查看更多>>摘要：In this study, a set of FCC-based (CrFeNi)(100-x)C-x (x = 0,4,8) medium-entropy alloys (MEAs) were prepared by spark plasm sintering (SPS) using atomized alloy powders as raw material. As designed MEAs are systematically evaluated for their tribological performance based on carbon content effects. The CFNC0 has a single FCC structure, while CFNC4 and CFNC8 have an FCC matrix with M23C6 carbides dispersed. The increased carbon content in the MEAs not only enhances the hardness of the alloy, but also reduce the scraping and penetration of the worn surface by the counterpart ball, which significantly reduces the wear rate. Under the normal load of 20 N, the specific wear rate of CFNC8 is just 8.02 x 10(-5) mm(3)/(N.m), which is about half that of CFNC0.
查看更多>>摘要：A novel process is proposed for the preparation of tungsten powder by the hydrogen reduction of tungsten trioxide. The process changes the traditional method of flow of hydrogen from the surface of WO3 followed by the slow diffusion of hydrogen to the WO3 to achieve reduction. Furthermore, a venting method from the bottom of WO3 was used, thus achieving enhanced reduction by hydrogen flow. The process promotes hydrogen contact with the WO3 and significantly reduces the partial pressure of H2O at the reaction interface, resulting in a higher reduction conversion than the traditional method under the same conditions. The average increase in the amplitude of the novel process is 45% when the conversions of the two processes are compared at temperatures between 750 degrees C and 950 degrees C. Besides, the tungsten powder produced by the novel process is finer and more uniform. Therefore, this process significantly improved the reduction efficiency and also has great potential in energy conservation and emission reduction.
查看更多>>摘要：In the current study, a novel approach is proposed as a method for producing Tungsten Copper composite with functionally gradient structure. In this process, a green tungsten powder compact is sintered using a special designed induction sintering set up with variable velocities to create a gradient structure. Then, the copper is infiltrated into the sintered graded tungsten scaffold at 1200 degrees C at hydrogen atmosphere. To investigate the applicability of the process, tungsten powder is pressed and the compact is sintered using induction selective sintering with variable speeds (V1-V4). Tungsten skeleton was infiltrated with copper and W-Cu composite is produced. The mechanical properties, microstructure, and electrical conductivity at different zones were examined. Corresponding to the gradient structure, the composite showed gradients in hardness and electrical conductivity. The microhardness is varied from 219 Hv to 330 Hv, and electrical conductivities changed from 15 to 33%IACS. This method seems to be very interesting and promising for future industrial applications.
查看更多>>摘要：Nanoindentation was carried out on pure tungsten carbide (WC) on the basal (0001) and prismatic (1010) planes, using Berkovich and spherical indenters, in both single load and multi-load testing. The work focuses on correlating the load-displacement curves, including elastic to plastic deformation, size effect and hysteresis with the deformation behaviour of WC. With different specimen preparation processes, the elastic to plastic deformation started at different threshold loads: This observation was found to be due to the variation in surface dislocation density. Staircase deformation was observed thought to be caused by dislocation motion and the formation of slip bands; sudden displacement discontinuities in the load-displacement response - associated with dislocation loop nucleation - occurred at, or near the theoretical shear strength. Furthermore, discontinuities in load-displacement curves were also used to confirm that hysteresis loops were a result of plastic deformation, as they were absent when the loading was purely elastic.
查看更多>>摘要：TZM alloy is one of the most important Mo-based alloys that has been used for high temperature applications due to its high melting point, good thermal conductivity and low thermal expansion coefficient. However, low oxidation resistance at elevated temperatures limits the high temperature applications of TZM. In this study, B4C reinforced TZM alloy was produced and its surface was borided in one step with spark plasma sintering (SPS) method. Homogeneously mixed TZM + B4C powders were molded between B4C powders and both sintering and boriding process were performed in a single step at different pressure and holding times. The effects of sintering parameters on densification, phase formation, microstructure, hardness and oxidation behavior were investigated. The thickness of the Mo-B based protective layers varied from 215 to 430 mu m depending on holding time and pressure. Boride layer of all samples had a lower hardness value than the matrix due to porosity formation near the surface. The hardness of the boride layers varied from 5.1 to 16.1 GPa depending on the amount of porosity. The highest hardness value was measured as 17.2 GPa on the matrix of TZM5B-10-70. Mo2B-Mo2C based matrix formation and Mo-B based protective layer improved the oxidation resistance of TZM alloy by reducing mass loss caused by exposed Mo.
查看更多>>摘要：In this paper, the mechanical response of nanoindentation process of nano-twinned cubic Boron Nitride (nt-cBN) materials is investigated by molecular dynamics method. The introduction of nano-twinned structure in cBN materials can lead to superior mechanical properties than those of single-crystal cBN (sc-cBN) materials. The mechanism of material strengthening is derived by studying the dislocation propagation during the indentation process. The limitation of dislocation propagation by nano-twinned layers leads to the accumulation of dislocations in the interlayer to form immovable dislocation multi-junctions, which leads to the increase of intracrystalline back stress and raises the yield stress of the material. In the high-pressure environment, dislocations can react with twin boundary layers and split into multiple dislocations, increasing the dislocation density and the number of dislocation multi-junctions in the material. The reaction between dislocations and twin-crystal plays an important role in twin-crystal strengthening.
