查看更多>>摘要：Yttria dispersion-strengthened tungsten composites with enhanced thermal and mechanical properties were fabricated by powder metallurgical methods in this work. The processing route involves the doping of yttria particles in the tungsten powders by liquid-liquid doping process, followed by hot pressing and two-step high-energy-rate forging (HERF). The microstructure, thermal conductivity, mechanical properties and thermal shock resistance of the hot-pressed W-Y2O3 composites and the HERF processed W-Y2O3 composites were comparatively investigated. The bending strengths of the HERF processed W-Y2O3 composites inc eased significantly compared with that of the hot-pressed W-Y2O3 billets. The HERF W-Y2O material presented a ductile behavior even at room temperature with a maximum flexural strain of 4.4% and a yield stress of 2324.5 MPa. At 100 degrees C, the material also exhibits evident plasticity with a flexural strain of 5.9% and the yield stress is up to 1931.9 MPa. The thermal conductivity of the W-Y2O3 composite is 170 W/mK, which is close to the deformed pure W. The thermal shock response of the HERF processed W-Y2O3 is also evaluated by applying edge-localized mode like high heat loads (100 pulses) with an energy density of 0.16-1 GWm(-2) at various temperatures in an electron beam facility. The thermal shock results indicate that the cracking threshold of the HERF processed W-Y2O3 at RT is is between 0.55 GWm(-2) and 0.66 GWm(-2). The cracking threshold of the HERF W-Y2O3 at 100 degrees C and above is higher than 1 GWm(-2).
查看更多>>摘要：High-entropy ceramics (HECs) become a focus as an innovative design of refractory materials due to their exclusive microstructure, huge adjustable components, some superior properties, and potential applications. Generally, they are manufactured by solid-state synthesis. However, in this paper, (Nb0.25Mo0.25Ta0.25W0.25)C (HEC-1) and (Nb0.2Mo0.2Ta0.2W0.2Hf0.2)C (HEC-2) high-entropy carbide ceramics were successfully fabricated through vacuum arc melting using transition metal carbides powders. X-ray diffraction, energy dispersive spectrometer and scanning electron microscope were performed to analyze the phase evolution and microstructure uniformity. The experimental data exhibited that the as-prepared HEC-1 and HEC-2 formed into a single-phase body-centered cubic (BCC) structure. HEC-1 revealed a high compositional uniformity at micro-scale, while a segregation of molybdenum element was observed in HEC-2. Vickers hardness test results showed that HEC-1 presented a great microhardness of 23.13 GPa at 0.98 N, which was higher than the highest hardness of the individual binary carbides. The significantly enhanced hardness could be attributed to the solid solution strengthening effect. HEC-2 exhibited a good combination of high microhardness and fracture toughness due to the doping of Hf. These broaden the potential applications of HECs towards, such as protective coatings, high temperature structure parts, and machining tools. This study provides a new approach to prepare bulk HECs, and the tuning in toughness by exploring a novel intricated compositions would be of great significance for the further development.
查看更多>>摘要：Extreme thermal environment, especially transient high thermal load, is one of the great challenges for plasma facing tungsten materials. In this work, the fine-grained W-Y2O3 composites fabricated by nano in-situ composite method were exposed to transient heat loading using electron beams. The microstructure evolution, damage behavior and failure analysis were investigated by experimentally combing with finite element modelling (FEM). Besides, the correlation among thermal shock resistance, microstructure and mechanical properties of the W-Y2O3 composites were initially analyzed. Results show the W-Y2O3 composites obtains superfine microstructure and coherent interface. The surface morphology evolves from smooth to roughness, cracks and melting as the power density increases. Simultaneously, four crack patterns of surface micro-cracks, crack networks, longitudinal cracks and internal transverse cracks were observed. The W-0.3 wt%Y2O3 composites exhibit superior resistance to crack formation attributed to excellent interfacial performance and diffuse Y2O3 hindrance. No obvious damage was found on the surfaces when the power density reached 600 MW/m2. With the increasing Y2O3 contents, the thermal resistance of W-Y2O3 composites significantly decreases. Finite element modelling (FEM) analysis shows that the surface damage is the most serious compared to the interior after thermal shock. The damage, especially cracks, are preferentially formed on the surface, and then migrate and expand to the interior. The damage and its evolution are attributed to the palstic thermal strain on the surface caused by thermal stress. Moreover, the higher the mechanical properties of W-Y2O3 composites, especially the high temperature yield stress and elongation, the better high thermal load resistance. These results should be of relevance for the optimum design of W plasma facing materials in future nuclear fusion reactors.
