查看更多>>摘要：Tungsten carbide-nickel (WC-Ni) composite coatings were firstly prepared on martensite stainless steel 2Cr13 by laser hot-wire deposition. The microstructural evolution and the dissolution characteristics of ceramic particles in the coatings had been systematically investigated. The results showed that the tungsten carbides' dissolution and microstructures in the coatings were close related to the dilution degree of the base metal. Apart form large retained particles, the coating with a lower dilution ratio (8.4%) was composed of Ni, Ni/Ni3B eutectics, and insitu W2C particles. The W2C phase was preferentially dissolved while the WC was retained in W2C/WC eutectoid-structured particles, which exhibited the typical thermal damage patterns including the dissolution-diffusion and partially/completely fragmentation-dissolution-diffusion. The amount of M6C carbides was increased with the increment of dilution ratio (from 9.5% to 24%), and the ceramic particles suffered from more severe heat damage which mainly exhibited the characteristics of dissolution-diffusion-precipitation. Not only was the W2C phase was dissolved, the WC phase was also decomposed and reacted into M6C carbides. Due to the reinforcement of retained particles and the precipitated carbides, these coatings showed higher hardness and wear resistance which was about 1.9- 2.3 and 12.1- 26.9 times that of 2Cr13 substrate.
Pervikov, A., VKrinitcyn, M. G.Glazkova, E. A.Rodkevich, N. G....
6页
查看更多>>摘要：In the present study, bimodal tungsten carbide WC powders are successfully prepared by a two-stage process: electrical explosion of wire (EEW) synthesis of bimodal tungsten powder followed by a carburization reaction. The conditions of the EEW synthesis have been determined, resulting in the preparation of the bimodal powder. During the carburization process, the tungsten carbide WC is formed through the transition phase W2C. The carbon content in the obtained WC powders is very close to the theoretical value. Also, it was found that a high C/W molar ratio at the carburization stage does not affect the carburization process significantly. Optimal conditions for the tungsten carbide WC synthesis have been determined: calcination at 1200 degrees C with 8 h dwell and C/W molar ratio to be 1.4. The yield of tungsten carbide is 99 wt% under these conditions. Thus, the carburization process of bimodal tungsten powder may provide a low-cost and high-efficiency route to prepare the WC powders for various applications.
查看更多>>摘要：The plastic deformation mechanisms of tungsten carbide at room and elevated temperatures influence the wear and fracture properties of WC-Co hardmetal composite materials. The relationship between residual defect structures, including glissile and sessile dislocations and stacking faults, and the slip deformation activity, which produce slip traces, is not clear. Part 1 of this study showed that {10 (1) over bar0} was the primary slip plane at all measured temperatures and orientations, but secondary slip on the basal plane was activated at 600 degrees C, which suggests that < a > dislocations can cross-slip onto the basal plane at 600 degrees C. In the present work, Part 2, lattice rotation axis analysis of deformed WC micropillar mid-sections has been used to discriminate < a > prismatic slip from multiple < c + a > prismatic slip in WC, which has enabled the dislocation types contributing to plastic slip to be distinguished, independently of TEM residual defect analysis. Prismatic-oriented micropillars deformed primarily by multiple < c + a > prismatic slip at room temperature, but by < a > prismatic slip at 600 degrees C. Deformation in the near-basal oriented pillar at 600 degrees C can be modelled as prismatic slip along < c > constrained by the indenter face and pillar base. Secondary < a > basal slip, which was observed near the top of the pillar, was activated to maintain deformation compatibility with the indenter face. The experimentally observed lattice rotations, buckled pillar shape, mechanical data, and slip traces are all consistent with this model.
