Doped Tungsten Wire Research Articles

Microstructure and High-Temperature Performance of High K-Doped Tungsten Fibers Used as Reinforcement of Tungsten Matrix

Tungsten (W) fiber-reinforced tungsten (Wf/W) composite with ultra-high strength and high-temperature resistance is considered an attractive candidate material for plasma-facing materials (PFM) in future fusion reactors. The main component of Wf/W composite is tungsten wire, which is obtained through powder metallurgy and the drawing process. In this paper, high potassium (K)-doped tungsten wires with 98 ppm of K and 61 ppm of impurities are prepared using traditional and optimized processing technologies, respectively, and a comparative study with conventional K-doped tungsten wires with 83 ppm of K and 80 ppm of impurities is conducted. The high-temperature mechanical properties as well as the microstructure’s evolution of the prepared tungsten wires are investigated. The results show that the high-temperature performance of K-doped tungsten wires is improved by increasing the K content and by simultaneously reducing the impurities. By adopting small compression deformation and low-temperature processing technology, the high-temperature performance of high K-doped tungsten wires can be further improved. A microstructure analysis indicates that the excellent high-temperature performance is attributed to a combination of the small K bubble size, high K bubble number density, and long K bubble string, which are produced through optimization of the processing technology. A study on the processing technology and the performance of tungsten wires with a high K content and a high purity can provide important information regarding Wf/W composites.

The effect of heat treatments on pure and potassium doped drawn tungsten wires: Part II – Fracture properties

Advanced tungsten fibre-reinforced composites (Wf/W), showing pseudo ductile behaviour even at room temperature, are a promising option for future fusion reactors as the intrinsic brittleness of tungsten can be mitigated effectively. The drawn tungsten wires used as reinforcements are the key components of the composites, thus their mechanical properties and thermal stability define the allowed operation/fabrication temperature of the composite material itself. In this work, the room temperature fracture behaviour of the pure and potassium doped tungsten wires was investigated, focusing on the evolution of the fracture micromechanisms in respect to annealing. Single-edge-notched specimens were used, with the crack growth direction perpendicular to the drawing axis of the wire. The occurrence of either a fractographic brittle or a ductile response in the as-received state of both materials is a strong indication that the ductile-to-brittle transition temperature is about room temperature. Pure, annealed tungsten wires experience a tremendous deterioration of the fracture toughness with a very prominent transition of the failure mode. The observed embrittlement by annealing can be related to the loss of the fibrous, elongated microstructure. In contrast to this, the results of the annealed, doped wires demonstrate that the microstructural stability and preservation of the initial, beneficial grain structure is directly reflected in the crack resistance of the material. Predominately ductile behaviour, with characteristic knife-edge necking, is seen even after annealing at 1600 °C.

The effect of heat treatments on pure and potassium doped drawn tungsten wires: Part I - Microstructural characterization

Advanced tungsten fiber-reinforced composites (Wf/W), showing pseudo ductile behaviour even at room temperature, are a promising option for future fusion reactors as the intrinsic brittleness of tungsten can be mitigated effectively. The drawn tungsten wires used as reinforcements are the key components of the composites, thus their mechanical properties and thermal stability define the allowed operation/fabrication temperature of the composite material itself. In this work, a comprehensive characterization of the pure and potassium doped tungsten wires was performed, focusing on the influence of various heat treatments on different microstructural features (nature of grain boundaries, grain shape and size, texture analyses) and mechanical properties. Annealing in the temperature range from 900 to 1600 °C enables the investigation of the microstructural stability of the two materials and arising annealing phenomena – recovery, recrystallization and grain growth. The results demonstrate that the pure tungsten recrystallizes fully in the temperature range 1300–1500 °C accompanied with tremendous coarsening and a complete loss of the initial fibrous, elongated grain structure. In contrast to this, potassium doped wire shows superior high temperature properties, where the performed heat treatments cause only milder microstructural changes, consequently suppressing recrystallization and grain growth to temperatures well above the investigated ones. Furthermore, hardness measurements and observed softening complement the discussion of the grain morphology evolution.

