Characterization and frictional behavior of nanostructured Ni–W–MoS2 composite coatings

Ni–W–MoS₂ composite coatings were obtained by pulse plating from a Ni–W electrolyte containing suspended MoS₂ particles. The coating composition, morphology, crystalline structure, microhardness and frictional behavior were studied as a function of MoS₂ concentration. The results obtained in this study indicate that co-deposited lubricant particles strongly influenced the composite Ni–W coating properties. It was found that increasing co-deposited MoS₂ diminished tungsten content in the coating and consequently increased the average grain size. Ni–W nanostructured coatings with high MoS₂ content have a porous sponge-like structure, high surface roughness and irregular frictional behavior. However, the friction coefficient of Ni–W coatings is reduced to half its value with low MoS₂ content.     

High-temperature corrosion of Cr2O3-forming alloys in CO-CO2-N2 atmospheres

The corrosion of Fe–28Cr, Ni–28Cr, Co–28Cr, and pure chromium in a number of gas atmospheres made up of CO–CO₂(–N₂) was studied at 900°C. In addition, chromium was reacted with H₂–H₂O–N₂, and Fe–28Cr was reacted with pure oxygen at 1 atm. Exposure of pure chromium to H₂–H₂O–N₂ produced a single-phase of Cr₂O₃. In a CO–CO₂ mixture, a sublayer consisting of Cr₂O₃ and Cr₇C₃ was formed underneath an external Cr₂O₃ layer. Adding nitrogen to the CO–CO₂ mixture resulted in the formation of an additional single-phase layer of Cr₂N next to the metal substrate. Oxidizing the binary alloys in CO–CO₂–N₂ resulted in a single Cr₂O₃ scale on Fe–28Cr and Ni–28Cr, while oxide precipitation occurred below the outer-oxide scale on Co–28Cr, which is ascribed to the slow alloy interdiffusion and possibily high oxygen solubility of Co–Cr alloys. Oxide growth followed the parabolic law, and the rate constant was virtually independent of oxygen partial pressure for Fe–28Cr, but varied between the different materials, decreasing in the order chromium >Fe–28Cr>Ni(Co)–28Cr. The formation of an inner corrosion zone on chromium caused a reduction in external-oxide growth rate. Permeation of carbon and nitrogen through Cr₂O₃ is thought to be due to molecular diffusion, and it is concluded that the nature of the atmosphere affects the permeability of the oxide.     

Nanocrystalline spherical iron–nickel (Fe–Ni) alloy particles prepared by ultrasonic spray pyrolysis and hydrogen reduction (USP-HR)

FeCl₂ and NiCl₂ were used for synthesis of nanocrystalline spherical Fe–Ni alloy particles by ultrasonic spray pyrolysis and hydrogen reduction (USP-HR). Spherical ultrafine Fe–Ni particles were obtained by USP of aqueous solutions of iron–nickel chloride followed by thermal decomposition of generated aerosols in hydrogen atmosphere. Particle sizes of the produced Fe–Ni particles can be controlled by the change of the concentration of an initial solution. The effect of the precursor solution in the range of 0.05, 0.1, 0.2 and 0.4 M on the morphology and crystallite size of the Fe–Ni alloy particles are investigated under the conditions of 1.5 h running time, 900 °C reduction temperature, and 1.0 L/min H₂ volumetric flow rate. X-ray diffraction (XRD) studies and Scherrer crystallite size calculations show that the crystalline size was nearly 28 nm. Energy dispersive spectroscopy (EDS) was performed to determine the chemical composition of the particles. Transmission electron microscope (TEM) was used to confirm the crystalline size, that was determined using XRD results. Scanning electron microscopy (SEM) observations reveal that the precursor solution strongly influences the particle size of the synthesized Fe–Ni alloy particles. Spherical nanocrystalline Fe–Ni alloy particles in the range of 80 and 878 nm were obtained at 900 °C.     

