Potassium-doped Tungsten Research Articles

Thermal stability of thin rolled potassium-doped tungsten sheets during annealing at temperatures up to 1400 °C

Microstructural changes will occur in tungsten when used as plasma-facing material in future fusion reactors due to the high operation temperatures. Potassium doping has proven to delay such undesired changes in drawn tungsten wires. The same strategy is adapted to thin cold-rolled tungsten sheets containing 80 ppm potassium. Their thermal stability is characterized by isochronal annealing for two hours as well as isothermal annealing at selected temperatures for longer times. Mechanical degradation is quantified by micro hardness testing; microstructure and texture evolution are investigated using scanning electron microscopy and electron backscatter diffraction. During annealing for two hours at temperatures between 800 °C and 1400 °C, the potassium-doped tungsten sheets undergo recovery. The strong rotated cube texture present in the as-rolled condition intensifies enormously during annealing for two hours, but does not change significantly during further annealing. Isothermal annealing at temperatures between 1300 °C and 1400 °C causes significant loss in hardness during the first two hours, but only smaller changes further on, indicating that solely extended recovery is occurring. No evidence for primary recrystallization is found as recrystallization is retarded by a combination of high angle boundaries being absent due to the strong texture and their immobilization by potassium bubbles. The recovery kinetics is quantified based on Kuhlmann’s recovery model and compared with observations on differently rolled thin plates of pure tungsten. Despite different amounts of recoverable hardness, the recovery parameters resolved for potassium-doped tungsten fit nicely to the master curve for recovery obtained earlier on pure tungsten plates confirming that recovery is indeed the dominating restauration mechanism and that doping with 80 ppm potassium does not alter the recovery mechanism fundamentally.

Strong rotated cube textures in thin cold-rolled potassium-doped tungsten sheets during annealing up to 1300 °C

Due to the high operation temperatures, microstructural changes will occur in tungsten when used as plasma-facing material in future fusion reactors. In drawn tungsten wires, potassium doping has proven to prevent such undesired changes effectively. This strategy has been adapted to thin cold-rolled tungsten sheets containing 80 ppm potassium. Their texture evolution during annealing is investigated using electron backscatter diffraction. In the as-rolled condition, a strong cube texture is present with a volume fraction of 67.0 % deviating maximal 15° from the rotated cube orientation. This texture intensifies strongly during isochronal annealing for 2 h at temperatures between 1000 °C and 1300 °C; the higher the temperature, the higher the rotated cube fraction. The volume fraction of 94.8 % obtained after 2 h at 1300 °C does not change significantly during further annealing. These are remarkably strong textures for a body-centered cubic metal compared e.g. to commonly reported high volume fractions of 30 % for Goss orientations in silicon steels. Reliable quantification of such strong texture components requires analysis of orientations individually.

Study of defects in potassium-doped tungsten alloy by positron annihilation technique

Tungsten alloy is known as a promising plasma-facing material (PFM) in IETR because of high strength, high-temperature stability, low sputtering erosion, low tritium retention, etc. However, tungsten has some disadvantages, such as high ductile-brittle transition temperature, low temperature brittleness, and radiation embrittlement. For the severe environment of PFM, various techniques have been adopted to improve W-based materials, among which the potassium doping is an effective bubble strengthening method, it can bring in nano-sized K bubbles, and enhance the toughness and strength, thermal shock performance, irradiation resistance of the materials. The K bubbles, which can pin grain boundaries (GBs) and dislocations, are the most characteristic defects in W-K alloy and have been widely reported. However, little attention is paid to other defects such as vacancies, GBs and dislocations. In fact, high-density dislocations exist in W-K alloy and vacancies play a considerable role in forming the K bubbles. Thus, positron annihilation technique (including the positron annihilation lifetime spectrum and slow positron beam Doppler broadening spectrum), which is a useful technique for detecting defects in solids, can be used to study these defects in W-K alloy samples. The positron lifetime of potassium bulk is about 376 ps and the positron lifetime of tungsten bulk is about 110 ps. But by simulating positron lifetime of defects in tungsten, it is found that potassium atoms in tungsten lattice do not exhibit the characteristic positron lifetime. Therefore, potassium is not considered in analyzing positron annihilation lifetime spectra of W-K alloy samples with different potassium content (46, 82, 122, 144 ppm). Three-state capture model is established in this paper, the dislocation density and vacancy cluster concentration of these samples are obtained. From the results, the dislocation densities in all samples are very high, but vacancy cluster concentrations are relatively low, and the vacancy cluster concentration in the sample with 82 ppm potassium content is the lowest in all samples. The behavior of potassium atoms in the sintering process is also discussed. Then the slow positron beam Doppler broadening spectra of W-K alloy samples and pure tungsten samples are measured and the obtained data are fitted by VEPFIT. It is noted that the defects in W-K alloy samples are much more than those in pure tungsten sample, and are distributed homogeneously with depth. The positron diffusion length information simultaneously obtained is compared with these values computed by dislocation density and vacancy cluster concentration, confirming the positrons trapped by potassium bubbles and grain boundaries are existent.

