液相烧结制备电力开关用W-7Cu-xNi合金微观组织及性能分析

为了提高电力开关用W-7Cu合金的综合性能,通过添加Ni元素方式对其进行加强,以液相烧结、机械球磨方式制备得到包含不同Ni含量的W-Cu合金。通过实验测试手段对其微观组织及物理性能进行了施压测试分析。研究结果表明：逐渐提高Ni含量后,形成了更大尺寸W颗粒,相邻颗粒间距降低。W-Cu-4%Ni合金界面形成更优润湿角,获得具有连续网状分布的Cu相,改善了W-Cu组织的分布均匀性,此时Ni元素已经完全与Cu相相溶。当Ni含量提高后,W-Cu合金获得了更大的硬度与致密度,热导率下降,相对密度增加。当加入4%的Ni时,致密度达到了95.6%,热导率从161 W/(m·K)降低为96.4 W/(m·K),获得了致密度更大合金。     

细晶W-Cu合金的高温拉伸力学行为与组织演变

研究了平均晶粒度在0.5μm以下细晶W-40Cu和W-50Cu合金在200~800℃范围内的高温拉伸力学行为,并结合SEM断口形貌分析了材料在高温状态下的断裂形式及其组织变化规律。结果表明:W-Cu合金拉伸强度随温度升高而迅速降低,其延伸率在室温至400℃温度区间时变化不大;当温度大于400℃时,合金延伸率迅速上升。拉伸断口特征表明:在室温条件下,细晶W-Cu合金的断裂主要包括W晶粒的沿晶断裂与Cu相的延性撕裂;温度在400℃时,Cu相开始软化,但合金材料受铜的"中温脆性"影响而使得材料的断裂延伸率变化不大;当温度达到800℃时,材料的断裂方式主要受Cu相的影响而表现出很好的延性断裂。     

轧膜成形制备W-Cu层状功能梯度材料及其性能研究

以W粉和电解Cu粉为原料,聚乙烯醇缩丁醛(PVB)为粘结剂,通过有机基轧膜工艺制备出3种组成的单层生坯(Cu质量分数分别为25%、50%、75%),再叠层共轧,制备出了具有不同粘结剂含量的W-Cu层状梯度材料生坯,之后在H2气氛中烧结,获得了W-Cu层状梯度材料,考察了粘结剂含量与制备工艺条件对材料显微组织和性能的影响。结果表明,通过单层轧制、叠层共轧共烧可以制得层状梯度W-Cu复合材料;粘结剂含量对W-Cu层状梯度材料的致密度和性能有着明显的影响。当粘结剂质量分数为6%时,轧膜坯有较好的成形性,且成形坯的孔隙率较低;所得多层生坯经1 150℃烧结后相对密度达93.11%;所得梯度W-Cu材料有良好的物理、力学性能。     

A Phenomenological Analysis of Sintering Mechanisms of W-Cu from the Effect of Copper Content on Densification Kinetics

The effect of addition of copper on the sintering of a W powder was systematically investigated by the analysis of dilatometric experiments on W and W-Cu compacts prepared with submicrometric powders. A pure W powder compact and a W-10 wt pct Cu powder compact with the same packing fraction of W particles were first studied, in order to analyze the effect of copper at fixed microstructure of the solid W particle packing. A more systematic set of experiments with different copper contents and W particle sizes was also qualitatively analyzed. A phenomenological model of sintering was developed and fitted in order to extrapolate the effect of copper content on sintering kinetics at fixed microstructure of the W particle skeleton. An interpretation of the sintering mechanisms was then proposed. Sintering of a W-Cu powder compact is the result of solid-state sintering of the W skeleton, enhanced by the capillary forces exerted by copper, with the superimposition of a particle rearrangement step after copper melting.     

Effect of cobalt addition on the liquid-phase sintering of W-Cu prepared by the fluidized bed reduction method

A new process, fluidized bed reduction (FBR) method, was applied for fabrication of uniform W-Cu sintered material. Liquid-phase sintering was carried out to obtain fully densified W-Cu composite, and the effect of cobalt addition on the sintering behavior was investigated. It was found that fully densified material could not be obtained even after sintering at 1200 °C for 4 hours in the case of 75W-25Cu, while more than 96 pct density could be obtained as soon as the sintering temperature reached 1200 °C when 0.5 wt pct cobalt was added prior to the sintering. It has been found that the wetting angle of the liquid copper is reduced significantly by the addition of cobalt, and the formation reaction of Co₇W₆ intermetallic compound at the surface of the tungsten powder is mainly responsible for the enhancement of the densification process.     