Electrochemical hydrogen storage of NdMg12–Ni composites modified with carbon nanotubes and BN particles

In this work, the electrochemical performance of NdMg₁₂–Ni composite electrode in alkaline solution and the effect of the surface modification with carbon nanotubes (CNTs) and boron nitride (BN) particles on the NdMg₁₂–Ni composite were investigated. The NdMg₁₂ alloy was synthesized by a salt-cover-melting and a subsequent quenching process. The NdMg₁₂–Ni–BN and NdMg₁₂–Ni–CNTs composites were prepared by ball-milling NdMg₁₂ alloy, Ni powders and CNTs or BN particles. It is found that CNTs or BN particles are mainly attached onto the surface of the NdMg₁₂–Ni composite after the ball-milling process. The electrochemical experiment results indicate that the NdMg₁₂–Ni composites modified with CNTs or BN particles have the improved electrochemical performance. In particular, the NdMg₁₂–Ni–5 wt.% CNTs and NdMg₁₂–Ni–3 wt.% BN composites have the higher initial discharge capacity of 416.6 mAh/g and 442.9 mAh/g, respectively, larger than the original NdMg₁₂–Ni composite. The large amount of grain boundaries and crystalline defects, induced during the ball-milling process, can accelerate the bulk hydrogen diffusion and provide more surface active sites for the electrochemical reaction of the composites. However, the cycle stability of the composites modified by CNTs or BN particles is still not satisfactory for the practical application.     

Pressure–composition isotherms in the Mg2Ni–H2 system

A relatively pure Mg₂Ni intermetallic compound was prepared by partial melting and sintering. Absorption and desorption pressure–composition isotherms for the Mg₂Ni–H₂ system were obtained. The relationships between the equilibrium plateau pressure (P_eq) and the temperature were

and

The procedure to obtain the pressure–composition isotherms was explained and a method to calculate the composition for pressure–composition isotherms (“the summation method”) was also suggested.     

Hot corrosion of a novel (Ni,Co)O/(Ni,Co)Fe2O4 composite coating thermally converted from an electrodeposited Ni–Co–Fe2O3 composite coating

Ni–Co–Fe₂O₃ composite coatings were successfully developed by sediment co-deposition. In order to improve their hot corrosion resistance, a pre-oxidation treatment was conducted at 1000 °C for 6 h. The corrosion behaviour of the oxidised composite coating was investigated at 960 °C in an atmosphere consisting of a mixture of Na₃AlF₆–AlF₃–CaF molten salts and air. They exhibited good hot corrosion resistance due to not only the pre-formed oxide scale with (Ni,Co)O and (Ni,Co)Fe₂O₄ phases after pre-oxidation, but also the formation of (Ni,Co,Fe)Al₂O₄ phases in the outer layer and a well-distributed NiFe₂O₄-enriched phase along the grain boundaries in the subscale area during the corrosion process.     

Metal Dusting of Fe–Cr and Fe–Ni–Cr Alloys Under Cyclic Conditions

Metal dusting is the disintegration of alloys into carbon and metal particles during high-temperature exposure to carbon-bearing gases. Model Fe–Cr and Fe–Ni–Cr alloys were studied to test the hypothesis that M₃C formation is necessary for metal dusting to occur. The alloys were exposed to a 68% CO–26% H₂–6% H₂O gas mixture at 680°C (a_c=2.9) under thermal cycling conditions. Equilibrium calculations predicted the formation of M₃C at the surface of Fe–25Cr, but not Fe–60Cr. All compositions were expressed in w/o, weight percent. Alloys of Fe–25Cr with 2.5, 5, 10, and 25 w/o nickel additions were also exposed to the same conditions to study the role of nickel in destabilizing the precipitation of M₃C and, hence, altering the resistance to metal dusting. Metal dusting was observed on all the alloys except Fe–60Cr. For Fe–25Cr, Fe–25Cr–2.5Ni, and Fe–25Cr–5Ni, the carbonization and dusting process was localized, and its incidence decreased in Fe–25Cr–2.5Ni, consistent with the increased destabilization of M₃C precipitation. However, Fe–25Cr–10Ni and Fe–25Cr–25Ni both underwent extensive dusting in the absence of protective Cr₂O₃ formation. The carbon deposits formed consisted of carbon filaments, which contained particles at their tips. These were shown by electron diffraction to be exclusively Fe₃C in Fe–25Cr, Fe–25Cr–2.5Ni, and Fe–25Cr–5Ni, and a mixture of austenite and (Fe,Ni)₃C in Fe–25Cr–10Ni and Fe–25Cr–25Ni.     