Tungsten–potassium: a promising plasma-facing material

Tungsten–potassium (potassium-doped tungsten or WK), initially known from the electric filament industry, is a promising plasma-facing material (PFM) in future fusion facilities like International Thermonuclear Experimental Reactor (ITER). However, the brittle nature of W and irradiation-induced defects of WK materials may result in a risk of deuterium–tritium reaction failure in fusion reactors. Previous studies revealed that advanced W with ultrafine grains and nanostructures might be able to address these problems. However, K-doped W, a rapidly developed material for PFMs, lacks a systematical summary. In this review, we firstly describe the powder metallurgy and plastic deformation for the preparation of WK. Then, the mechanical properties of WK and thermal shock resistance results are reviewed. Important issues such as irradiation damages from neutron, heavy ion, and plasma (H isotope or He) irradiation are also discussed. Hitherto, WK under irradiations shows comparable or even better performances compared with other counterparts such as ITER grade pure tungsten. This review could be benefitial to the future efforts of improving the ductility and irradiation tolerance of WK materials.

Plastic deformation of recrystallized tungsten-potassium wires: Constitutive deformation law in the temperature range 22–600 °C

Recent efforts dedicated to the mitigation of tungsten (W) brittleness have demonstrated that tungsten fiber-reinforced composites acquire extrinsic toughening even at room temperature, which is due to the outstanding strength of W wires. However, high temperature operation/fabrication of the fiber-reinforced composite might result in the degradation of the mechanical properties of W wires. To address this, we investigate mechanical and microstructural properties of potassium-doped tungsten wires, being heat treated at 2300 °C and tested in temperature range 22–600 °C. Based on the microscopic analysis, the engineering deformation curves are converted into actual stress - strain dataset, accounting for the local necking. The analysis demonstrates that local strain in the necking region can reach up to 50% and the total elongation monotonically increases with temperature, while the ultimate tensile strength goes down. Preliminary transmission electron microscopy analysis using FIB-cut lamella from the necking region revealed the presence of curved dislocation lines in the sample tested at 300 °C, proving that plastic deformation occurred by dislocation glide.

Microstructure and bubble formation of Al–K–Si doped tungsten prepared by spark plasma sintering

Dense potassium-doped tungsten (W–K) ingots with homogeneously dispersed nanoscaled bubbles were fabricated by spark plasma sintering (SPS) using Al–K–Si (AKS) doped tungsten powder. Parameters such as sintering temperature, holding time and sintering pressure were considered to tune the grain size, density, microhardness, bubble formation and impurity contents. In addition, a two-step sintering scheme was designed to reduce Al/Si impurity contents and enhance the density. The sample prepared by the two-step sintering scheme displays both inter- and intra-granular nanocomposites, which is expected to have better creep resistance, high bending strength and fracture toughness. Potassium was detected inside the bubbles, determining the crucial role in the bubble formation process, and thus leading to compatible properties as plasma-facing material.

Effects of temperature and strain rate on the tensile properties of potassium-doped tungsten

Tensile tests were performed on pure and K-doped tungsten at temperatures from 25 to 700°C and strain rates between 10−5 and 10−1s−1 in vacuum. The yield strength of both materials increased with increasing strain rate and decreasing temperature. The amount of change in the yield strength decreased with increasing temperature. The determination of activation volumes for plastic deformation highlighted that the rate-controlling process of the deformation behavior at lower temperatures was the same for both materials, namely, kink-pair formation on screw dislocations, and the process was not affected by potassium addition. The fracture strain of both materials increased with increasing strain rate and decreasing temperature, in the temperature range where the materials showed measurable ductility. K-doped W showed higher yield strength and a lower ductile-to-brittle transition temperature than pure W. No negative effect of K addition on strain rate- and temperature-induced changes in tensile properties was found. The analysis also highlighted the effectiveness of K addition, and of the grain refinement induced by it, for improving the mechanical properties of tungsten.