查看更多>>摘要：Nanostructured W-31wt.%Cu composite was for the first time produced via magnetic pulse compaction from bimetallic particles obtained using electric explosion of intertwisted copper/tungsten wires in argon and then characterized for microstructures, mechanical strength and tribological behavior at high temperatures. Microstructure of the composite is characterized by recrystallized copper grains with mean grain size of 59 +/- 3 nm and unreacted spherical tungsten particles. The composite density was in the range 93-99%. Flexural and compression strengths were 560 +/- 10 and 1035 +/- 150 MPa, respectively. Tribological high temperature tests showed that this composite develops reduced wear starting from the testing at 250 degrees C. Such an adaptation mechanisms is related to generation of copper tungstate CuWO4 on the worn surfaces.
查看更多>>摘要：This study focuses on diffusion bonding of WC-Co cemented carbide to a steel using Co and composite Ni/Co interlayers. The typical microstructure and microstructural evolution with variable bonding temperatures were investigated, and mechanical properties of the joint were evaluated. The results illustrated that increasing temperature promoted interdiffusion of interfacial atoms, leading to the elimination of interfacial voids and the enhancement of Fe-Co-Cr interdiffusion zone on steel side. As the temperature increased, so did the thickness of Co6W6C formed near the WC-Co substrate (using Co interlayer). On the other hand, the Co6W6C was inhibited due to the addition of Ni foil, and replaced by the WC and (Ni, Co) solid solution resulting in satisfactory shear strength of the diffusion-bonded joint, where referring to using composite Ni/Co interlayer. The maximum shear strength of 418 MPa was achieved when the joint was diffusion-bonded at 1100 degrees C for 60 min using composite Ni/Co interlayer. In addition, ductile fracture occurred in the joint indicating propagation of cracks in the Ni-Co interdiffusion zone, and passing through the WC-Co substrate. Added up, this study provided experimental support for reliable joining cemented carbides to Fe- or Co-based alloys.
查看更多>>摘要：Hard coatings play an important role in increasing the ability of deformation and wear resistance due to the superior mechanical and tribological properties. Nano-structured coating is now being used for cutting tool and exhibits extremely fascinating and useful properties. In milling process, a typical failure mechanism of coating is the cyclic mechanical impact between the workpiece and cutting tool. This could induce intensive fatigue failure of coated tools. In this work, a novel cyclic impact tester has been designed by using the piezoelectric ceramic actuator to study the fatigue resistance of bilayer micro-structured and multilayer nano-structured TiSiN/TiAlN coating. Besides, the mechanical, tribological and adhesion properties of coatings were evaluated. The multilayer nano-structured coating showed higher hardness yet toughness, better adhesion and tribological properties, as well as more superior properties of fatigue impact resistance. The influence of coatings on the wear behavior and cutting performance of cutting tools was investigated in milling Ti-6Al-4V. The milling forces applied on the coated tools were measured and analyzed. It found that the friction force and normal force applied on the multilayer nano-structured coated tool were lower than that of bilayer micro-structured coated tool. The correlation between the mechanical properties in cyclic impact tests and cutting performance in milling experiments of coatings was studied. The multilayer nano-structured coated tool was much less susceptible to fatigue fracture and this could be attributed to the better fatigue resistance in the high frequency cyclic impact tests.
查看更多>>摘要：Nano-potassium bubble strengthened tungsten (W-K) alloy with excellent overall performance has been proven to be one of the most desirable plasma facing materials (PFMs). However, some key properties, e.g., thermal heat resistance, ductile-brittle transition temperature (DBTT), strength and toughness need to be further improved to meet the requirements of fusion reactor. Herein, pure yttrium (Y) was doped in W-K alloy to achieve multiple strengthening effects. The grains of W-K alloy have been significantly refined, and the tensile strength was improved along with the introducing of Y. Thermal shock cracking threshold has increased to exceed 0.62 GW/ m(2). Interestingly, the existing form and size of the precipitates differed with the change of doping amount, leading to the nonlinear evolution of DBTT. The W-K alloy with 0.1 wt%-Y doping content (W-K-0.1Y) showed the best match of strength and toughness. Furthermore, coupled with relatively good thermal conductivity, W-K0.1Y alloy displayed prominent thermal shock resistance. This work may pave a way for designing PFMs with higher performance.
