic portion and oxidation of the coating during anodic polarization are accelerated in the chloride solution relative to that in the sulfate solution. Electrolyte agitation exacerbates anomalous deposition and reduces the current efficiency by enhancing mass transport of Co(II) and H⁺ to and from the electrode. The origin of anomalous deposition and effects of the chloride ion are examined in terms of coordination chemistry and ligand field theory. This analysis suggests that oxidation of the Co–Ni coating in the chloride solution during anodic polarization of the PR and CV cycles when cobalt preferentially dissolves is crucial to suppressing the anomalous behavior. Examination of the coatings shows that the anion type, degree of agitation of the electrolyte, and electroplating technique significantly affects their microstructure and roughness.     

Effect of nano-Al2O3 particles and of the Co concentration on the corrosion behavior of electrodeposited Ni–Co alloys

Incorporation of nano-Al₂O₃ particles into a Ni–Co alloy by electrodeposition influences the corrosion properties, morphology, and structure of the layers. The resistance against corrosion of Ni–Co/Al₂O₃ composite films deposited on stainless steel was investigated in a 0.1-M NaCl solution by potentiodynamic polarization. The presence of nanoparticles improves the corrosion resistance of Ni–Co/nano-Al₂O₃ deposits when compared to pure Ni–Co alloy. Moreover, by increasing the pH of the electrodeposition bath and the content of Co in the alloy, the resistance against corrosion is furthermore improved. The morphology of the deposits before and after their corrosion was analyzed by scanning electron microscopy. The presence of the embedded alumina particles in the Ni–Co alloys was one of the key factors that limited further propagation of corrosion on the metallic surface. Preferential corrosion attack, in the form of a pitting corrosion, was located mainly at the grain boundaries.     

Spontaneous passivity of amorphous bulk Ni–Cr–Ta–Mo–Nb–P alloys in concentrated hydrochloric acids

Rod-shaped amorphous bulk Ni–Cr–Mo-22 at.%Ta-14 at.%Nb–P alloys resistant to concentrated hydrochloric acids were prepared by copper-mold casting. Alloys of amorphous single phase and mixture of nanocrystalline phases in the amorphous matrix were all spontaneously passive in 6 and 12 M HCl and were immune to corrosion in 6 M HCl, although the corrosion weight loss was detected for heterogeneous alloys in 12 M HCl. Spontaneous passivation is due to presence of stable air-formed films in which chromium was particularly concentrated in addition to enrichment of tantalum and niobium. The angle resolved X-ray photoelectron spectroscopy revealed that chromium and molybdenum are rich in the inner part of the film. The major molybdenum species is in the tetravalent state, although penta- and hexavalent state molybdenum is also included. The high corrosion resistance was interpreted in terms of the high stability of the outer triple oxyhydroxide, Cr_1−x−yTa_xNb_yO_z(OH)_3+2x+2y−2z, and the effective diffusion barrier of the inner Mo⁴⁺ and Cr³⁺ oxide layer. Contribution to the Fall Meeting of the European Materials Research Society, Symposium D: 9th International Symposium on Electrochemical/Chemical Reactivity of Metastable Materials, Warsaw, 17th-21st September, 2007.     

Ammonia oxidation on electrodeposited Pt–Ir alloys

Ammonia electro-oxidation on Pt–Ir alloys has been studied applying cyclic voltammetry and differential electrochemical mass spectrometry (DEMS), and the results were compared with pure Pt. Bimetallic alloys were prepared by electrodeposition and characterized using X-ray diffractometry (XRD) and Auger spectroscopy, before and after oxidation of ammonia. Pt/Ir atomic composition was 70:30 obtained from 1:1 solutions. Substitution alloys were established where Ir atoms replace Pt positions in the face-centered cubic structure. Preferential crystal orientations were detected in the electrodeposits with the development of a crystallographic texture. DEMS showed that N₂ is the main product during ammonia oxidation for both Pt and Pt–Ir, but the formation of nitrogen oxides is observed for E > 0.8 V_RHE. The yield of N₂ is higher for the alloy, which also displays lower poisoning of the surface when increasing ammonia concentration. These results confirm Pt–Ir alloys as alternatives to Pt electrodes concerning ammonia oxidation. Finally, it was observed that XRD patterns, as well as texture coefficient values, change after using the electrodeposits for ammonia oxidation, with the less compact planes the more affected ones. Dedicated to Prof. Dr. Teresa Iwasita on the occasion of her 65th birthday in recognition of her numerous contributions to interfacial electrochemistry.     

Magnetic properties of electrodeposited amorphous nickel–phosphorus alloys

The influence of the sodium hypophosphite concentration in a sulfate–citrate electrolyte of nickel plating on the phosphorus content in electrodeposited amorphous Ni–P alloys was analyzed. The effect of the thermal treatment on the magnetic properties (coercive force and specific magnetization) of the obtained Ni–P alloys was considered; the Curie temperatures of the alloys were determined.     

