查看更多>>摘要：? 2022 Elsevier LtdThe present study provides a detailed description of all phenomena related to fabrication of dense vanadium carbide ceramics from nanopowders. The solid-state synthesis and characterization of initial microcrystalline and milled nanocrystalline VCx particles with x of 0.70–0.86 are documented. Densification kinetics of milled powders is investigated and sintered ceramics are analyzed in terms of hardness, microstructure, and phase composition. The synthesized and milled powders demonstrate order-disorder transitions during heat treatment, but the transition point is shifted for milled samples due to decarburization and partial dissolution of oxygen impurities. Overall, milled VC0.86 was found to be the most promising material because it provided single-phase ceramics with 97% density and 19 GPa hardness after sintering at 1300 °C for 30 min under vacuum.
查看更多>>摘要：? 2022 Elsevier LtdDual-grain WC/W2C-Cu-Ni composites were fabricated by selective laser melting using a mixture of Ni-coated WC-W2C and Cu[sbnd]Ni powders. The phase composition and microstructure of the processed composites were investigated by X-ray diffraction, scanning electron microscopy, electron backscattered diffraction and transmission electron microscopy techniques. The results revealed that the microstructure of composites was consisting of both partially melted WC-W2C particles and nano-WC grains evenly distributed in the Cu[sbnd]Ni binder phase. The existence of WC1-x phase was detected between nano-WC and Cu[sbnd]Ni phases. An interface formed between protruding parts, existing around the partially melted WC-W2C particle, and Cu[sbnd]Ni resulted in the increase of the contact area and bonding strength between the partially melted WC-W2C particles and the matrix. The mechanical properties (hardness, bending strength and toughness) of the composites processed in this research were investigated and compared to those of parts fabricated by pressureless infiltration. The dual-grain structure of the composites enhanced their mechanical properties compared to those of traditional WC composites processed by a pressureless infiltration method.
查看更多>>摘要：? 2022 Elsevier LtdNegatively charged silicon-vacancy (SiV?) colour centres are considered to be promising room-temperature single-photon sources. The controlled preparation of high-grade industrial diamonds containing a certain amount of SiV? colour centres remains a significant technical challenge. Herein, micron-sized industrial-grade diamonds containing different concentrations of SiV? colour centres were prepared by adjusting the doping amount of Si powder under the pressure conditions of 5.0–5.2 GPa and temperatures of 1420–1500 °C. Al was used as the nitrogen getter in FeNiCo?C systems through the diamond film growth method (FGM). The results show that the intensity of the SiV? colour centres in synthesized diamond increases with increases in the Si doping amount. It has also been proved that nitrogen impurities inhibit the formation of SiV? colour centres in diamond. In addition, the diamond Raman peak positions exhibit a redshift (from 1330.88 cm?1 to 1329.55 cm?1), and an increase in the Raman full width at half maximum (FWHM) (from 4.78 cm?1 to 5.17 cm?1), indicating that Si enters the diamond lattice, but the diamond crystals still have a single sp3 structure. Our research provides a fast, efficient and low-cost method for the controlled batch preparation of industrial-grade diamond containing SiV? colour centres.
查看更多>>摘要：? 2022 Elsevier LtdSurface degradation due to slurry erosion is the common phenomenon encountered in fluid handling and material processing industries. Slurry erosion research initiated at the latter half of the twentieth century. Huge economic losses incurred through slurry erosion, to such an extent that components can't be repaired. Erosion is a complex process, because it operates with many factors and these factors contribute individually and in combined form, they affect severely. To evaluate materials' behavior under erosion environment, slurry erosion test rigs are developed. Numerous jet erosion test rigs are developed, yet standardization of the test method is unattainable due to its intricacy. Jet type slurry erosion test rigs are the preferable choice, as they offer flexibility with the test parameters and it could provide impact characteristics. Erodents' introduction is a tedious task, as particles have to be introduced into the flowing fluid. In the present article, a novel device named modified Venturi device is developed. Performance of the test rig is evaluated by varying the operating test parameters and by evaluating Cast Iron sample as the candidate material. From the results, it is noticed that developed test rig can be successfully employed to evaluate the erosion behavior. Erosion depends on the kinetic energy and the drag force on the particle. Numerical approach through CFD has been adopted and found that the experimental results are in good correlation with Finnie erosion model. SEM Micrographs reveal the material removal mechanism in the test rig.
查看更多>>摘要：? 2022 Elsevier LtdIn this work, a novel low-cost WC-CoCuFeNi cemented carbide has been produced by spark plasma sintering technology. The CoCuFeNi medium-entropy alloy binder has a face center cubic structure with more slip systems and shows good bonding behavior with WC. The newly-developed cemented carbides exhibit a good combination of hardness and fracture toughness. The hardness and fracture toughness of WC-20 wt% CoCuFeNi are up to 1506.7 HV30 and 10.33 MPa·m1/2, respectively. These results show the WC-CoCuFeNi cemented carbides could be a promising substitute for WC-Co.