The kinetics of growth and the critical conditions for the formation of the most-stable oxide in the oxidation of binary alloys

During the growth of the most-stable oxide BO _v in the oxidation of binary alloys containing nonnoble components A and B, the oxygen pressure prevailing at the alloy-scale interface is higher than the corresponding value for equilibrium between BO _v and pure B. The effects of this change on the rate constant for the growth of BO _v and on the critical concentration of B in the bulk alloy required for the stability of BO _v on its surface are examined and discussed. The general treatment is then applied to the growth of NiO on Cu–Ni alloys and of Cr₂O₃ on Fe–Cr, Co–Cr, and Ni–Cr alloys by using appropriate defect models for these oxides, considering also the possibility of oxide doping.     

Preparation and microwave absorption properties of Ni–B alloy-coated Fe3O4 particles

The Fe₃O₄ particles that were activated with Pd-Sn catalytic nuclei by single-step activation were used as cores for obtaining nickel- and boron-decorated particles with shell/core structure via electroless plating. The materials were characterized by TEM, SEM, EDX, X-ray diffraction, VSM and microwave network analyzer. The results showed that Fe₃O₄ particles coated with Ni–B alloy films had reduced metal deposition in the reaction solution. There was no peak of amorphous alloy Ni–B by XRD but the inverse spinal Fe₃O₄. The calculated results indicate that the Fe₃O₄ densely coated with Ni–B exhibits better absorption performance than the pure Fe₃O₄ in the range of 2–18 GHz, such as more powerful absorbing capability and a wider electromagnetic wave absorbing frequency band.     

Properties of mechanically alloyed P/M composites of Al–Mg2Si–oxide systems

With a purpose of obtaining light-weight materials of high strength, mixture of aluminum powder and Mg₂Si powder at the composition of Al–20mass%Mg₂Si₂ was mechanically alloyed with addition of oxide (Cr₂O₃, Fe₂O₃, MnO₂) powders. Mechanical alloying was conducted by using an Attritor-type ball mill under argon atmosphere. The mechanically alloyed powders were consolidated to the P/M materials by vacuum hot pressing and hot extrusion. Solid-state reactions during mechanical alloying and subsequent thermomechanical processing were studied. Their structures and mechanical properties were examined and compared with hyper-eutectic Al–Si based P/M materials. All added oxides were decomposed and aluminide compounds were formed during heating of the extruded P/M materials. Mg₂Si was only partially decomposed after heating P/M materials. All the P/M materials of Al–Mg₂Si–oxide showed high compressive strength above 900 MPa. Among them, the highest strength of 1090 MPa was obtained for Al–Mg₂Si–Fe₂O₃. Even the P/M material of Al–Mg₂Si without oxide addition showed compressive strength of 795 MPa. The Al–Mg₂Si based P/M materials showed higher compressive strength and higher ductility than hyper-eutectic Al–Si based P/M materials.     

Microstructure and martensitic transformation of Ni–Fe–Ga–Si ferromagnetic shape memory alloys

The effects of the fourth element Si on the martensitic transformation and magnetic properties of Ni–Fe–Ga magnetic shape memory alloys were investigated. A complete thermoelastic martensitic transformation in Ni–Fe–Ga–Si alloys was observed in the temperature range of 218–285 K. The martensitic transformation temperatures of Ni–Fe–Ga alloys are obviously decreased by the substitution of Si for Ga element, that is, the substitution of 1 at.% Si for Ga leads to a decrease of martensitic transformation temperature of about 39.6 K. Moreover, the substitution of Si for Ga leads to a decrease of the saturation magnetic field and the magnetic anisotropy constant K₁ obviously.     

Strong bonding of infrared brazed α2-Ti3Al and Ti–6Al–4V using Ti–Cu–Ni fillers

Infrared dissimilar brazing of α₂-Ti₃Al and Ti–6Al–4V using Ti–15Cu–25Ni and Ti–15Cu–15Ni filler metals has been performed in this study. The brazed joint consists primarily of Ti-rich and Ti₂Ni phases, and there is no interfacial phase among the braze alloy, α₂-Ti₃Al and Ti–6Al–4V substrates. The existence of the Ti₂Ni intermetallic compound is detrimental to the bonding strength of the joint. The amount of Ti₂Ni decreases with increasing brazing temperature and/or time due to the depletion of Ni content from the braze alloy into the Ti–6Al–4V substrate during brazing. The shear strength of the brazed joint free of the blocky Ti₂Ni phase is comparable with that of the α₂-Ti₃Al substrate, and strong bonding can thus be obtained.     