Production of alloys in the Fe–Co system with an oxygen concentration below 10 ppm

A fundamentally new solution of the challenge of producing alloys in the Fe–Co system with an oxygen concentration below 10 ppm (10^–3%) has been for the first time justified and demonstrated. A thermodynamic analysis showed that decreasing the pressure of the gas phase over the melt significantly increases the deoxidizing power of carbon. At cobalt and carbon contents characteristic of soft- and hard-magnetic alloys and a total pressure of 0.01 atm, the oxygen concentration was 10–1 ppm (10^–3–10^–4%). With increasing cobalt content of the melt, the deoxidizing power of carbon increases. The curves of the oxygen solubility in carbon-containing iron–cobalt melts pass through a minimum, which shifts toward lower carbon contents with increasing cobalt content of the melt.     

Magnetic properties of electrodeposited Ni‒P alloys with varying phosphorus content

The effect thermal treatment has on the magnetic properties (magnetization, saturation magnetization, and coercivity) of Ni?P alloys prepared via electrodeposition is studied. The process of amorphous Ni?P alloys devitrification is investigated by differential scanning calorimetry. The effects of chemical composition and thermal treatment on magnetic properties of the alloys are revealed.     

Enthalpies of formation of SmCo alloys in the composition range 10–22 at. % Sm

Several intermetallic compounds exist in the composition range 10–22 at.% Sm(Sm₂Co₁₇, SmCo₅, Sm₂Co₇) but their preparation as single-phase specimens is very difficult. In order to determine the enthalpies of formation of these compounds, measurements were carried out on four alloys containing respectively 12.9 at.% Sm, 16.4 at.% Sm, 17 at.% Sm and 19.8 at.% Sm, annealed in the temperature range 950–1100 °C. The compositions of the phases present in each specimen were deduced from the characterization of the measured alloys by scanning electron microscopy, electron microanalysis and X-ray diffraction.The heats of formation were deduced from solution calorimetry in molten tin. The variation of the experimental results as a function of the samarium content enabled the enthalpy of formation of SmCo₅ ( − 40.8 kJ mol⁻¹) to be determined. The same ΔH_f value as determined for the phase quenched from 950 °C was measured for SmCo₅ kept at room temperature after very slow cooling. This result did not confirm the eutectoid decomposition previously reported for SmCo₅.The extrapolation of the measured values for the higher and lower samarium contents leads to the evaluation of the enthalpies of formation of Sm₂Co₁₇ (−152 kJ mol⁻¹) and Sm₂Co₇ (−99kJ mol⁻¹).     

Effect of TMAB concentration on structural,mechanical and corrosion properties of electrodeposited Ni–B alloys

The influence of trimethylamine borane (TMAB) concentration as a boron (B) source on the structural and corrosive properties of Ni–B alloy coatings produced by electrodeposition was investigated. The crystal structure of the Ni–B coatings is influenced by the B content in the coating, having a slight (220) preferred orientation. The B content in the coating increased from 16 to 34 at.% with the corresponding increase in the concentration of TMAB from 1 to 20 g/L, while interestingly retaining a crystal structure. The hardness of the coatings increased with increasing B content owing to the formation of smaller crystallites. An increase in B content in the alloy coatings, led to a shift in the E_corr values to more anodic potentials, indicating increase corrosion protection for the Ni–B coatings. This study achieved to reach 34 at.% B content in Ni–B alloy coatings produced by electrodeposition while preventing amorphization of the coating layer.     

Thermodynamics of oxygen solutions in titanium-containing Fe–Co melts

Performed for the first time, the thermodynamic analysis of oxygen solutions in titanium-containing Fe–Co melts showed that the deoxidizing power of titanium with increasing cobalt content of the melt first decreases, reaches a minimum at a cobalt content of 20%, and then increases. The titanium contents [%Ti]* at equilibrium points between the oxide phases TiO₂, Ti₃O₅, and Ti₂O₃ were determined. The curves of the oxygen solubility in titanium-containing iron–cobalt melts pass through a minimum, which shifts toward lower titanium contents with increasing cobalt content of the melt. Further alloying with titanium leads to an increase in the oxygen concentration of the melt so that the higher cobalt content of the melt, the steeper the increase in the oxygen content after the minimum as titanium is added to the melt.     

Study of martensite transformation and microstructural evolution of Cu–Al–Ni–Fe shape memory alloys

In this study, variations in the transformation temperature, crystal structure, and microstructure of the arc melted alloy having nominal composition of Cu–13%Al–4%Ni–4%Fe (in mass%) were investigated for two different treatment conditions, homogenized and heat treated at 950 °C for 1 h. For both conditions, transformation temperature of the alloy was examined by DSC and it was determined as ~200 °C, similar to the value for Cu–Al–Ni alloys given in the literature. The crystal structure of the martensite Cu–13%Al–4%Ni–4%Fe (in mass%) alloy was identified as 18R using XRD. By heat treatment performed at 950 °C, diffraction peaks become more distinct. The microstructure of the alloy was studied with the help of optical microscope as a result of which parallel martensite plates and precipitates were detected. Microhardness value of the alloy was found as 361 and 375 Hv for homogenized and heat-treated conditions, respectively.     