查看更多>>摘要：? 2022 Elsevier LtdThe brittleness of tungsten (W) is a significant limitation on its widespread applications as a high-temperature structural material. Laminated composite technology proved to be an effective method to improve the toughness of W. In this work, W/Ta and W/TiN/Ta laminated composites were prepared by spark plasma sintering (SPS) to overcome the brittleness of W. The toughening properties of the W/Ta and W/TiN/Ta laminated composites were evaluated by electron beam thermal shock and laser thermal diffusivity testing. The toughening mechanism was clarified by scanning electron microscopy, electron probe micro-analysis, and transmission electron microscopy. After the three-point bending test, interfacial debonding was observed predominantly at the incoherent W-TiN rather than at the semi-coherent Ta-TiN interfaces. At 0.30 GW/m2 of electron beam thermal shock, the average width and depth of thermal shock cracks in the W/Ta laminated composite were ~ 9.4 μm and ~ 30.0 μm, respectively, which were much smaller than those in the W/W laminates and W/TiN/Ta laminated composite.
查看更多>>摘要：? 2022Magnetron sputtered WB2 coatings doped with 8, 11 and 16 at.% zirconium were analysed using energy dispersive spectroscopy, X-ray diffraction and nanoindentation under the load of 4, 7 and 10 mN. It has been observed that these coatings crystallize in the α-AlB2 and ω-W2B5 prototype structure. Phenomenon responsible for this is an increase of the zirconium content which causes an increase in the ω-W2B5 phase. All the deposited coatings have a hardness of about 45 GPa while Young's modulus drops down from 497 to 480 GPa with increasing zirconium content. Coatings without doping and doped with 16 at.% zirconium were annealed at 650 °C and subjected to cyclic thermal loads using a maximum temperature 600 °C and cooling with a compressed air. It has been observed that addition of zirconium improved coatings phase stability.
查看更多>>摘要：? 2022The effects of Al content on the oxidation resistance of AlMo0.5NbTa0.5TiZr refractory high entropy alloys (RHEAs) were investigated. The oxidation obeys a power-law passivating oxidation at the early stage, followed by a transformation to linear oxidation. Increasing Al content reduces the mass change of forming the full coverage of passivating scale and prolongs the passivation duration of alloys. Higher Al content benefits the formation of AlNbO4 and AlTaO4 instead of Nb2O5 and Ta2O5, which has a positive effect on oxidation resistance of alloys. The fracture of scales generally occurs at the mass gain of 15 mg?cm?2 roughly and leads to the transformation to linear oxidation.
查看更多>>摘要：? 2022 Elsevier LtdMolybdenum or tungsten borides are excellent high-temperature ceramics or electrocatalysis materials. In this paper, Mo or W borides were prepared by boronizing Mo or W powder in Al melt with B4C as boron source. By thermodynamic calculation, the feasibility of preparing various borides and the stability of borides in Al melt were evaluated. Through high temperature reaction at 1523 K and the following acidic leaching, single phase MoAlB, W2B5, WB, and W2B5 powders were successfully synthesized. Owing to the decarburizing function of Al melt, C contents of various boride products were below 1 wt%. In addition, the phase transition processes of various products in the synthesis stage were revealed.
查看更多>>摘要：? 2022 Elsevier LtdProcessing of tungsten carbide?cobalt (WC-Co) by laser-based powder bed fusion (PBF-LB) can result in characteristic microstructure defects such as cracks, pores, undesired phases and tungsten carbide (WC) grain growth, due to the heterogeneous energy input and the high thermal gradients. Besides the processing conditions, the material properties are affected by the initial powder characteristics. In this paper, the impact of powder composition on microstructure, phase formation and mechanical properties in PBF-LB of WC-Co is studied. Powders with different cobalt contents from 12 wt.-% to 25 wt.-% are tested under variation of the laser parameters. Furthermore, the impact of vanadium carbide (VC) and chromium (Cr) additives is investigated. Both are known as grain growth inhibitors for conventional sintering processes. The experiments are conducted at a pre-heating temperature of around 800 °C to prevent crack formation in the samples. Increasing laser energy input reduces porosity but leads to severe embrittlement for low cobalt content and to abnormal WC grain growth for high cobalt content. It is found that interparticular porosity at low laser energy is more severe for low cobalt content due to poor wetting of the liquid phase. Maximum bending strength of σB > 1200 MPa and Vickers hardness of approx. 1000 HV3 can be measured for samples generated from WC-Co 83/17 powder with medium laser energy input. The addition of V and Cr leads to increased formation of additional phases such as Co3W3C, Co3V and Cr23C6 and to increased lateral and multi-laminar growth of the WC grains. In contrast to conventional sintering, a grain growth inhibiting effect of V and Cr in the laser molten microstructure is not achieved.
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