Dopant particle characterization and bubble evolution in aluminum-potassium-silicon-doped molybdenum wire

The present article describes the creation of dopant inclusions in aluminum-potassium-silicon (AKS)-doped molybdenum powder and the generation of potassium bubbles in doped molybdenum wire. Molybdenum wire is used extensively in the incandescent lamp industry for coiling mandrels, filament support wires, and foil seals. The AKS-doped molybdenum wire is an important product, because it possesses greater high-temperature strength and a higher recrystallization temperature than undoped molybdenum; both of these properties are important for structural applications in lamps. The AKS-doped molybdenum wire is produced in a similar manner to AKS-doped tungsten wire, but lower processing temperatures are typically used for the production of molybdenum wire. Previous studies on AKS-doped tungsten wire have shown that the dispersion which provides the interlocking grain structure in recrystallized tungsten wire is bubbles of elemental potassium; these enhance incandescent lamp filament life. However, there is little previous work on the potassium-containing dispersion in AKS-doped molybdenum wire. In AKS-doped molybdenum, the dispersion can be either potassium bubbles, or solid oxide particles, depending on the processing method. This article will describe a series of analyses of doped molybdenum wire and its precursors, namely, doped powder and sintered ingots. The roles of high- and low-temperature sintering are also described.     

掺杂Si-Al-K对钼粉及其烧结制品组织、性能的影响

研究了不同含量掺杂元素对钼粉形貌、烧结坯及钼丝微观组织的影响。研究表明，随着掺杂元素含量的升高，钼粉粒度逐步细化：在烧结过程中，掺杂元素在烧结坯形成非晶相的第二相粒子，并且随着掺杂元素含量的升高，第二相粒子的分布密度增大，使烧结坯晶粒得到细化；在加工过程中，烧结坯中的第二相粒子碎化为粒子串，粒子串分布于纤维晶界处，可改善钼丝性能。     

Study of grain boundary fracture surfaces in doped tungsten-rhenium alloys

The combination of Auger electron spectroscopy and scanning electron microscopy has identified the source of the unique interlocked elongated grains responsible for the high temperature sag resistance in doped tungsten and tungsten-rhenium alloys as due to bubbles which are formed by the volatilization of potassium during sintering. By pinning grain boundaries these bubbles raise the recrystallization temperature (from 1300†C to 2100†C) and their distribution within the material controls the recrystallized grain morphology. There is a thin layer of potassium remaining on the bubble surfaces. The size and distribution of the bubbles is related to the amount of material deformation during processing. Increasing rhenium content does not affect the concentration or distribution of residual potassium. It has no noticeable effect on bubble size, distribution or density. The presence of a thermal gradient during annealing does affect bubble density and recrystallization temperature. Formerly with the Air Force Materials Laboratory, Wright-Patterson AFB, Ohio Formerly with the Aerospace Research Laboratory, Wright-Patterson AFB, Ohio     

液—固掺杂垂熔钨坯中改性元素的行为

对传统粉末冶金方法(液—固混合制成掺杂钨粉,经过压制、烧结、旋锻、拉拔等工序)制成的钨坯和钨丝,用扫描电子显微镜进行了断面及磨面分析。结果发现:在晶界孔穴内,元素Si和Al的含量丰富;在晶内孔穴内,除元素Si、Al外,还富集了元素K。只用钾泡理论解释改性元素改善高温钨丝的抗下垂性能是不够的,应以分子筛理论和SiO_2-Al_2O_3系统相图予以补充。     

Effect of the conditions of sintering W-Ni-Fe-Co heavy alloy nanopowders on the structure and density of compacted samples

A compact material made of a heavy tungsten alloy W-Ni-Fe-Co nanopowder is produced. The nanopowders are synthesized by the treatment of a solid tungstic acid in aqueous solutions of Ni, Fe, and Co salts followed by the reduction of the solid residue by hydrogen at 800°C (the average size of the powder conglomerates is ∼300 nm, and the conglomerates consist of 100-nm particles). Solid-phase sintering is performed in stages. An increase in the temperature at the last stage from 1300 to 1350 and 1450°C increases the density from 16.7 to 17.2–17.4 g/cm³ and the average tungsten grain size to 2.4–4.6 μm. The samples after solid-phase sintering at 1350°C have no porosity. Liquid-phase sintering of nanopowders with high surface and interface energies occurs at 1480°C. Original Russian Text ? K.B. Povarova, M.I. Alymov, O.S. Gavrilin, A.A. Drozdov, E.V. Evstratov, A.I. Kachnov, A.E. Sal’ko, 2007, published in Metally, 2007, No. 6, pp. 65–72.     

