The sintering behavior of quasi-spherical tungsten nanopowders

In this work, the sintering behavior of quasi-spherical tungsten nanoparticles was investigated by analysis the sintered compacts obtained at different sintering temperatures and dwell time, and the influence of microstructures on the density and Vickers microhardness of sintered products was also studied. Experimental results show that particle shape and size distribution are critical to the sintering activity and mechanical properties of obtained compacts. 91.3% of theoretical density (TD) of the compact could be obtained at low sintering temperature of 1500 °C, and the highest hardness of 606 VHN could be achieved when sintered at 1100 °C due to formation of uniform, densely packed sintered compacts with grain size of 235.7 nm. Importantly, unusual linear correlation between grain size and relative density was observed in our experiment, and a cut-off point exists at 85.6% of TD. The kinetic analysis revealed that surface diffusion is responsible for the mass transport during the initial sintering stage.     

Effect of heating mode on sintering of tungsten

Microwave heating is recognized for its various advantages, such as time and energy saving, very rapid heating rates, considerably reduced processing cycle time and temperature, fine microstructures and improved properties. The present paper investigates the feasibility of consolidating tungsten powders through microwave sintering. A comparative analysis has also been attempted between the sintering response of pure tungsten powder compact in a microwave and conventional furnace.     

Thin intergranular films and solid-state activated sintering in nickel-doped tungsten

Nickel-doped tungsten specimens were prepared with high purity chemicals and sintered. Although activated sintering starts more than 400 °C below the bulk eutectic temperature, the nickel-rich crystalline secondary phase does not wet the tungsten grain boundaries in the solid state. These results contrast with the classical activated sintering model whereby the secondary crystalline phase was presumed to wet grain boundaries completely. High resolution transmission electron microscopy and Auger electron spectroscopy revealed the presence of nanometer-thick, nickel-enriched, disordered films at grain boundaries well below the bulk eutectic temperature. These interfacial films can be regarded as metallic counterparts to widely observed equilibrium-thickness intergranular films in ceramics. Assuming they form at a true thermodynamic equilibrium, these films can alternatively be understood as a class of combined grain boundary disordering and adsorption structures resulting from coupled premelting and prewetting transitions. It is concluded that enhanced diffusion in these thin intergranular films is responsible for solid-state activated sintering.     

Effects of low-content activators on low-temperature sintering of tungsten

Tungsten compacts are processed by the activated sintering technique at low temperatures near 1200 °C, whereby either nickel, iron or a combination of the two are employed as additives. Despite very small contents of additives of a few monolayers and below, the processing technique is found viable for providing enhancing effects to the sintering kinetics of tungsten, and the sintered materials remain in the partially densified stage. Nickel is proved to be a superior activator, which induces a reduction of the activation energy as its content increases.     

Consolidation and properties of binderless sub-micron tungsten carbide by field-activated sintering

The rapid sintering of nano-structured WC hard materials in a short time is introduced with a focus on the manufacturing potential of this spark plasma sintering process. The advantage of this process allows very quick densification to near theoretical density and prohibition of grain growth in nano-structured materials. A dense pure WC hard material with a relative density of up to 97.6% was produced with simultaneous application of 60 MPa pressure and electric current of 2800 A within 2 min. A larger current caused a higher rate of temperature increase and therefore a higher densification rate of the WC powder. The finer the initial WC powder size the higher is the density and the better are the mechanical properties. The fracture toughness and hardness values obtained were 6.6 MPa m^1/2 and 2480 kg/mm², respectively under 60 MPa pressure and 2800 A using 0.4 μm WC powder.     

The modeling of electric-current-assisted sintering to produce bulk nanocrystalline tungsten

Nanocrystalline tungsten has the potential to have superior strength and hardness properties versus conventional tungsten. While tungsten nanopowders are becoming more commonly available, processing through conventional press and sinter techniques induce unacceptable grain growth. As an alternative, electric-current-assisted sintering (commercially known as SPS or PPC) allows extremely high heating rates (>1,000°C/min.) to be achieved which accelerates the consolidation process and preserves the nanocrystalline structure of the material. The high heating rates can, however, lead to non-uniform density and compromised properties. This work employs numerical simulations of the process as a means to understand and reduce these gradients and optimize the process.     