Synthesis of nanostructured WC-12 pct Co coating using mechanical milling and high velocity oxygen fuel thermal spraying

A nanostructured WC-12 pct Co coating was synthesized using mechanical milling and high velocity oxygen fuel (HVOF) thermal spraying. The variation of powder characteristics with milling time and the performance of the coatings were investigated using scanning electron microscope (SEM), X-ray, transmission electron microscope (TEM), thermogravimetric analyzer (TGA), and microhardness measurements. There is no evidence that indicates the presence of an amorphous phase in the sintered WC-12 pct Co powder, and the binder phase in this powder is still crystalline Co. Mechanical milling of up to 20 hours did not lead to the formation of an amorphous phase in the sintered WC-12 pct Co powder. During the initial stages of the milling, the brittle carbide particles were first fractured into fragments and then embedded into the binder phase. This process gradually formed polycrystal nanocomposite powders of the Co binder phase and W carbide particles. The conventional cold welding and fracturing processes primarily occurred among the Co binder powders and polycrystal composite powders. The nanostructured WC-12 pct Co coatings, synthesized in the present study, consist of an amorphous matrix and carbides with an average particle diameter of 35 nm. The coating possesses an average microhardness of 1135 HV and higher resistance to indentation fracture than that of its conventional counterpart.     

Effect of liquid content on distortion and rearrangement densification of liquid-phase-sintered W-Cu

Tungsten and copper exhibit negligible solubility for each other, so densification during liquid phase sintering of W-Cu is limited to rearrangement of the W particles and solid-state sintering of the W skeleton. Experiments are conducted to evaluate the effects of Cu volume fraction on liquid phase sintering of W-Cu. Sintered microstructures are quantitatively analyzed and are used to define critical microstructural parameters that prevent distortion and rearrangement densification. Slumping is prevented first by capillary forces, then by formation of a rigid W skeleton at critical values of contiguity and connectivity, which depend on the dihedral angle. A refined expression for the dependence of contiguity on volume fraction that includes the effect of the dihedral angle is developed. An analysis of gravitational, capillary, and bonding forces acting on W particles in liquid Cu explains the ability to achieve high sintered densities through rearrangement despite a lack of distortion with up to 80 vol pct liquid. Capillary forces are sufficient to break weak solid-solid bonds that form during heating, enabling rearrangement to occur even without dissolution of these bonds. At higher solid volume fractions, sufficient particle contacts form to prevent rearrangement by these capillary forces, thus limiting sintered densities.     

粉末温轧工艺对W-20Cu合金密度和显微组织的影响

在W-Cu混合粉末中加入0.1%~2.0%(质量分数)的有机添加剂,在60~150℃温度下温轧成生板坯,然后进行液相烧结,获得W-20Cu合金板材。通过正交试验研究粉末轧制速度、轧制温度与添加剂含量对生板坯密度的影响,并对烧结板材的密度和显微组织进行分析与表征。结果表明,轧制温度与添加剂含量对粉末轧制板坯密度有显著影响,二轧制速度对生板坯密度的影响较小。随轧制温度升高,W-20Cu生板坯的密度增大,烧结板材的孔隙尺寸逐渐减小,孔隙率逐渐降低,烧结密度相应提高;随添加剂含量增加,板坯密度先升高后降低。在轧制温度为150℃,添加剂含量为0.3%时,生板坯的相对密度达到最大值85.38%,液相烧结后获得相对密度为99.65%的W-20Cu合金板材,金属Cu元素在钨基体中均匀、弥散分布。     

The high-temperature electron emission and vaporization of tungsten-osmium alloys

