查看更多>>摘要：? 2021In this study, we have firstly proposed a high-speed electron beam sintering technique for producing fully dense submicron-grained WC-8Co hardmetals. A distinctive feature of the sintering process is the controlled stress-strain state by varying thermal stresses during local electron beam heating at different rotation speed (0.1; 1.0; 2.5 mm/s) of a cylindrical powder compact. As a result, the sintering time of porous compacts to full density does not exceed 150 s. The experimental and calculated results indicate that the main requirement for obtaining well-sintered material is the shrinkage rate should be less or equal to the rate of thermal expansion of the sample segments subjected to repeated local heating.
查看更多>>摘要：? 2021For studying the evolution of the η phase in cemented carbides with the CoNiFeCr high entropy alloy binder, we prepared low-carbon, medium-carbon, and high-carbon cemented carbides with a 0.2 wt% difference in the C content using SHIP. The results showed that the η phase morphology was sensitive to the carbon content. When the carbon content was 4.5%, the η phase was abundant and uniformly dispersed. When the carbon content increased to 4.7%, the η phase amount decreased and became locally aggregated in large clusters, showing typical segregation. When the carbon content was further increased to 4.9%, the η phase disappeared. The η phase clusters exhibited a sharp contrast between inner and outer parts. The inner part showed the presence of the η phase, WC with small and round grains, and a small amount of binder. The outer part had only WC with large grain and sharp grains, and a large amount of binder. In addition to the inherent η phase brittleness, the η phase segregation resulted in a large hardness dispersion and a further toughness decrease.
查看更多>>摘要：? 2021Wear resistance of the rock drill bit is one of the important factors affecting drilling efficiency and exploration cost. Hence, the development of tool materials with high wear resistance has been widely concerned by researchers. The composites of cemented carbides and Ti-coated diamond (Ti/Dia) were successfully prepared by spark plasma sintering with excellent wear resistance. The characterization of microstructure and phase composition revealed that diamond particles distributed homogeneously without obviously graphitization. With 2–6 wt% Ti/Dia, the relative density, hardness and flexural strength of the composites were all improved. However, with the further increase in Ti/Dia content, the relative density and flexural strength sharply decreased. The composites with 2 wt% Ti/Dia shown the best wear resistance and the wear rate was reduced by 50 times compared with that of the sample without diamond addition. The diamond plays an important role in the rock cutting and grinding for the sample with 2 wt% Ti/Dia. However, for the samples with 18 wt% Ti/Dia, the cutting edge is easy to fracture and the diamond is more likely to pull-out whole because of the weak retention ability of the matrix to diamond.
查看更多>>摘要：We synthesized a series of Cu-Ti based crystalline and amorphous brazing filler metals (BFMs) and further investigated the microstructure characteristics and melting features for comparison. The presence of precursor film together with latent heat of crystallization showed a significant effect on the enhanced wettability of amorphous BFMs/tungsten heavy alloy (WHA) system. The wetting and spreading mechanism were addressed in a view of atomic adsorption and diffusion. The brazed joints showed effective interface bonding and high shear strength, especially at elevated temperature (up to 364 MPa at 400 degrees C). This study demonstrates the great superiority and feasibility of amorphous brazing filler metals in the joining of WHAs.
查看更多>>摘要：The CoCrFeNiMo0.4 high-entropy alloys (HEAs) were synthesized by a high-pressure solid-state reaction at 3-5 GPa and held for 3-10 min at 1200 degrees C. The microstructure and mechanical properties of the sample were influenced by the pressure and holding time, which was confirmed when the Co, Cr, Fe, Ni, and Mo atoms diffused to form HEAs. The increased pressure hindered the diffusion between atoms and thus inhibited the formation of HEAs. Under the influence of the enthalpy of mixing between elements, Fe had the strongest repulsive force on the system and the weakest diffusion effect, but increasing the holding time promoted the diffusion of Fe atoms. Compared with the microhardness reported in other studies, the maximum hardness in our work was 449.1 HV, which represents a maximum increase of approximately 80%. The synergistic effects of the solid-solution strengthening of the system and the serious lattice distortion effect caused by large Mo atoms and high pressure were the dominant mechanisms responsible for the enhanced microhardness of the sintered HEAs under extreme conditions.
Gurgel, Diego Piresde Medeiros, Regina Bertilia DantasBarreto, Lucas Pires de PaivaGomes, Uilame Umbelino...
7页
查看更多>>摘要：Tribological performance of polycrystalline diamond-tantalum (PCD-Ta) composites manufactured by powder metallurgy and HPHT sintering was studied. Different PCD/Ta concentrations were prepared and tested in a pinon-disc configuration. The tribological interactions were investigated through depths of penetration, Coefficients of Friction (CoF), wear rates, and microstructural evaluation of pairs. The overall results indicated that tantalum percentage directly influenced the tribological behavior of composite pairs, and higher wear performance was observed in samples with lower binder content. Furthermore, microstructural analyses revealed that all pairs exhibited abrasive and adhesive wear mechanisms, and their intensities were closely dependent on the Ta concentration.