掺杂氧化铝对纳米钨粉烧结过程的影响

以碳热预还原和氢气深还原两步制备的纳米钨粉作为烧结原料,即先通过碳黑还原脱除三氧化钨中的大部分氧,再以氢还原脱除残留的氧。该方法制备的钨粉颗粒呈球形形貌,平均晶粒度可达90 nm。同时,向钨粉中掺杂质量分数为1%和2%的氧化铝,探究了氧化铝对钨粉烧结行为的影响。通过烧结样品的断口形貌和晶粒的平均尺寸分析发现,氧化铝对烧结后期的晶粒长大有明显的抑制作用,相同的烧结温度下晶粒的尺寸随着氧化铝含量的上升而减小。在1600 ℃时,纯钨粉烧结坯的晶粒平均尺寸为2.75 μm,但添加质量分数为1%和2%氧化铝的烧结样品晶粒平均尺寸约为1.5 μm,这是由于氧化铝能有效地抑制烧结后期的钨粉晶粒长大。纯钨粉和掺杂氧化铝钨粉的烧结坯的硬度随温度升高具有不同的趋势。掺杂钨粉烧结坯的硬度随着温度的升高而升高,且其最大值高于800 HV。但是,纯钨粉烧结坯的硬度随烧结温度增加而先增加后降低,在1400 ℃时取得最大值（473.6 HV）,这是由纯钨粉烧结坯的晶粒在高温下急剧长大所导致。在烧结温度为1600 ℃时,纯钨粉、掺杂质量分数1%和2%的氧化铝掺杂的钨粉的烧结坯的相对密度依次为98.52%、95.43%和93.5%。      

Phase transformations in reactive sintering of tungsten

Abstract

It has been demonstrated recently that tungsten (T _m = 3410±20°C) can be sintered by reactive sintering in a reductive atmosphere such as hydrogen. This alternative technique to the conventional sintering (T _s>2000°C) makes use of a small amount of aluminium addition which acts as a sintering aid and hence lowers the sintering temperature significantly (T _s1200°C). This study explores the phase transformations that take place during reactive sintering of tungsten in view of the mechanisms involved. DSC, SEM and TEM have been used for a fundamental understanding of this system.     

Sintering a disperse mixture of W-Sc2O3 powders

The effect of additions of scandium oxide (1, 5, and 10 vol. %) on compaction during sintering of a disperse mixture of W-Sc₂O₃ powder at 2000°C has been studied. It has been found that the scandium oxide particles activate compaction during heating and conversely retard shrinkage of tungsten in the stage of isothermal soaking. The kinetics of compaction during sintering is determined by the geometry of the heterophase system. Depending on how much the tungsten and scandium oxide particles increase in size, the nature of thestructure changes markedly from matrix-statistical to statistical.     

Phase Formation in Sintering of Materials Based on Silicon Nitride with Li2CO3 and AlN Additives

We have studied the phase formation processes occurring in sintering of materials based on silicon nitride with lithium carbonate and aluminum nitride or Al powder additives. We have studied samples of different compositions that were sintered at 1450, 1550, and 1750°C under a nitrogen atmosphere. We have established that the phase composition depends on the sintering temperature, the composition of the starting charge, and also the amount and the nature of the Al-containing additives.     

The effect of tungsten particle size on the processing and properties of infiltrated W-Cu compacts

Three tungsten powders with average particle sizes of 8.7, 23.2, and 65.2 μm were used to make W-15Cu compacts. The compacting pressure and sintering temperature were adjusted for each powder to attain the desired skeleton density. Sintered skeletons were then infiltrated with oxygen-free copper at 1200 °C in hydrogen and in vacuum. Results showed that as the tungsten particle size decreased, higher compacting pressures and sintering temperatures were required for the same desired skeleton density. The processing parameters and the tungsten particle size caused variations in the amount of closed pores and the W-W contiguity, which in turn resulted in different infiltrated densities and resistivities. Direct infiltration on green compacts was also examined, and higher infiltration densities and lower electrical resistivities were obtained compared to those obtained by infiltrating sintered compacts. These results are discussed based on infiltrated density, differences in microstructure, and the W-W contiguity.     