查看更多>>摘要：The impact characteristic and crack propagation mechanism for large-size Mo-3Nb single crystal with a diameter of about 30 mm have been investigated and disclosed comprehensively by Charpy impact test. The fracture analysis shows that the single crystal crack propagates radially, resulting in a multi-step distribution on the fracture surface. The fracture of Mo-3Nb polycrystal is composed of cleavage and quasi-cleavage, exhibiting a trans-granular fracture feature. The impact toughness of single crystal shows orientation dependence, which is attributed to the crack propagation path. Compared with the straight path, the waved path can delay the crack propagation and absorb higher impact energy. The impact toughness of Mo-3Nb single crystals is lower than that of Mo-3Nb polycrystal at room temperature because the single crystal lacks the coordinated deformation of grain boundary. The analysis of dislocation arrangement for Mo-3Nb single crystal shows the different crack propagation paths is caused by the mobility of screw segments, which are controlled by the double cross-slip multiplication mechanism. However, the waved fracture morphology of Mo-3Nb polycrystal is caused by the effect of grain boundaries instead of the cross-slip of screw dislocation.
查看更多>>摘要：Laser Surface Texturing (LST) is widely used to modify hard material surfaces improving their physic-chemical and mechanical properties. This technology is particularly relevant for tungsten carbides, a material that requires high complexity methods when other micro-machining processes are used. LST allows innovative cutting tool designs that improve the machining behavior and enlarge the cutting tool lifetimeThis research analyses the influence of LST parameters on the track dimensions, roughness, microstructure, hardness, and lubricant retention ability of the modified surfaces. Twelve combinations of energy density of pulse and scanning speed created different geometrical patterns on WC-Co surfaces. LST parameters were related to specific shape and dimensions of the linear grooves. Energy density was proven as the most influential parameter for dimensional characteristics and roughness values. Specific channel morphologies increased the lubricant expansion area up to 50%, leading the lubricant to a linear track direction. Low scanning speed and high energy density also increased the surface hardness up to 20%. The surface composition was also modified. The thermal effect of the laser treatments and the non-protective atmosphere increased the oxygen on the surface and modified the WC-Co microstructure. However, the thermal affected zone is considerably low compared to other texturing processes.
查看更多>>摘要：This paper concerns the influence of increasing the milling time, which is performed by a high-energetic rotaryvibratory mill and WC-Co balls, on boron carbide sintering. The commercial boron carbide powder was characterised regarding the morphological, grain size and phase composition after 0-16 h of milling. The prepared green bodies were sintered using dilatometry to determine the WC contamination impact on the onset and end process temperature. The increasing tungsten boride phase content was confirmed by XRD analysis. The content of WC contamination on the boron carbide laser sintering process was also qualitatively analysed. The obtained polycrystals were analysed in terms of phase composition, microstructure and tungsten element distribution.