Combustion synthesis of TiC–TiB2 composites

An experimental study on formation of TiC–TiB₂ in situ composites with a broad range of compositions was conducted by self-propagating high-temperature synthesis (SHS) using the reactant compacts from different combinations of Ti, B₄C, C, and B powders. Direct reaction of Ti with B₄C at stoichiometry of Ti:B₄C = 3:1 yields a TiB₂-rich composite with TiC:TiB₂ = 1:2. Formation of the products containing 20, 33.3, and 50 mol% of TiB₂ was achieved by the Ti–B₄C–C reactants. In addition, the test specimen composed of Ti, B₄C, and B was employed for the synthesis of a composite with 80 mol% TiB₂. Among three different types of the powder compacts, the boron-containing sample was characterized by the fastest combustion wave and the highest reaction temperature. The lowest combustion temperature and wave velocity were observed in the Ti–B₄C compact. When fine Ni particles were added to the Ti–B₄C reactant, it was found that the propagation rate of the reaction front was increased and the densification of the end product was enhanced significantly. This was attributed to formation of the Ti–Ni eutectic liquid during the reaction. As a result, the relative density of a TiC + 2TiB₂ composite increases from 30 to 86% with the Ni content from 0 to 20 mol%. Based upon the XRD analysis, small amounts of TiNi₃ and TiB were detected in the Ni-reinforced TiC–TiB₂ composites.     

The third-element effect in the oxidation of Ni–xCr–7Al (x = 0, 5, 10, 15 at.%) alloys in 1 atm O2 at 900–1000 °C

The oxidation in 1 atm of pure oxygen of Ni–Cr–Al alloys with a constant aluminum content of 7 at.% and containing 5, 10 and 15 at.% Cr was studied at 900 and 1000 °C and compared to the behavior of the corresponding binary Ni–Al alloy (Ni–7Al). A dense external scale of NiO overlying a zone of internal oxide precipitates formed on Ni–7Al and Ni–5Cr–7Al at both temperatures. Conversely, an external Al₂O₃ layer formed on Ni–10Cr–7Al at both temperatures and on Ni–15Cr–7Al at 900 °C, while the scales grown initially on Ni–15Cr–7Al at 1000 °C were more complex, but eventually developed an innermost protective alumina layer. Thus, the addition of sufficient chromium levels to Ni–7Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum. This effect is interpreted on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most reactive component in binary alloys.     

Characterization of Ni-W solid solution alloy powders and sintered compacts synthesized via mechanically activated hydrogen reduction of NiO-WO3 mixtures

Blended nickel oxide — tungsten oxide powders corresponding to the compositions of 70 wt% nickel — 30 wt% tungsten were mechanically alloyed (MA’d) for different durations such as 0 h, 6 h, 12 h and 24 h and reduced/alloyed at 550 °C for 1 h followed by 600 °C for 0.5 h under hydrogen (H₂) atmosphere. H₂ reduction of the MA’d fine oxide powders resulted in the fabrication of nanocrystalline Ni(W) solid solution alloy powders, whereas a mixture of Ni and WO₂ powders were obtained via hydrogen reduction of asblended oxide mixtures, which revealed the activation of the reduction process by MA. Obtained powders were sintered at 1300 °C for 1 h under H₂ and Ar gas flowing conditions. X-ray diffraction patterns taken from the sintered samples revealed the presence of the Ni(W) solid solution phase for all samples, whereas the presence of elemental W phase was observed in the sintered as-blended and reduced powders. The lowest relative density value of 92.04% and microhardness value of 1.27 GPa were measured for the sintered as-blended and reduced powders, which increased to between 97.62% and 98.72% and 2.19 GPa and 2.23 GPa, respectively, with the applied MA.     

Subsolidus phase relations of the ZnO–Li2O–P2O5 system

As a systematic search for suitable flux to grow zinc oxide single crystals, the subsolidus phase relations of the ternary system ZnO–Li₂O–P₂O₅ were investigated by means of X-ray diffraction (XRD). There are 6 binary compounds, 5 ternary compounds and 17 three-phase regions in this system. A new compound, Li₆Zn(P₂O₇)₂, is found in this system based on XRD experiments. The phase diagrams of the pseudo-binary systems Li₃PO₄–ZnO and LiZnPO₄–ZnO are investigated. It shows that the compounds, Li₃PO₄ and LiZnPO₄, are not suitable as flux for the growth of ZnO single crystals below 1250 °C.     