The identification of second phases within bubbles in annealed doped tungsten wire

Elemental potassium was detected within bubbles formed in annealed, commercially doped tungsten wire by transmission electron microscopy and selected area diffraction. The bubbles in specimens which had been annealed in hydrogen contained liquid potassium, solid potassium, and small quantities of potassium hydride. Only solid potassium was observed within bubbles when specimens were annealed in < 10^-5 mm Hg vacuum. The potassium solidified epitaxially on the tungsten bubble surface and had virtually the same lattice orientation. The potassium hydride usually displayed a specific orientation relationship with the solid potassium and the tungsten.     

Impurity segregation to voids in doped tungsten ribbon filaments

Filaments of Al-, Si-, and K-doped tungsten foil, 0.00254 cm thick, were heated to 2300°C in vacuo until a hot spot developed. Within the hot spot were voids varying in size from about 50 to 6000Å. The larger voids were faceted with the major facets parallel to {110}. The relative surface tensions in the doped foil were in the order {110} < {112}< {111} < {100} as determined from a model of the voids. Selected area diffraction showed that the void surfaces were coated with a thin epitaxial layer of potassium with [311] W ‖[311] K in the majority of cases.     

Characterisation of powder metallurgy H13 steels prepared from water atomised powders

ABSTRACT

In this paper, powder metallurgy (PM) H13 steels were investigated based on the characterisation of water atomised H13 powders. Vacuum sintering was carried out from 1150 to 1265°C for sintering densification. The relative density of the as-sintered sample reached to 95% and mechanical properties were released above 1200°C. To eliminate the residual pores, subsequent forging was performed. The relative density increased to above 99%. As the concentration of Si elements on the powder surface was higher than the nominal composition, there was Si segregation in the form of silicon oxides (1–3?μm). Although oxide particles such as SiO₂ were inevitable, the mechanical performance of PM H13 steels was still comparable to that of ingot metallurgy products. The tempered PM H13 steels exhibited high tensile strength of 1460?MPa in YS, 1737?MPa in UTS and 8.7% in elongation. Besides, the impact toughness was as high as 14.7?J?cm^?2.     

垂熔制度对掺杂钨性能的影响

烧结的目的是通过高温作用得到接近理论密度和无孔隙的制品。而在掺杂钨中，需要形成合适的钾泡来稳定细微孔隙，以使钨丝具有高温蠕变强度；同时需去除钨中的杂质和无效掺杂剂。本文的研究表明，通过合理的垂熔烧结制度可使钨条密度最佳。     

Hard Alloy WC – 24% Ni Obtained in the Solid Phase from Ultrafine WC, NiO, and C Powders. Part 1. Density and Structure of Specimens

We have studied the density and structure of specimens of the alloy WC – 24 mass% Ni, obtained by combining into one step the processes of synthesis of the metallic phase and compaction of the ultrafine mixture of WC – Ni powders by high-energy pressing and sintering. We have established that reduction of nickel monoxide by carbon occurs at temperatures of 650-750°C and does not affect the shrinkage process which in the case of sintering begins only at a temperature of 1050°C. High-energy pressing of briquettes sintered at the indicated temperature reduces their porosity from 30-25% down to 8-4%. Specimens of porosity <1% can be obtained by pressing at 1150°C or 1050°C in the case of triple pressing. Raising the temperature at which the briquettes are heated is accompanied by enlargement of the pores together with a decrease in the total porosity, but at temperatures of 1300°C (sintering) and 1250°C (pressing), the pore dimensions are sharply reduced. The high density of the specimens pressed at low temperature does not provide low electrical resistance, which suggests the presence of weakly connected boundaries. When the specimens are sintered and pressed in the solid phase, we observe the growth of tungsten carbide particles. It is most rapid at 1150-1250°C, while at 1050°C the particle growth process slows down. Reduction of the metal oxide when the powders are heated promotes formation of structure in the higher temperature range.     