超细钨粉的粒度表征

对供给态和研磨态超细钨粉的颗粒粒度进行了表征。将供给态粉和研磨态粉由激光衍射法、FSSS法和BET法测量出颗粒粒度。结果表明 :利用激光衍射法和FSSS法所测量出来的供给态粉和研磨态粉的粒度结果是错误的 ,这是因为测量系统的缺陷和测量原理的不合适导致的。可以使用吸附等温线来获得表面粗糙度的分数维维数D和微孔的表面积St,且用D和St 来修正dBET的计算公式。使用修正公式得到的 4种粉末的平均粒度值与扫描电镜的观测值相一致     

TiC颗粒增强钨基复合材料的烧蚀性能

用自制的氧乙炔烧蚀装置对ＴｉＣ颗粒增强钨基复合材料的烧蚀性能进行了测试,同时用多波长比色高温计对烧蚀试样表面温度和用热电偶对试样背面温度进行了在线监测。复合材料的质量烧蚀率和和线烧蚀率由低到高的排列顺序为：Ｗ〈３０％ＴｉＣｐ／Ｗ〈４０％ＴｉＣｐ＝Ｗ。ＴｉＣ颗粒加入到Ｗ中可明显提高材料的抗烧蚀性能,而且ＴｉＣ颗粒含量越高,材料的抗烧性能越好。ＴｉＣｐ／Ｗ复合材料的烧机理是Ｗ和ＴｉＣ的氧化烧和燃气流的     

Effects of tungsten particle shape and size on dynamic deformation and fracture behavior of tungsten heavy alloys

Dynamic torsional tests were conducted using a torsional Kolsky bar for five alloys, one of which was fabricated by double-cycled sintering process, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. The double-sintered tungsten alloy specimen whose tungsten particles were very coarse and irregularly shaped showed cleavage fracture in the central area of the gage section with little shear deformation, whereas shear deformation was concentrated in the central area of the gage section in the other alloys. The deformation and fracture behavior of the double-sintered alloy correlated well with the observation of the impacted penetrator specimen and thein situ fracture test results,i.e., microcrack initiation at coarse tungsten particles and cleavage crack propagation through tungsten particles. These findings suggested that the cleavage fracture mode would be beneficial for the serf-sharpening effect and, thus, the improvement of the penetration performance of the double-sintered tungsten heavy alloy would be expected.     

Anisotropic shrinkage induced by particle rearrangement in sintering

A microscopic model is presented which describes the anisotropic shrinkage induced by particle rearrangement during sintering. Two types of topological transitions in the rearrangement, that is the formation of a new contact, are illustrated in three-dimensional simulation of sintering of four spheres arranged in a rhombus. The shrinkage rate is shown to become anisotropic as the particle coordination number changes with rearrangement. A micromechanical principle is discussed with the use of sintering force and effective viscosity. The anisotropic shrinkage is caused by the additional sintering force acting on the new grain boundary formed at the rearrangement.     

强化石灰烧结法中硅的行为研究

针对我国铝土矿资源特点和我国烧结法生产氧化铝的现状,对强化石灰烧结法中硅的行为进行研究。采用铝硅比为3.84的矿石,按照钙铝比为1.3~1.5进行配料,在1270℃以上进行熟料烧成。实验结果表明:熟料烧成无困难,熟料中氧化铝含量达到了48%~53%,熟料中主要物相为CaO.Al2O3和2CaO.Al2O3.SiO2,烧结熟料溶出性能好;烧结熟料溶出后粗液中硅含量仅为20mg/L~30mg/L,硅量指数可达1000以上,可省去专门的脱硅工序;提出用"纯氧化铝溶出率"作为烧结法的经济指标,强化石灰烧结法用A/S比为3.84的矿物达到了强化碱石灰烧结法A/S7的指标,且碱耗更低。     

Enhancement of densification and sintering behavior of tungsten material via nano modification and magnetic mixing processed under spark plasma sintering

In the present study, the influence of nano additives (Ni, Fe) and different mixing (turbular and magnetic) on the densification, microstructure and micro-hardness of the tungsten material under spark plasma sintering is analyzed. After turbulent mixing the nanoparticles are distributed widely in the W interparticle gaps but after magnetic mixing the nanoparticles are distributed not only on the gaps of the W particles but also on the broken surfaces. Ni incorporated tungsten materials achieved the maximum density of 98.3% at 1400 °C (turbular mixing) and 97.9% at 1300 °C (magnetic mixing). Fe incorporated tungsten material showed density of 97.7% at 1600 °C and 97.2% at 1400 °C after turbular and magnetic mixing. The influence of nanoparticles in the densification process was explained by Laplace force, boundary slip and Agte-Vacek effect. The microstructural analysis showed that nano-modification reduced the degree of porosity, and provides a compact material at low temperatures. X-ray fluorescence analysis reveals that magnetic mixing shows more uniform distribution of nanoparticles than turbular mixing. The nanoparticles incorporation increased the micro hardness of tungsten material. Hence, it is clear that magnetic mixing and nano modification greatly improved the densification and sintering behavior of the tungsten material.     