查看更多>>摘要：Metallographic microscope, scanning electron microscope (SEM), electron backscatter diffraction (EBSD), X-ray diffraction analysis (XRD) and mechanical properties testing methods are used to investigate the mechanical properties and microstructure of the annealed powder metallurgical (PM) rhenium (Re) at five different tem-peratures (room temperature, 800 degrees C, 900 degrees C, 1000 degrees C, 1100 degrees C). The elongation and reduction of cross section of the annealed PM Re are found to decrease with increasing temperature. Specifically, the elongation and reduction of cross section are 12.5% and 14.28% at room temperature, and drop to 0.25% and 3.975% at 1100 degrees C. The ultimate tensile strength of the annealed PM Re is also found to decrease with the increasing temperature, while the yield strength is found to first increases and then decreases with the increasing tem-perature. The ultimate tensile strength and yield strength are 884 MPa and 220 MPa at room temperature, 643 MPa and 550 MPa at 800 degrees C, 485 MPa and 467 MPa at 1100 degrees C, respectively. Observed by SEM and metallo-graphic microscope, the fracture mode is identified to be transgranular quasi-cleavage at room temperature and 800 degrees C, and a mixed fracture of inter-particle debonding and transgranular quasi-cleavage at 900 degrees C-1100 degrees C.
查看更多>>摘要：In this study, we investigated the microstructure, mechanical and wear properties of WC-12Co cemented carbides fabricated via selective electron beam melting (SEBM). Subsequently, some of the samples were treated with vacuum sintering heat treatment at 1400 degrees C at 5 MPa under argon. Apart from the WC and Co phases, the Co3W3C phase was found in the SEBM-fabricated cemented carbides. After heat treatment, WC grain growth and the Co6W6C phase appeared. Interestingly, eta phase content decreased after heat treatment. The longitudinal section of the samples exhibited a layered structure, which was interlaced with coarse and fine WC. A compressive strength value of 1839 MPa was achieved after heat treatment, while the compressive strength without heat treatment was 1770 MPa. The nanoindentation test showed that the microhardness of the heattreated samples decreased compared to the SEBM- fabricated sample (18.92 GPa). The SEBM-fabricated WC12Co cemented carbides had an average friction coefficient of 0.548 and a wear rate of 0.0055 mm3, which increased by 59.7% and 142% in the samples after heat treatment.
查看更多>>摘要：Bonding between W and steel using a 90W-9Cu-1Ni powder mixtures/Ni multi-interlayer was carried out in vacuum for 60 min at 1120 & nbsp;C with a bonding pressure of 20 MPa and a bonding pressure of 5 MPa assisted with presetting Co powder on the surface of W. The microstructures, composition distribution, and fracture characteristics of the joint were studied by SEM, EDS. Joint properties were evaluated by shear experiments tests. The results showed that the joints comprised W/W-Cu-Ni composites/Ni/steel. The W-Cu-Ni composites with a homogeneous and dense microstructure formed by liquid phase sintering of 90W-9Cu-1Ni powder mixtures prepared by high energy ball mill method. Under the bonding pressure of 20 MPa, a bonding line with a width of about 0.5 mu m and high element tungsten content formed between W-Cu-Ni composites and W, which resulted in high shear strength (275 +/- 12 MPa) of the W/steel joints and failure during testing was always located at W near the W-Cu-Ni composites/W interface. For the bonding process of bonding pressure of 5 MPa assisted with presetting Co powder on the surface of W,mu-(Co, Ni)(7)W-6 intermetallic compounds layers were formed in W-Cu-Ni composites and W, which atomic ratio of Co to Ni is 2 to 1. The average shear strength of W/steel joints bonded by this bonding process was 242 +/- 9 MPa and all joints were fractured in the Ni layer near the W-Cu-Ni composites/Ni interface.
查看更多>>摘要：In this paper cemented carbide cutting insert with the variation of cobalt alloy has been developed through sintering via microwave radiation processing technique. The Compact samples of uniform mixed WC-2Co and WC-4Co alloy powder has been placed in a microwave sintering furnace cavity and heated at an equal heating rate i.e. 40?C/min upto 1450?C. The sintered sample has been characterized for properties like densities, Vickers hardness, and fracture toughness. The relative densities of the sample are found to be 90-95% of the theoretical density. The highest Vickers hardness and toughness are found to be 1830 HV and 12.28 MPa. (m)-(1/2) for WC-2Co and WC-4Co samples. The hardness and toughness properties are found to increase with an increase in the temperature. With an increase in the percentage of binder, i.e. Co in the alloys the toughness characteristic of sintered sample is found to increase whereas hardness decreases. The sintered specimen has been modified for cutting edges accordingly to act as a cutting insert. The machining of duplex stainless steel has been performed by using sintered samples. The machining performance for the prepared tool has been compared with the standard tool. The maximum cutting force required during the machining is approximately 650 N and it is 10-12% higher than the standard tool. Surface roughness during the machining has been found to be 2.83 jam during the machining. Thus, a comparable cutting insert with high-quality mechanical properties has been developed using microwave sintering.
NSTL
Elsevier