Structure and properties of WC-Co alloys in solid-phase sintering. I. Geometrical evolution

The paper examines the geometrical evolution and particulate interaction in WC-Co alloys produced by solid-phase sintering. It is shown that solid-phase sintering actively modifies the geometric structure: carbide particles grow, become facetted and grow together, and the cobalt phase is redistributed. The WC particles grow irregularly over different temperature ranges. The low rate of growth is characteristic of temperature ranges between 1050 and 1200°C. When alloys are sintered at 1200°C and higher temperatures, the growth of tungsten carbide particles intensifies substantially (by four times). Hence, the temperature 1200°C separates two structurization areas in solid-phase sintering. At this temperature, there is a bend on the specific resistivity curve, which is evidence of higher-quality grain and phase boundaries.     

Porous tungsten with controlled porosity by low temperature sintering

Abstract

Porous tungsten as a high current density cathode is one of the important applications of the metal, which is mostly used in high temperature conditions due to its exceptional resistance to melting (T _m = 3410±20°C). Its porous form has been a crucial component of dispenser cathodes used in electronic valves and high power lamps. Porous tungsten skeleton forms the matrix, which is then impregnated with an electron emissive compound. Upon every emission from the surface, new material has to be fed into the surface pores via the open pore channels. Hence it may be proposed that a uniform porosity is needed for a better performance. However, a controlled porosity has not been achieved yet. Moreover, sintering of tungsten has always been difficult due to the extreme process conditions. A high sintering temperature (T _s≥2000°C) and a strong reductive atmosphere (hydrogen) have been the absolute necessity in making these parts. This study further explores an alternative sintering technique being developed. The idea is based on the reactive sintering concept. The energy output from the exothermic reactive system of tungsten oxide and aluminium has been the heat source for sintering porous tungsten. As a result, sintering temperature and time have been reduced considerably. Higher homogeneity, thus more uniform pore distribution, was observed. A better control of porosity related to the pressing and sintering conditions was achieved by the characterisation method previously developed. Microhardness has been a useful monitor of the scatter in porosity of the parts. Throughout the study, SEM was used to observe the porous structures and powder morphologies. DSC and XRD were useful to follow the microstructural evolution in the reactive system.     

EFFECTS OF PARTICLE SIZE ON THE SINTERING KINETICS IN TUNGSTEN POWDER

Abstract

An experimental study has been made of the effects of initial particle size on sintering kinetics in tungsten powder within the temperature range 1100–1500°C. Particle size, compacting pressure, sintering time and temperature all influence the rate of sintering. Isothermal changes in density and volume have been measured. The results indicate grain-boundary diffusion as the mechanism principally responsible for material transport in the case of particle sizes <4 μ Surface diffusion appears to bethe mechanism of material transport in compacts with particle sizes of 14– 16 μ The temperature-dependence of the rate of sintering is characterized by activation energies of 101± 2 and 72± 2 kcal/mole for fine particles (< 4 μ) and coarse particles (14–16 μ), respectively.     

Sintering mechanism of iron powder with microadditions of boron

The effect of boron on the sintering of iron powder was investigated. Boron (0–400 ppm) was added to high-purity iron powder of the German firm “Mannesmann.” Powder mixtures were pressed to compacts of identical density and sintered at different temperatures in different atmospheres. The results indicated the absence of a liquid phase and no influence of boron in the high-temperature stage of sintering. However, boron additions substantially improved sintering at low temperatures (up to 800°C) due to an effect on the interparticle contacts. With a properly selected sintering regime, microadditions of boron substantially increase the density of sintered ingots.     

The process of bubble formation in the hot isostatic pressing treated,doped molybdenum wire

Entrapped materials in the bubbles formed in the K- and Si-doped and hot isostatically pressed molybdenum wire were examined by means of scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) spectroscopy. Solidified particles entrapped in the bubbles were identified as Al-K-Si leucite-type oxide. After reheating these solidified particles at 1900 °C for 5 minutes under atmospheric pressure, however, a portion of the particle was homogeneously volatilized. The remainder was identified as an Al-K-Si mullite-type oxide. Therefore, it can be concluded that the added dopants form a single phase upon annealing, and the formation of bubbles in the doped molybdenum wire is due to volatilization of Al-K-Si oxides, not pure elements constituting the oxides.