查看更多>>摘要：Molybdenum metal have high brittleness and ductile-to-brittle transition temperature (DBTT) due to the existence of interstitial oxygen impurities, so it is of great scientific significance to strictly control and accurately analyze the oxygen content. In this work, the effects of oxygen on microstructure and evolution mechanism of body-centred-cubic metallic molybdenum are studied. The X-Ray diffraction (XRD), oxygen nitrogen analyzer, scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and electron probe micro-analyzer (EPMA) are used to analyze the chemical composition, microstructure, oxygen existence and distribution form. The results demonstrate that Mo-O specimens with up to 3300 ppm of oxygen content have single-phase body-centered cubic microstructure with no precipitates. When the oxygen content in molybdenum increases from 44 ppm to 2200-3300 ppm, more oxygen exists in the pores between molybdenum grains, and the fracture mode changes from intergranular fracture to intergranular and transgranular mixed fracture. The microhardness increases with increase of oxygen content. These results provide new ideas for controlling the change trend of oxygen in molybdenum and regulating the properties of molybdenum.
查看更多>>摘要：Hardness and fracture strength of WC-Ni-Co-Cr-Ti-Al cemented carbides have been measured at room temperature in as-HIPed and solution-aged conditions. These treatments are applied to modify the size of gamma prime precipitates, which are intrinsically formed within the metallic binder of these ceramic-metal composites during the sintering process. Compositions containing approx. 28-29 vol% metal content exhibit hardness values in the range of those reported for similar grades of WC-Co hardmetals. Optimized aluminum additions lead to materials with fracture strength values only 15% lower than those reported for the same WC-Co commercial references. These results suggest gamma prime precipitation hardening as a potential strategy for improving the performance of WC-Co materials at high temperatures. Regarding Fe-Ni-Co-Cr alloys are potential candidates for partial substitution of Co content in WC-based hardmetals. It has been investigated WC coarse grade with 15 wt%(FeNiCoCr). The Cr content has been adjusted in order to avoid the precipitation of M7C3 carbides. Within the corresponding carbon windows, fracture strength values range from 2.8 GPa to 3.0 GPa. These values are within the range of standard WC-Co grades with similar binder contents and WC grain sizes.
查看更多>>摘要：WC-Co hardmetals are utilized as tool materials in metal cutting applications in which they are exposed to high mechanical cyclic loads and elevated temperatures. A better understanding of the failure mechanisms of WC-Co hardmetals under these application conditions and the ability to diagnose the damage evolution state are key factors to understand the limits of endurable cyclic load at a certain temperature. The aim of the current work was the experimental determination of stress-strain-hysteresis loops for the investigation of damage indicators in uniaxial cyclic tests at a stress ratio of R = sigma(min)/sigma(max) = -1 for two WC-10 wt% Co hardmetals at 700 degrees C and 800 degrees C in vacuum. An increase in the stress-strain-hysteresis loop area and tension-compression-strain asymmetry was recorded with increasing number of load cycles at 800 degrees C, with earlier failure than at 700 degrees C. The relationship between the stress-strain-hysteresis loop parameters and the damage evolution state at the microstructure level, as well as the deformation behavior of WC- and Co-phases with increasing number of load cycles, were analyzed. To this end, the microstructure for one WC-Co hardmetal grade was analyzed by scanning electron microscopy and electron backscatter diffraction after cyclic testing up to defined numbers of load cycles at 800 degrees C. It was observed that the hysteresis loop area and strain asymmetry coincide with the formation of nanopores at WC/WC interfaces and WC/Co phase boundaries, which enlarge to form larger cavities with increasing number of load cycles. Additionally, electron backscatter diffraction data showed that the fcc Co-phase partially transformed into hcp Co under cyclic loading. All specimens, in which an increase in the stress-strain hysteresis loop area or strain asymmetry was observed, ultimately failed when a sufficiently high number of load cycles was applied. Thus, these results indicate that the investigated parameters are reliable indicators for bulk material damage.
NSTL
Elsevier