查看更多>>摘要：? 2022As an important refractory metal, tungsten (W) has unique physical-chemical properties, and the applications of high-quality tungsten NPs with uniform particle size, high purity, and good particle dispersion in industrial fields are highly desirable. Herein, the integration of tungsten metallurgical purification and nanoparticle preparation is achieved by the hydrogen arc plasma method. In the process of purification, on the condition of arc current of 250 A, hydrogen concentrations of 30 vol%, purification time of 40 min, and gas pressure of 60 kPa, the highest impurity removal rate is achieved. The active H+ is able to react with impurities to generate metal hydride (MH, M = Al, Cr, Mg, and Fe), which promotes the removal rate of impurity elements. During the preparation of nanoparticles (NPs), the as-prepared tungsten NPs with an average particle size of 44.1 nm have a regular spherical structure, high purity, and a relatively uniform particle size distribution. When the hydrogen concentration is controlled at 50 vol%, the extremely high temperature promotes the evaporation of tungsten. Meanwhile, the active H+ reacts with tungsten to form tungsten hydride (WH), further increasing the evaporation rate of tungsten. According to the above results, the hydrogen arc plasma promotes the simplification of the production process steps of high purity tungsten NPs and reduces the industrialized production cost of high-quality refractory metal NPs.
查看更多>>摘要：? 2022 Elsevier LtdThe nanostructural oxide dispersion strengthened (ODS) Mo alloy was successfully fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). In this study, the MA and SPS conditions were systematically studied, and the characteristics of oxide particles were investigated. The results show that the MA process can refine the powder and promote the dissolution of interstitial atoms, but introduce the ZrO2 and excess O impurities. To recycle ZrO2 impurity as a source of active element Zr for alloy preparation, the content of Zr and O with milling time was quantitatively evaluated. In the SPS process, the precipitation of oxide particles retard the densification of as-MA Mo powders. Based on the calculated linear shrinkage, the densification behavior is controlled by sintering temperature, which is driven by grain boundary or dislocation climbing. A high relative density of 99.28% in ODS Mo alloy is achieved using the optimized MA (250 rpm/18 h) and SPS (1400 °C/5 min) parameters. Additionally, the ultrafine Mo grain structure (300 nm) and uniformly dispersed nanoscale Y-Zr-O oxide particles (20.32 nm) inside grains greatly improve the mechanical properties (569.63 ± 28.08 HV0.3) of ODS Mo alloys. Non-stoichiometric Y-Zr-O particles, with different Y/Zr ratios (Y/Zr <1), possess a special fluorite crystal structure.
查看更多>>摘要：? 2022In this study, the influence of carbon content on fatigue strength of fine grade cemented carbides (WC-12Co) was investigated. Thereby, the carbon content was varied systematically in the two- and three-phase region. Specimens from the different alloys were fabricated and tested in flat bending fatigue experiments. Results show no significant influence of the carbon content as long as no graphite was precipitated. In accordance with findings in the literature, the fatigue strength decreases significantly when graphite precipitations occur. Both, wavelength dispersive x-ray spectroscopy (WDS) and energy dispersive spectroscopy (EDS) were used for analysis of the chemical composition of the binder phase. No significant influence of the global carbon content on the morphology of WC-grains as well as the local carbon content in the cobalt-binder was found.
查看更多>>摘要：? 2022 Elsevier LtdMicrostructure damage created by high-energy particle irradiation has an inevitable effect on the performance of material. Helium (He) as the transmutation element in irradiated solid prefers to occupy the vacancy (V) center to form He[sbnd]V complex, which in turn induces the formation of He bubble. Herein, the external applied stress driven irradiation-induced He bubble formation and evolution in single-crystalline tungsten (W) has been investigated using the phase-field method. Importantly, the trapping and resolution kinetic behavior between He atom and He bubble is described by the diffusion coefficient in terms of rate theory continuum model, and the effect of external stress on the movements of vacancy, self-interstitial and He atom has been implemented into their diffusivities as well. We first concentrate on the intrinsic lattice distortion (stress) patterns of void and bubble to validate the developed model by comparing the reported numerical results. Then, the accelerated growth velocity of bubble along the loading direction is predicted, which agrees well with the previously observed stress pattern around the void/bubble. The evolution behaviors of the stress-assisted single, bi-dispersed and multiple bubbles vary with the direction and magnitude of external loading, indicating a strong correlation between the size and density of bubble and the loading level. It is found that with the contribution of increasing external stress, more bubbles can be generated at the formation stage and visible bubbles tend to reduce at the coarsening stage due to bubble coalescence.
查看更多>>摘要：? 2022 Elsevier LtdVarious components employed in hydro machines are rigorously affected by the presence of sand particles throughout their operation which leads to degradation of the target surface. To counteract this menace, laser cladding has been emerged as a prominent technique to produce dense coatings along with enhanced surface properties like hardness and wear resistance. Therefore, in this study, hard layers of NiCrSiBC + 50WC powder were deposited on SS410 steel by laser cladding. Further, the effect of laser beam power, scanning speed, and powder feed rate on quality of clads was investigated. The laser cladded samples were examined for their micro structural characteristics, micro-hardness, and slurry erosion. Performance of NiCrSiBC + 50WC coatings under slurry erosion was evaluated for harsh conditions of various factors namely sand particle size, slurry concentration, impingement velocity, and impingement angle. Laser cladding parameters namely scanning speed and laser beam power were found to have a significant influence on micro-hardness of developed clads, while powder feed rate did not exhibit a significant effect on micro-hardness variations. Owing to higher hardness of deposited materials, slurry erosion resistance of NiCrSiBC + 50WC clad surfaces was greatly enhanced as that of uncoated SS410 steel. From SEM images, plastic deformation, micro-cutting, crater formation, and ploughing were observed as the major wear mechanisms for the slurry eroded clads. Further, from the obtained results, it has been noticed that a blend of high scanning speed, high laser beam power, and intermediate powder feed rate is suitable to develop coatings having superior wear resistance.
da Costa Morais L.Beneduce F.Magnabosco R.da Silva A.L.N....