Effective work functions from the polycrystalline surfaces of three arc-melted binary tungsten-osmium alloys of varying composition and one sintered tungsten-osmium alloy were examined. Alloy content varied from 4.76 to 13.05 wt pct osmium. Emphasis was placed on obtaining data over a wide temperature range and maintaining as high a vacuum as possible. Work functions were obtained by the thermionic method with high-temperature electron emission measured by a guard-ringed vacuum emission vehicle (VEV) designed to use a Schottky technique. The surface osmium activity was analyzed by employing a high-temperature quad-rupole mass spectrometer designed to detect subliming vapors from a freely vaporizing solid material. The work function of the sintered W-5 pct Os and arc-melted W-5 pct Os did not vary with an increase in temperature. Likewise, the arc-melted W-9 pct Os and W-13 pct Os both exhibited a constant work function. The highest observed work function (4.92 eV) was obtained at temperatures of 2100 and 2200 K from the W-13 pct Os sample. The W-13 pct Os sample yielded high work functions (4.85 to 4.92 eV) over the entire temperature range studied(i.e.), 1700 to 2800 K).     

Mechanical Properties of β–Ti-35Nb-2.5Sn Alloy Synthesized by Mechanical Alloying and Pulsed Current Activated Sintering

A developed Ti-35?pct Nb-2.5?pct Sn (wt pct) alloy was synthesized by mechanical alloying using high-energy ball-milled powders, and the powder consolidation was done by pulsed current activated sintering (PCAS). The starting powder materials were mixed for 24 hours and then milled by high-energy ball milling (HEBM) for 1, 4, and 12 hours. The bulk solid samples were fabricated by PCAS at 1073?K to 1373?K (800 °C to 1100 °C) for a short time, followed by rapid cooling to 773?K (500 °C). The relative density of the sintered samples was about 93?pct. The Ti was completely transformed from ?? to ??-Ti phase after milling for 12 hours in powder state, and the specimen sintered at 1546?K (1273 °C) was almost transformed to ??-Ti phase. The homogeneity of the sintered specimen increased with increasing milling time and sintering temperature, as did its hardness, reaching 400?HV after 12 hours of milling. The Young??s modulus was almost constant for all sintered Ti-35?pct Nb-2.5?pct Sn specimens at different milling times. The Young??s modulus was low (63.55 to 65.3 GPa) compared to that of the standard alloy of Ti-6Al-4V (100 GPa). The wear resistance of the sintered specimen increased with increasing milling time. The 12-hour milled powder exhibited the best wear resistance.     

一种制备W-Cu复合材料的新工艺

随着微电子信息技术的发展,W-Cu复合材料被用作基片、连接件和散热元件等热沉材料,因而具有更广泛的用途.采用喷雾干燥-氢还原法制备了W-10Cu、W-15Cu和W-20Cu(Cu的质量分数依次为10%、15%和20%)超细钨铜复合粉,并经过成形和烧结制得W-15Cu合金,测量了烧结后的合金导热性能.结果表明,在一定的还原条件下,可以获得粒度细小、氧含量低、钨铜复合均匀的W-Cu复合粉末;W-15Cu合金的相对密度可以达到99.34%,结构组织高度均匀、一致,热导率为184.0 W/m·K,已达到其做为热沉材料的热性能要求.     

A study on the improvement of the sintered density of W-Ni-Mn heavy alloy

Liquid phase sintering behavior of 90W-6Ni-4Mn heavy alloy has been studied. The present work takes into account the thermodynamic oxidation/reduction reactions of the constituent elements W, Ni, and Mn. The sintering cycle consists of heating under high purity nitrogen gas, holding at reduction temperatures after the atmosphere is changed to dry hydrogen, and sintering at 1260 °C for 1 hour. As the reduction temperature increases from 1050 °C to 1200 °C, the relative sintered density increases from 92 pct, reaching 100 pct at temperatures above 1150 °C. The relative density increases with increasing holding time at the reduction temperature and remains unchanged with heating rate. The sintered microstructure has been analyzed by a scanning electron microscope and energy dispersive X-ray spectroscopy. The relative density was compared with those obtained from other investigators. It is found that the formation of manganese oxide due to reducing reactions of W and Ni powders should be avoided in order to obtain a W-Ni-Mn heavy alloy without pores.     