The effect of yttrium oxide on the sintering behavior and hardness of tungsten

In this study, the relative density and hardness of Y₂O₃ dispersed tungsten alloy were investigated as functions of the Y₂O₃ content and sintering temperature. The sintering temperature and the amount of the second phase were varied from 1800 to 2500°C and 0 to 2.0 weight pct, respectively. The relative density of the alloys is higher than that of pure tungsten in the range from 2000 to 2500°C, whereas the density is lower at 1800°C. As the Y₂O₃ content increases, the sintered density increases at a given temperature. The transition temperature (T_tr), where the relative density of the alloys exceeds that of pure tungsten, is reduced with increased Y₂O₃ particle content. In order to examine the effect of the second phase on the mechanical property, the hardness of pure tungsten and the alloys are measured. The hardness is mainly dependent upon the relative density of the alloys, rather than the amount of the second phase and tungsten grain size. The relationship between hardness and density is discussed according to the plasticity theory of porous materials.     

Direct solid-state synthesis of tungsten carbide nanoparticles from mechanically activated tungsten oxide and graphite

Solid-state carbothermic reduction of tungsten oxide (WO₃) to nano-sized tungsten carbide (WC) particles was achieved by calcining mechanically activated mixtures of WO₃ and graphite at 1215 °C under vacuum condition. By experiments and thermodynamic calculations, the intermediate phases, WO_2.72, WO₂ and metallic tungsten (W), were observed at 741 °C, which decomposed to synthesize the final product (WC). Homogeneity increase and associated decrease in the diffusion path by mechanical milling and formation of these intermediates are mainly responsible for the successful production of WC. The process indicates that solid-state synthesis of WC nanoparticles directly from as-milled mixtures of tungsten oxide and graphite powder is possible.     

Mechanical properties of tungsten heavy alloy and damage behaviors after hypervelocity impact

93W-4.5Ni-1.5Fe-1Co(W) was prepared by powder metallurgic method, and then dynamic mechanical properties of this material were tested at high temperature by means of high-temperature split Hopkinson pressure bar(SHPB). The results show that the material possesses highdynamic mechanical properties, significant temperature effects, and strain hardening behaviors. Used two-stage light gas gun, the penetration test of 93 W projectile was finished. After the completion of the test, through the microstructure observation of the residual 93 W projectiles with the aid of scanning electron microscopy(SEM) and transmission electron microscopy(TEM), it can be found that there are obvious signs of hot melt existing on the surface of the projectile, a lot of adiabatic shear bands inside the projectile, and microcracks exist at the end of adiabatic shear bands. The test results show that adiabatic shear is the main form to cause the projectile failure and it is the emergence of the adiabatic shearing phenomenon that makes 93 W display good self-sharpening property in the process of hypervelocity penetration. At the same time,the results of TEM observation show that there are highdensity dislocations at the interface between W and Ni–Fe–Co-based alloy inside the 93 W.     

A study on the sintering of ultrafine grained tungsten with Ti-based additives

Tungsten (W) is a brittle material at room temperature making it very difficult to fabricate. Although the lack of ductility remains a difficult challenge, nano-sized and ultrafine grain (UFG) microstructures offer potential for overcoming tungsten's room temperature brittleness. One way to manufacture UFG W is to compact and sinter nanosized W powder, however, it is a non-trivial task to control grain growth during sintering. In an effort to inhibit grain growth, the effect of Ti-based additives on the densification and grain growth of nano-W powders was investigated in this study. The addition of 1 wt.% Ti into tungsten led to more than a 63% decrease in average grain size of sintered samples at comparable density levels. It was also found that sintering in Ar yielded a finer grain size than sintering in H₂ at similar densities. Compared to conventional high temperature sintering, a lower temperature sintering cycle for a longer hold time resulted in both near-full density and fine grain size. The roles of the Ti additive include not only the inhibition of grain growth, but also the potential absorption of oxygen from W particles.     