Thermodynamic activity measurements in the B2 phases of the Fe–Al and Ni–Al systems

The vaporisation of Fe–Al and Ni–Al alloys has been investigated in the temperature range 1140–1600 K and 1178 to 1574 K, respectively, by Knudsen effusion mass spectrometry (KEMS). Eleven different Fe–Al and also eleven Ni–Al compositions have been investigated in the composition ranges 30–51 at.% Al and 38–53 at.% Al, respectively. The Fe–Al samples have been investigated mostly in the B2 region of the phase diagram. The partial pressures and thermodynamic activities were evaluated directly from the measured ion intensities formed from the equilibrium vapour over the alloy and the pure element. From the temperature dependence of the activities the partial and integral molar enthalpies and entropies of mixing have been obtained. These are the most accurate data obtained by mass spectrometry on Fe–Al and Ni–Al systems so far. Nearly temperature independent integral enthalpies and entropies of mixing over the wide temperature range investigated were found, with the mixing entropies being large and negative.     

Effect of an Enamel Coating on the Oxidation and Hot Corrosion Behavior of an HVOF-Sprayed Co–Ni–Cr–Al–Y Coating

The oxidation and hot-corrosion behavior of a Co–Ni–Cr–Al–Y coating produced by high-velocity oxygen fuel (HVOF) with and without an enamel coating were investigated in air at 900°C and in molten 75 wt.% NaCl+25 wt.% Na₂SO₄ at 850°C. The results show that the enamel coating possesses excellent hot corrosion resistance in the molten salt, in comparison with the HVOF-sprayed Co–Ni–Cr–Al–Y coating alone. In the hot-corrosion test, breakaway corrosion did not occur on the samples with the enamel coating and the composition of the enamel did not significantly change. The oxidation resistance of the Co–Ni–Cr–Al–Y coating, which had good adhesion, was also improved by the enamel coating.     

Structural study on Zr0.39Ni0.61 and (Zr0.39Ni0.61)D0.59 amorphous alloys by neutron and X-ray diffraction

Neutron and X-ray diffraction and reverse Monte Carlo (RMC) simulation were performed on Zr_0.39Ni_0.61 and (Zr_0.39Ni_0.61)D_0.59 amorphous alloys to investigate the rearrangement of metal atoms due to the deuterium absorption. The RMC models showed that the Zr–Zr distances slightly increase but the Zr–Ni and Ni–Ni distances remain virtually unchanged after the deuterium absorption. Additionally, the Voronoi polyhedral analysis of the RMC configurations showed that there is not much difference in the local structure around Zr and Ni between Zr_0.39Ni_0.61 and (Zr_0.39Ni_0.61)D_0.59 amorphous alloys.     

Martensitic transformation and anomalies in resistivity of (Ti–50Ni)1−xCx (x = 0.1, 0.5 at.%) shape memory alloys

Phase transformation of solid solution (Ti–50Ni)_1−xC_x (x = 0.1, 0.5 at.%) alloys have been studied by using differential scanning calorimetry, physical property measurement system and optical microscope. The transformation temperature decreases due to the existence of titanium carbide (TiC) particles compared with that of near-equiatomic Ti–Ni shape memory alloy. The resistivity vs. temperature curves show hysteresis. Thermoelastic martensitic transformation occurred in two alloys despite the difference in TiC content. Nevertheless, the resistivity results show different martensitic transformation routes. A one-step B2 → B19′ transformation occurred in the low TiC content alloy and an R transformation appeared in another alloy, suggesting that the martensitic transformation routes depended on the TiC content. The cumulative effect of the TiC particles causes the local stress field and lattice distortion to restrain the transformation of the B19′. On the other hand, the TiC content has an effect on the temperature coefficient of electrical resistivity (TCR) of alloys. The Ti–Ni–0.5C alloy shows a negative TCR in the range 100–300 K during which transformation occurs. Another alloy shows the opposite result. The cause of the negative TCR is briefly discussed.