用机械活化与化学活化方法制备 W-Cu 合金

为了改进制备工艺和提高W－Cu合金的性能,对原材料粉末做了机械活化和化学活化处理,通过成形和烧结制备了W－Cu合金,考察了经机械活化和化学活化后粉末的变化,观察了烧结合金的组织、测试了合金的密度等性能。结果表明,机械活化可以使粉末颗粒变细,至亚微米乃至纳米级、比表面增大、缺陷增多,并使铜在钨中具有一定溶解度。化学活化可以在粉末颗粒表面形成微量合金元素的较均匀分布,并通过反应形成高活性层。二者都能使粉末的活性提高,对经活化处理的粉末施以烧结可以获得较高密度和性能的W－Cu合金。     

The Effects of Composition and Microstructure on the Thermal Conductivity of Liquid-Phase-Sintered W-Cu

Liquid-phase sintering of high-purity, submicron, co-reduced W-15Cu powders at temperatures of 1463 to 1623 K (1190 to 1350 °C) produces W grain sizes ranging from 0.6 to 1.2 μm while maintaining less than 2 pct porosity. Measured thermal conductivities of 185 to 221 W/(m·K) are related to the grain size and contiguity, which ranged from 0.51 to 0.62. The effects of composition and microstructure on thermal conductivity are further investigated with a model based on a computational cell that allows adjustment of the grain shape to produce selected matrix volume fractions and contiguities. The model considers porosity, the effects of transition metal impurities on the thermal conductivities of the W and Cu phases, and the role of an interfacial resistance between W grains. The effects of grain size and contiguity on thermal conductivity are shown for thermal boundary conductances ranging from 0 to 1.7 × 10¹⁰ W/(m²·K). Comparison of the model predictions with those of prior models, the experimental results, and previously reported thermal conductivities shows that impurities are highly detrimental to the thermal conductivity, but the thermal boundary conductance is a significant factor for high-purity W-Cu.     

Structural Evolution of Nanocrystalline Nickel-Tungsten Alloys Upon Mechanical Alloying with Subsequent Annealing

In the current study, the two alloys, Ni-20 at. pct W and Ni-35 at. pct W, were mechanically alloyed and subsequently heat treated to evaluate their structural variations using X-ray diffraction, scanning, and transmission electron microscopy, and differential thermal analysis. In addition, the effect of Fe contamination on the progress of mechanical alloying was investigated. The results showed that the Ni-20 at. pct W contained only Ni(W) solid solution even after prolonged milling times, while the Ni-35 at. pct W was amorphized after 40 hours of milling. The composition of the amorphized alloy was estimated to be Ni-31 at. pct W. Furthermore, it was demonstrated that the nanocrystalline NiW intermetallic compound was stable at temperatures greater than 1303 K (1030 °C) and did not completely vanish upon peritectoid reaction. Consequently, an exceptional grain coarsening resistance was observed at high temperatures near the melting points. The mechanisms involved in this outstanding thermal stability were also probed.     

Phase equilibria effects on the enhanced liquid phase sintering of tungsten- copper

The sintering behavior and mechanical properties of W-Cu are improved by the addition of elements that have solubility for W,e.g., Co, Ni, Fe, and Pd. The degree of enhancement with small concentrations of additive is dependent on specific phase diagram features, and the ranking of effectiveness does not follow the trend observed for the activated solid-state sintering of W. These observations are explained through a combination of liquid phase sintering and activated sintering theories that considers the combined W, Cu, and activator phase equilibria effects. In small concentrations, Ni and Pd have little effect on densification because they go into solution with Cu, resulting in only a slight increase in the solubility of W in the liquid phase. In this case, the sintered density, strength, and hardness increase with increasing additive concentration due to enhanced densification through solution-reprecipitation. Cobalt and Fe are the most ef-fective activators due to their limited solubility in Cu and the formation of a stable intermetallic phase with W at the sintering temperature. This promotes the formation of a high-diffusivity interboundary layer which enhances solid-state sintering of the tungsten grains at temperatures at which a liquid phase is present. With Co and Fe additions, the sintered density, strength, and hardness peak with activator concentrations of 0.35 to 0.5 wt pct. An evaluation of models for activated solid-state sintering and liquid phase sintering indicates a substantial solid-state contribution to densification when a high-diffusivity interboundary layer is present and the sol-ubility of W in the liquid phase is small.     