Investigation on sintering mechanism of nanoscale tungsten powder based on atomistic simulation

Atomistic simulations focusing on sintering of crystalline tungsten powders at the submicroscopic level are performed to shed light on the processing on the nanoscale powders. The neck growth and shrinkage were calculated during these sintering simulations, making it possible to extend these results to the evolution of global physical properties that occurs during sintering. The densification and grain growth during sintering were calculated with variations in temperature, pressure, particle configuration and crystalline misalignment between particles. These findings lay the foundation for a virtual approach to setting the processing cycles and materials design applicable to nanoscale powders.     

喷雾造粒制备SiC-AIN复合粉体特性及烧结性能

采用水基料浆与流态化喷雾造粒相结合制备SiC-AIN复合粉体,分析复合粉体的粉体特性、成形性能及烧结特性,探讨SiC-AIN复相陶瓷的增强增韧机制.结果表明:喷雾造粒后,复合粉体的流动特性显著提高,粒度级配合理;随着压强增加,坏体密度在40～80、80～160和160～220 MPa范围内呈现阶梯式增长,160 MPa以上成形后素坯均匀致密,无硬球颗粒存在;无压烧结SiC-AIN复相陶瓷具有优越的烧结性能和力学性能,这是由于AIN对SiC晶粒形成生长势垒,并反应生成2H型固溶体,从而细化晶粒,导致裂纹扩展产生了绕道与偏转效应,呈现晶粒撕裂与拨出现象,协同改善了复相陶瓷的强度及断裂韧性.     

Effects of sintering conditions on mechanical properties of mechanically alloyed tungsten heavy alloys

The effects of sintering conditions on the microstructural evolution and mechanical properties of mechanically alloyed tungsten heavy alloys were investigated. W, Ni and Fe powders were mechanically alloyed in a tumbler ball mill at a milling speed of 75 rpm, ball-to-powder ratio of 20∶1 and ball filling ratio of 15%. The mechanically alloyed powders were compacted and solid-state sintered at a temperature of 1300°C for 1 hour in a hydrogen atmosphere. The solid-state sintered tungsten heavy alloy was subsequently liquid-phase sintered at 1470°C with varying sintering times from 4 min to 90 min. The solid-state sintered tungsten heavy alloy showed fine tungsten particles of 3 μm in diameter and high relative density above 99%. The volume fraction of the W-Ni-Fe matrix phase was measured, as 11% and tungsten/tungsten contiguity was 0.74 in solid-state sintered tungsten heavy alloys. Mechanically alloyed and two-step sintered tungsten heavy alloys showed tungsten particles of 6–15 μm and a volume fraction of the W-Ni-Fe matrix phase of 16% and tungsten/tungsten contiguity of 0.40. The solid-state sintered tungsten heavy alloy exhibited a yield strength of about 1100 MPa due to its finer tungsten particles, while it showed low elongation and impact energy due to its large tungsten/tungsten contiguity. The yield strength of two-step sintered tungsten heavy alloys increased with the decreasing of tungsten particle size and volume fraction of the W-Ni-Fe matrix. This article is based on a presentation made in “The 4th International Conference on Fracture and Strength of Solid”, held at POSTECH, Pohang, Korea, August 16–18, 2000 under the auspices of Far East and Ocean Fracture Society (FEOFS)et al.     

钢铁厂烧结机头灰中铅的配位浸出行为与综合回收

针对钢铁厂烧结机头灰中富含铅、铁、碳、钾、氯等多种有价元素的特点,根据氯离子与铅配位的特性,采用配位浸出的方式实现铅与铁、碳等元素的选择性分离回收。SEM-EDS、XRD等研究分析表明,烧结机头灰中铅主要以絮状的KPb₂Cl₅等物相吸附于铁氧化合物、硅铝酸盐和碳颗粒表面,铁主要以Fe₂O₃和Fe₃O₄物相存在。实验考察了溶液pH值、温度、氯离子浓度、浸出时间和液固比等因素对铅浸出率的影响。研究表明,在溶液pH值为3.0,浸出温度为80℃,氯离子浓度为6 mol/L,液固比(mL/g)为10:1,浸出时间为2 h的优化条件下,烧结机头灰中铅化合物与氯发生配位溶解反应生成PbCl_i^2-i(i=1～4)等易溶解的络合离子,实现铅的浸出,铅浸出率为95.7%;而烧结机头灰中对钢铁冶炼有用的铁、碳、硅、铝等元素不被浸出,富集在浸出渣中,较好地实现了选择性浸出。浸出液中的铅经冷却结晶、洗涤纯化后,获得纯度为99%的氯化铅产品...