查看更多>>摘要：The densification mechanism of boron carbide is revealed by considering the effect of electromigration induced by electric current during spark plasma sintering (SPS). Different electric current values are obtained by choosing different heating rate, resulting in the different dislocation density as well as densification behavior. The apparent pressure-particle neck area relationship is applied to estimate the instantaneous electric current density, then a densification model was proposed which taking it into account. Higher electric current responds to lower dislocation density and lower apparent activation energy of the mechanism controlling densification. The densification mechanism exhibits a transformation from atom-diffusion-controlled creep deformation mechanism for low electric current to dislocation-glide-controlled mechanism for high electric current.
查看更多>>摘要：The aim of the current work was the experimental determination of limit stresses in the uniaxial cyclic compression test that lead to advancing ratcheting in different WC-Co hardmetal grades at elevated temperature. At stresses below the limit stress the plastic strain per cycle reduces and plastic strain accumulation stops after a characteristic number of load cycles. Special attention was paid to the microstructural influence on the onset of advancing ratcheting and the associated damage development at the microstructure level. WC-Co hardmetals are used in various areas such as forming and forging tools, where they are exposed to high temperatures and pressure loads. Their good high-temperature properties allow them to be used under these conditions, but these properties are influenced by the microstructure. Investigations of the mechanical properties of hardmetals had been carried out under monotonously increasing loads and cyclic tests at room temperature and elevated temperatures. In these tests, the effect of different stress ratios R = sigma(min)/sigma(max) on the fatigue behaviour of hardmetals were studied, However, no studies are known for WC-Co hardmetals and their limit stresses in relation to strain ratcheting under cyclic compressive loading at elevated temperature. Hence, in the current work the influence of increasing stresses as a function of microstructure and their effect on the evolution of the strain of six different WC-Co hardmetal grades are discussed. For this purpose, the materials were investigated by uniaxial cyclic compression tests at a stress ratio of R = -infinity at 700 degrees C and 800 degrees C in vacuum. The investigated hardmetal grades differ on the one hand in their WC grain size, which varies between 0.4 mu m and 2.0 mu m, and on the other hand in their Co-content, which varies between 6 wt% and 12 wt%. The residual strain value epsilon(res) at zero applied stress was observed to stabilize with increasing number of load cycles at low applied stress ranges. Strain ratcheting occurred above a critical stress range, referred to as the limit stress for strain ratcheting. Strain ratcheting is the accumulation of plastic strain with increasing number of load cycles in which no strain stabilization occurs. Further, for all investigated hardmetal grades, the limit stresses were observed to decrease with increasing temperature. In the following, the microstructure of one hardmetal grade was analysed after loading below and above the limit stress by scanning electron microscopy and electron backscatter diffraction (EBSD). The influence of strain stabilization and ratcheting was analysed with regard to damage development and deformation behaviour of the WC and Co-phases. Strain ratcheting was observed to result in the formation of cavities and nanopores at phase boundary triple points and WC/Co interfaces. Additionally, the EBSD data showed that the fcc Co phase was transformed into hcp Co. Therefore, it is assumed that on the one hand, a certain strain value needs to be exceeded for strain ratcheting to occur and, on the other hand, that besides dislocation movement, microdefect formation and phase transformation significantly contribute to the increase in strain.
查看更多>>摘要：Wear resistance of the rock drill bit is one of the important factors affecting drilling efficiency and exploration cost. Hence, the development of tool materials with high wear resistance has been widely concerned by researchers. The composites of cemented carbides and Ti-coated diamond (Ti/Dia) were successfully prepared by spark plasma sintering with excellent wear resistance. The characterization of microstructure and phase composition revealed that diamond particles distributed homogeneously without obviously graphitization. With 2-6 wt% Ti/Dia, the relative density, hardness and flexural strength of the composites were all improved. However, with the further increase in Ti/Dia content, the relative density and flexural strength sharply decreased. The composites with 2 wt% Ti/Dia shown the best wear resistance and the wear rate was reduced by 50 times compared with that of the sample without diamond addition. The diamond plays an important role in the rock cutting and grinding for the sample with 2 wt% Ti/Dia. However, for the samples with 18 wt% Ti/Dia, the cutting edge is easy to fracture and the diamond is more likely to pull-out whole because of the weak retention ability of the matrix to diamond.
查看更多>>摘要：The novel oscillating pressure sintering (OPS) technique was used to design and prepare 90 W-10Cu (mass fraction) refractory alloys under lower sintering temperature, and the effects of sintering temperature on microstructure, density, grain size and Vickers hardness were invested. When the sintering temperature was 1200 degrees C, the density and Vickers hardness of the OPS sample are 99.38% and 168.7 HV1, respectively, which are higher than the hot-pressed sintered samples at the same temperature. The density of W-Cu refractory alloys could be effectively promoted under lower temperature by the oscillating pressure sintering. Moreover, the grain size could be reduced activity within 4 mu m during the sintering process. This is mainly due to the liquid phase binder fully filling the W grains under oscillating pressure to form the closer network microstructure, which promotes improvement of the density and performance. The results show that the oscillating pressure sintering with low temperature is beneficial to the preparation of W-Cu refractory alloy with higher density, fine-grained and well property.
NSTL
Elsevier