查看更多>>摘要：In this study, the WC-CoCr coating and Al2O3-TiO2 coating were deposited by high-velocity oxygen-fuel (HVOF) spraying and atmospheric plasma spraying (APS), respectively. Both coatings were immersed in 3.5 wt% NaCl solution with 20 ppm Na2S to investigate their long-term corrosion resistances in the simulated seawater environment. The corrosion behaviors of both coatings for different times were investigated by potentiodynamic polarization, EIS, XRD, EDS and SEM. The electrochemical measurement results showed that the corrosion resistance of both coatings decreased obviously after long-term immersion in sulphide-containing solution, while the Al2O3-TiO(2 )coating exhibits more stable corrosion performance during the whole immersion process compared with the WC-CoCr coating. The corrosion resistance of the WC-CoCr coating in sulphide-containing solution after long-term immersion was inferior to that of the Al2O3-40TiO(2) coating.
查看更多>>摘要：This work aims to explore the effects of different sintering temperatures on microstructure, mechanical properties and magnetic properties of (Ti,W)C-xCoCrFeNiMo (wt%, x = 15, 30) cermets. Results showed that bending strength, hardness and fracture toughness of sintered cermets first increased and then decreased with the increase of sintering temperature. They reached peak values of about 967.2 MPa, 92.8 HRA and 9.1 MPa.m(1/2) for x = 15, and 1278.3 MPa, 91.2 HRA and 13.6 MPa.m(1/2) for x = 30, after sintering at 1450 ?, respectively. The Ms, Mr, and chi(max) of cermets first increased, then decreased and finally increased. They reached minimum to 1.08 emu/g, 0.09 emu/g and 6.58 emu/(g.KOe) for x = 15 at 1450 ? and 1.18 emu/g, 0.02 emu/g and 8.49 emu/ (g.KOe) for x = 30 at 1410 ?, respectively. This was related to the higher content of non-ferromagnetic elements and the weak magnetic phase Ni3Mo3C in the binder phase
Sala, N.Abad, M. D.Sanchez-Lopez, J. C.Crugeira, F....
7页
查看更多>>摘要：Nanostructured TiB2 and TiBC thin films with carbon contents up to 11 at. % were prepared by physical vapor deposition using high power impulse magnetron sputtering (HiPIMS) technology. The influence of carbon incorporation during the deposition of TiB2 coatings was investigated on the chemical composition, microstructure and mechanical properties by means of scanning electron microscopy, atomic force microscopy, x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), nanoindentation, scratch test, calotest and adhesion Daimler-Benz test. The results indicated that small additions of carbon up to 3 at. % improved the mechanical behavior and increased the adhesion of the TiB2 thin films. Hardnesses up to 37 GPa were reached and the adhesion of the coating to AISI D2 steel substrates increased from 11 to 18 N. XRD and XPS results showed that the carbon atoms are either occupying interstitial sites within the hexagonal structure of the TiB2 or forming bonds with titanium and boron atoms. The preferred orientation of the films determined by XRD also changed with the increasing carbon content in the (001) crystalline plane.
查看更多>>摘要：Molybdenum is an important high-temperature structural material but has poor processability. Additive man-ufactured unalloyed Mo is generally very small and the mechanical properties is seldomly studied. In this work, wire arc additive manufacturing was adopted, and crack-free molybdenum parts with high-density (99.0%) and characteristic size of 20 mmx20 mmx120 mm were successfully fabricated by short-track scanning. The microstructure and mechanical properties of samples in both the as-deposited and heat-treated states were studied and compared. Large columnar grains were observed, which were basically along the <001> direction. Heat treatment leads to grain coarsening, and the elimination of some sub-grain boundaries. Due to the weak-ened effect of grain and sub-grain boundary hardening, the mechanical properties of heat-treated specimens were worse than that of as-deposited specimens at room temperature. Both of them exhibit brittle fracture features. Under high temperature, the ductile fracture is observed, and the as-deposited specimen has similar strength and ductility, compared with the heat-treated specimens, suggesting a weak role of grain and sub-grain boundary at high temperature. A large number of fragments were observed at the fracture surface after high-temperature tests, which was MoO3 by energy dispersive spectroscopy test.
NSTL
Elsevier