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查看更多>>摘要：? 2022 Elsevier LtdThe current study reports the influence of TiC addition on carbide particle growth of NbC-Ni cemented carbides. Microstructural analysis after liquid phase sintering in vacuum at 1420 °C for 40 min showed a decrease in NbC grain size as TiC content increased, associate to a change in NbC carbide morphology from rounded to equiaxed. Chemical composition from EDS analysis revealed a mixed carbide (Nb,Ti)C formation, with a gradient of Ti in carbide region for samples with TiC above 15%. Dictra? simulations of spherical particles immersed in Ni matrix evidenced a retardation in carbide dissolution and a decrease in the volume dissolved as TiC is added. It was also noted that liquid phase composition for 140 min showed less Nb in liquid phase available for reprecipitation during cooling process, promoting carbide particle growth inhibition. Ti profiles from Dictra? and carbide composition from EDS suggest a thin Ti rich layer formation in carbide/binder interface, which can be related to the carbide particle growth inhibition mechanism. TiC addition promote an increase in hardness and toughness values, and the highest values for both properties was found for 7%TiC-NbC-12%Ni composition (mass percent).
查看更多>>摘要：? 2022In this work, WCoB was introduced into ultra-coarse cemented carbides for improving the oxidation- and wear-resistance at elevated temperatures. At the initial oxidation stage, WCoB addition had a slight effect on the oxidation rate, but enhanced the wear resistance obviously by restraining the WC grains from pull-out and fracture as the Co binder phase was preferentially removed during the wear process. At the rapid oxidation stage, the wear rate of the WCoB-containing cemented carbide was drastically decreased compared to that of the straight WC-Co. The wear of the cemented carbide was mainly caused by the removal of the oxide layer and the spalling of materials under the surface layer. WCoB inhibited the oxidation and enhanced the wear resistance of the ultra-coarse cemented carbides by forming liquid B2O3 in the oxidation layer and sharing the stress with WC matrix before oxidation.
查看更多>>摘要：? 2022Scandium?tungsten (Sc-W) cathodes have garnered a lot of research attention due to their high emission current density. This study investigates the electronic properties of Sc-W cathodes using the VASP code to explain the operating mechanism of hot electron emission. The results reveal that the work function is decreased by 0.58 eV, whereas the charge density in the crystal structure, the number of high valence electrons and the energy of electrons increase when Sc content is increased from 1% to 2%. Moreover, the bandgap and energy required for electrons to jump from the valence band to the conduction band decreased with the increase of Sc content, enhancing the current emission density and reducing the work function of the cathode.
查看更多>>摘要：? 2022Tungsten carbide (WC)-based matrix powder, basalt fiber (BF) and diamond were used as raw materials to prepare WC-based diamond composites by hot-press sintering, and then the sintered samples were tested for thermal conductivity and wear resistance. The surface, fracture surface and wear surface of the samples were observed using scanning electron microscope (SEM), x-ray diffraction (XRD) and energy disperse spectroscopy (EDS). The results showed that the addition of BF significantly weakened the wear resistance of the diamond composites, and the abrasive ratio of the samples was reduced by 34.4% at 5% mass fraction BF addition compared to the samples without BF addition. At the same time, the addition of a moderate amount of BF increased the relative density of the matrix and reduced the number of pores in the matrix material. When 1 wt% BF was added, the WC-based diamond composite had the highest thermal conductivity, reducing the risk of possible thermal damage to the diamond.
查看更多>>摘要：? 2022 Elsevier LtdHydroelectric turbine machinery experience extreme wear and tear due to slurry erosion and is one of the leading causes for the unplanned shutdown of hydro power plants. The coatings produced by High-velocity oxy-fuel (HVOF) thermal spray process on different hydro-turbine parts is known for improving wear resistance. In the present study, WC-10Co-4Cr and Cr3C2 ? 25(Ni20Cr) coatings were sprayed by HVOF process on F6NM stainless steel of Dul-Hasti hydro-power plant turbine material. The slurry erosive wear studies of developed coatings was performed using high speed slurry erosive wear tester. Sediments collected from Dul-Hasti power plant dam site was taken as erodent material. The effect of four parameters specifically rotational speed (500–1250 rpm), slurry concentration (25–40 wt%), sand particle size (125–425 μm), and test duration (450–900 min) on slurry erosion of coated materials was studied. It was observed that among all the developed three coatings, the WC-10Co-4Cr coated samples has shown superior erosion resistance compared to Cr3C2 ? 25(Ni20Cr) coated samples due to its improved micro-hardness. Both the developed coatings were observed to be useful in enhancing the erosion resistance of F6NM steel significantly. Erosion mechanism were studied in detail using SEM. The properties such as hardness and porosity were also examined.
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