Densification of potassium-doped tungsten during sintering

Tungsten is traditionally sintered at very high temperatures. The master sintering curve (MSC) for densification is a functional model that describes sintering under an arbitrary time–temperature regime of a particular material during heating. The MSC for potassium-doped tungsten (W-K) has been determined by fitting experimental relative density data results versus work of sintering data with a modified sigmoid function. Five independent parameters of the fitting function are identified by minimizing error in terms of mean residual square. For measurement of relative density during sintering, we developed a non-contact high temperature dilatometer experiment using an optical method. Densification was continuously recorded at constant heating rates of 5, 10, 20, 30 and 40°C/min. The work presented here was carried out to predict and control densification evolution of W-K during free sintering. The results demonstrate, that the MSC model of W-K describes densification independently of selected temperature regime.

Microstructural and mechanical characterization of annealed tungsten (W) and potassium-doped tungsten foils

Here we show that potassium-doped tungsten foil should be preferred to pure tungsten foil when considering tungsten laminate pipes for structural divertor applications. Potassium-doped tungsten materials are well known from the bulb industry and show an enhanced creep and recrystallization behaviour that can be explained by the formation of potassium-filled bubbles that are surrounding the elongated grains, leading to an interlocking of the microstructure. In this way, the ultra-fine grained (UFG) microstructure of tungsten foil can be stabilized and with it the extraordinary mechanical properties of the foil in terms of ductility, toughness, brittle-to-ductile transition, and radiation resistance.In this paper we show the results of three-point bending tests performed at room temperature on annealed pure tungsten and potassium-doped tungsten foils (800, 900, 1000, 1100, 1200, 1300, 1400, 1600, 1800, 2000, 2200, and 2400°C for 1h in vacuum). The microstructural assessment covers the measurement of the hardness and analyses of fractured surfaces as well as a comparison of the microstructure by optical microscopy.The results show that there is a positive effect of potassium-doped tungsten foils compared to pure tungsten foil and demonstrate the potential of the doped foil.

Fabrication and Helium Irradiation of Potassium-Doped Tungsten

Potassium-doped tungsten (W-K) and pure tungsten samples were fabricated by the spark plasma sintering (SPS) method and irradiated by He plasma. The samples were characterized by relative density, optical microscopy, and atomic force microscope (AFM) and scanning electron microscope (SEM) measurements. Through tuning the sintering parameters, samples with different K content but similar density and size distribution were obtained in this study. It is found that a sample with higher K content shows better resistance to helium irradiation than that containing less K. In addition, both W-K samples show better performance than pure W in low energy He resistance, indicating that the potassium doping is helpful for improving He-plasma-irradiation resistance for tungsten-based materials.

Synthesis of single-crystalline potassium-doped tungsten oxide nanosheets as high-sensitive gas sensors

The single-crystalline potassium-doped tungsten oxide nanosheets were prepared by simply heating a treated foil of tungsten. The as-prepared products were characterized using X-ray powder diffraction (XRD), scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and selected-area electron diffraction (SAED). XRD and SAED results revealed that the products are pure, single-crystalline. The nanosheets width reaches to 30 μm and thickness is about 80 nm. The nanosheets have high sensitivity, fast response time and good stability to H 2S, acetone and other gases.

A Simple Method for the Synthesis of Highly Oriented Potassium‐Doped Tungsten Oxide Nanowires

Very long and highly oriented potassium‐doped tungsten oxide nanowires (see Figure) can be synthesized using the simple technique presented here. The mechanism of nanowire formation is investigated and a vapor–liquid–solid mechanism is proposed. This novel method can be easily scaled up and can also be extended to the preparation of nanowire arrays of other materials.