W-15Cu合金制备中钨骨架孔隙控制的研究

钨铜复合材料是由高温烧结钨骨架经渗铜制成的两相假合金,钨骨架中的孔隙形态和大小将直接影响材料的性能.本文分别以添加硬脂酸和诱导铜的方法对钨粉进行处理,W-15Cu骨架孔隙在压制成形过程中得到了精确控制,钨骨架经熔渗后制成W-15Cu合金.用扫描电镜观察材料的显微结构,并测出材料的密度、气密性,研究了钨粉成形性与钨铜合金密度的关系.结果表明,添加硬脂酸和诱导铜不会带入杂质,能改善钨粉的成形性能,可以获得组织均匀、致密度高的W-15Cu复合材料.     

Solid-state contributions to densification during liquid-phase sintering

Densification via liquid-phase sintering generally requires transport of substantial amounts of dissolved solid through the liquid. However, in composite systems, such as W-Cu, solid solubility in the liquid is almost negligible, and densification is hindered by the low amount of total mass transport. In this case, solid-state sintering of the skeletal solid structure in the presence of the liquid is a significant densification mechanism. In this article, the relative contributions to densification of both liquid and solid mass transport mechanisms are considered. A computer simulation is constructed to predict the densification behavior and concurrent microstructural development of liquidphase sintered composites for realistic heating cycles. Governing differential equations for densification are derived from idealized models of the microstructure, considering grain size, diffusion distance from vacancy source to sink, pore size, and pore morphology. Temperature-dependent terms, including the diffusivity, solubility, and surface energy, govern densification and microstructural parameters, such as the grain size, dihedral angle, and contiguity. Predictions for the sintered density, grain size, and contiguity are compared to experimental results for the W-Cu and W-Cu-Ni systems with approximately 20 vol pct liquid. For W-Cu, which has almost no intersolubility, solid-state sintering of W in the presence of liquid Cu is the dominant densification mechanism. Nickel additions increase solid solubility in the liquid and improve typical liquid-phase sintering contributions to densification. Alternatively, high sintered densities can be achieved in the absence of solubility with a sufficiently small particle size due to the solid-state contribution.     

Preliminary evaluation of tensile and stress-rupture behavior of W + 24 At. Pct Re + 0.4 At. Pct HfC wire

The tensile properties of hafnium carbide-dispersed tungsten-rhenium alloy wire, W + 24 at. pct Re + 0.4 at. pct HfC (W24ReHfC), were studied from liquid nitrogen temperature (LN₂) to 1750 K and its stress-rupture behavior determined from 1144 to 1500 K. These results are compared to previous data on W + 4 at. pct Re + 0.4 at. pct HfC (W4ReHfC) and W + 0.4 at. pct HfC (WHfC) wire.[5] The room-temperature (RT) tensile strength of the W24ReHfC wire was about 3250 MPa and higher than that of the W4ReHfC (3160 MPa) and WHfC (2250 MPa) wires. The RT ductility of the W24ReHfC wire was quite high with a 50 pct reduction of area, whereas the W4ReHfC wire and the WHfC wire had RT ductilities of 28 and 2 pct, respectively. At temperatures of 1144 to 1366 K, the W24ReHfC wire had tensile strengths favorably comparable to the W4ReHfC and WHfC wires. However, above 1366 K, the W4ReHfC wire had both a greater tensile strength and stress-rupture strength than the W24ReHfC wire. The main contributions to the strengthening of the W24ReHfC wire were the fine and elongated fibrous grain microstructures and the dispersion of the HfC particles in the W-Re matrix. These properties suggested that the W24ReHfC wires hold promise as potential fiber reinforcements in composites from RT to about 1350 K.     

The effects of impurities and heat treatment on the internal friction of tungsten at high temperatures

An improved internal friction technique was used to study the relaxation processes in pure tungsten and the following alloys: commercially doped tungsten (218W), W-1 pct ThO₂, 218W-3 pct Re, and 218W-20 pct Re. Internal friction experiments were performed on worked and recrystallized specimens in the temperature range of 300° to 3000°K. The effects of impurities and alloying additions on the damping and recrystallization behavior of tungsten are demonstrated. It is further shown that the internal friction curves can conveniently be used to determine the temperature of primary and secondary recrystallization. The important effect of impurities on the microstructure and high temperature strength is also revealed by the temperature dependence of shear modulus.