Formation of short-range order in Al–Ge–Fe melts: influence of composition

ABSTRACT

The short-range order in Al–Ge–Fe melts has been studied by X-ray diffraction and reverse Monte Carlo simulations in wide concentration range. Influence of the replacement of one component by another while the content of third component is constant on the formation of a local structure of ternary melts has been discussed. It has been shown that at Ge content less than 30 аt. % Ge atoms are uniformly distributed in the volume of the Al–Ge–Fe melts and atomic clusters with structure similar to the liquid germanium are formed at content more than 30 аt. % Ge. The addition of the third component (Ge or Al) to the binary Al–Fe or Fe–Ge melts, correspondingly, results in competition between Al and Ge atoms in formation of the local structure around Fe atoms. The obtained concentration dependences of the nearest neighbour distances point out that the ternary interactions take place in the Al–Fe–Ge melts.     

Influence of Fe concentration on the properties of the electrodeposited Zn–Fe coatings

Electrodeposition of Zn–Fe alloys on a copper substrate from a sulfate bath with different Fe²⁺ concentration (0.05, 0.10 and 0.20 mol L^?1) at room temperature was investigated using cyclic voltammetry. The influence of the Fe²⁺ content in the plating bath on the surface morphology, structural and magnetic properties of the coatings were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Vibrating Sample Magnetometer (VSM). The results show that the morphology of Zn–Fe films changes with different Fe²⁺ concentration. The EDS analysis demonstrated that the Fe content of the coatings increased with increasing the Fe concentration in the bath. XRD measurements shows the presence of ?Zn (hcp), δ1ZnFe (hcp) and the ΓZnFe (bcc) phases with a (101) preferential orientation in all the electrodeposited films. The magnetic analysis of Zn–Fe films indicated that the saturation magnetization was largely enhanced in comparison to pure Zn, especially with 0.2 at. % Fe, while the coercivity decreased.     

The electrochemistry of nanostructured Ni–W alloys

This work reports on the features that Ni–W nanostructured alloys, electrodeposited on carbon steel by different current pulse programs, may present depending on their surface morphology and surface composition. The Ni–W nanostructured coating, with a cauliflower structure, lack of fragility, and high WO₃/W surface composition ratio, is a stable electrode to catalyze hydrogen evolution reaction, exceeding bulk and electrodeposited Ni catalytic activity. Also, the nanostructured alloys must have a low WO₃/W surface composition ratio for Ni and its oxides to provide protection and improve corrosion resistance in sulfate media.     

Electrochemical and microgravimetric characterization of magnetron-sputtered Fe–Cr–Ni–Ta and Fe–Cr–Ni alloy films in neutral and strongly acidic media

The Fe–Cr–Ni and Fe–Cr–Ni–Ta alloy films were deposited on quartz substrates by magnetron-sputtering using targets of AISI 316 stainless steel and in combination with pure tantalum. The conventional melting of the Fe–Cr–Ni–Ta alloy formed is virtually impossible because the melting point of tantalum is higher than the boiling points of the other components. Elemental content of the films was determined by XPS analysis. Corrosion behaviour of both alloy films was studied in 5% NaCl and 10 M HCl by electrochemical quartz crystal microgravimetry (EQCM), electrochemical impedance spectroscopy (EIS) and dc-voltammetry. The corrosion resistance of Fe–Cr–Ni–Ta appeared to be significantly higher than that of Fe–Cr–Ni in both neutral (5% NaCl) and strongly acidic (10 M HCl) media. The Fe–Cr–Ni–Ta specimen exhibited an extremely high corrosion resistance in 10 M HCl, where the corrosion rates were about one order of magnitude lower than those of Fe–Cr–Ni in neutral solution. EQCM measurements in NaCl solution indicated accumulation of corrosion products on the Fe–Cr–Ni–Ta surface, which was evident from a distinctive increase in electrode mass. By contrast, the mass of the tantalum-free alloy film decreased with a constant rate, which indicated alloy dissolution to prevail. The corrosion current calculated from the mass decrease was in good agreement with that derived from voltammetric measurements. The EQCM data showed that the corrosion resistance of the Fe–Cr–Ni–Ta alloy film in 10 M HCl was about two orders of magnitude higher than that of the Fe–Cr–Ni.     

Thermal stability of metastable nano-composites in planar flow cast Ti–Zr–Ni alloys

The influence of the solidification rate, the thermal stability, and devitrification process of the rapidly solidified Ti₄₅Zr₃₈Ni₁₇ alloy have been examined on ribbons prepared by the planar flow casting method. Differential scanning calorimetry in the continuous heating mode, X-ray diffraction, and electron microscopy techniques were applied. Comparison of the so-obtained microstructures revealed the competition between icosahedral quasi-crystalline and β-Ti(Zr) phases, both dispersed in an amorphous matrix. It has been found that the decomposition process of rapidly quenched ribbons consists of the sequence of several independent exothermic and endothermic reactions involving the additional precipitation of quasi-crystalline nanoparticles and both irreversible and reversible changes between the unstable high-temperature β and stable low-temperature α phases. The formation of the intermetallics as well as the transformation of quasi-crystals into Laves phase has been observed at higher temperatures in all ribbons.