Molybdenum-tungsten interdiffusion and the influence on potassium bubbles in tungsten lamp wire

The present article describes the effects of molybdenum contamination on the microstructure of lampgrade potassium-doped tungsten wire after exposure to temperatures above 2500 °C. Molybdenum is generally used as the mandrel material in the coiling of single and double coil lamp filaments. During high-temperature heat treatments used during filament manufacture, significant amounts of molybdenum can diffuse into the tungsten. In this study, tungsten-molybdenum diffusion couples were prepared and heat treated at temperatures of 2500 °C and above to generate molybdenum contamination of the tungsten. After dissolution of the molybdenum from the diffusion couple, additional high-temperature heat treatments of the tungsten were performed to simulate lamp burning; equiaxed grains and excessive potassium bubble growth were observed in the tungsten. Explanations for these microstructural changes are discussed. Electron beam microprobe data were also obtained to characterize tungsten-molybdenum interdiffusion, and a Boltzmann-Matano analysis was performed to evaluate the contribution of the concentration dependence of the interdiffusion coefficient to the measured diffusion profiles.

Improving analytical precision by utilizing intrinsic internal standards for determining minor constituents by inductively coupled plasma atomic emission spectrometry

In order to improve analytical precision, intrinsic internal standardization was applied to two routine analyses. Matrix emission lines that closely matched the noise patterns of the emission lines of the analytes were chosen. These noise patterns were induced by varying the ICP rf power and argon carrier flow rate. The changes in these two experimental conditions affect the plasma temperature, amount of aerosol injected into the plasma and analyte residence time in the plasma. Consequently, they contribute significantly to the over-all variability. Two lighting materials were chosen for this study: potassium-doped tungsten powder and europium-activated Y2O2S powder. Experimental results showed that with the intrinsic internal standard lines, the precision for determining the minor constituents (potassium in tungsten powder and iron in Y2O2S powder) was improved by 25–50%.

Mechanisms of Grain Growth in Bubble-Fence-Delineated Films

ABSTRACTThe strength of polycrystalline metallic films at high temperatures is limited by grain boundaries. Boundaries transverse to the film surfaces typically fail by decohesion, while boundaries parallel to the film surfaces are susceptible to sliding. In light of these facts, multilayer films are now being made that resemble stacks of columnar-grained thin films[l]. Nearly planar arrays of insoluble bubbles, called "bubble fences," maintain the integrity of the individual layers by pinning the interlayer boundaries. However, the transverse boundaries that delimit the columnar grains within an individual layer are able to move. Processing aims to produce a highly-overlapping microstructure that isolates the weak-link transverse boundaries and increases the material's time-to-failure. In potassium-doped tungsten this strategy is an extension of current lamp filament technology.We have studied the mechanisms of grain growth within the grain layers in order to provide a basis for improving the synthesis and processing of these materials. For this type of grain growth to occur, the boundaries between grain layers must remain pinned by the bubble fences. Theory and 3D Evolver[2] simulations of a simple model provide a possible explanation for the depinning seen in recent experiments[3]. Reducing the ratio of intra-fence bubble separation to bubble diameter is predicted to eliminate the depinning. Ongoing study of a complete 3D model will further aid in tuning the synthesis and processing to create grain architectures with superior high-temperature properties.

The microstructure of rolled and annealed tungsten rod

This paper reports a study of the microstructural changes that occur when potassium-doped tungsten ingots are rolled at elevated temperatures. The effect of annealing on the microstructure of the rolled material is also considered. All samples were rolled on a Kocks mill. At low levels of deformation, the grain boundaries are primarily high-angle boundaries, and many grains are dislocation free. Both of these features probably result from dynamic recrystallization during rolling. As deformation increases, the grains become more elongated, and more low-angle boundaries are found within the material. Also, the potassium gets drawn into narrower and longer tubes. When these rolled rods are annealed at temperatures between 1275 ‡C and 1950 ‡C, several changes occur in the microstructure. The material undergoes abnormal grain growth. The temperature at which this occurs depends on the length of the anneal, the amount of de-formation the rod has received, and the spatial location in the rod. This spatial distribution most likely results from strain gradients that exist in the rolled rod. The abnormal grain growth is accompanied by a decrease in hardness. The potassium-containing tubes in the rod also break up into bubbles during annealing. The temperature at which this breakup occurs again depends on the length of the anneal and the amount of deformation.

A Model for the formation of the non-sag structure of potassium-doped tungsten wire

During recrystallization, potassium-doped tungsten wire develops a favorable stable structure of elongated grains. This is caused by a special bubble structure which is produced by the doping agent, potassium. The paper aims at a coherent semiquantitative model describing the formation of sinter pores, of potassium bands during deformation, of the bubble structure, and of driving and dragging forces during recrystallization.