Development of tungsten fibre-reinforced tungsten composites towards their use in DEMO—potassium doped tungsten wire

For the next step fusion reactor the use of tungsten is inevitable to suppress erosion and allow operation at elevated temperature and high heat loads. Tungsten fibre-reinforced composites overcome the intrinsic brittleness of tungsten and its susceptibility to operation embrittlement and thus allow its use as a structural as well as an armour material. That this concept works in principle has been shown in recent years. In this contribution we present a development approach towards its use in a future fusion reactor. A multilayer approach is needed addressing all composite constituents and manufacturing steps. A huge potential lies in the optimization of the tungsten wire used as fibre. We discuss this aspect and present studies on potassium doped tungsten wire in detail. This wire, utilized in the illumination industry, could be a replacement for the so far used pure tungsten wire due to its superior high temperature properties. In tensile tests the wire showed high strength and ductility up to an annealing temperature of 2200 K. The results show that the use of doped tungsten wire could increase the allowed fabrication temperature and the overall working temperature of the composite itself.

Contradictions and new aspects of the bubble model of doped tungsten wires

The interpretation of a series of small angle neutron scattering studies on lamp grade tungsten rods and wires is in contradiction with an extended body of the results that were obtained by fractographic methods and thin foils studies by means of scanning microscopy and transmission electron microscopy. In order to facilitate the re-interpretation of the scattering studies, the state of agglomeration of potassium in the dope phase has to be clarified, due to the peculiar scattering properties of the two-phase inclusions. These efforts yielded to the following results: (1) There are independent pieces of experimental evidence that the dope inclusion are filled with liquid potassium in heavily drawn lamp grade tungsten wires and rods in various stages of the processing. (2) A slight volume compression of liquid inclusion upon wire drawing is compatible with the scaling laws of the Taylor model in a good approximation, because the expected deviations are less than the accuracy of the required measurements. (3) The size increase of the inclusion cannot be explained as the expansion of individual bubbles that do not interact with each other. One has to take into account also various sorts Ostwald type ripening processes.

Analysis of AKS- and lanthana-doped molybdenum wire

The lamp industry uses molybdenum wire in many high temperature, structural applications, for which doped molybdenum wire is an important product because it possesses greater high-temperature strength and a higher recrystallization temperature than undoped molybdenum. Prior studies on aluminum–potassium–silicon (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. In doped molybdenum the dispersion can be either potassium bubbles, or solid oxide particles, depending on the processing method. Lanthana-doped molybdenum has been reported to have recrystallization temperatures above those obtained through AKS-doping. Lanthana particles are stable within molybdenum to elevated temperatures because lanthanum has very limited solubility in molybdenum. This paper will describe a series of analyses of lanthana-doped and AKS-doped molybdenum wires of a range of sizes.

Development of the bubble structure from selectively deforming potassium-pores in doped tungsten wires

As the AKS-doped sintered ingot is processed into wire, the smaller potassium-containing pores appear to be less deformable than the larger ones. In the present work the consequences of this selective pore deformation were investigated on the characteristics of the developing bubble dispersion in different heat-treated samples. The results indicated that the refinement of the potassium distribution during thermomechanical processing occurs more effectively if the potassium in the sintered ingot is enclosed in relatively large pores. It was concluded from a simple estimation that the size-dependent deformation of the potassium-pores exerts an equalizing effect on the potassium distribution manifesting itself in the fact that an initial broad size distribution of potassium-pores in the sintered ingot will be moderated during thermomechanical processing to a relatively narrow size distribution of the bubbles in the wire. In light of the size-dependent pore deformation, the occurrence of the isolated single bubbles in AKS-doped tungsten wires was also briefly discussed.

Effects of impurities on doped tungsten wires: L. Uray. (Hungarian Academy of Sciences, Budapest, Hungary.) Int. J. Refract. Metals/Hard Mater., Vol 20, No 4, 2002, 319–326

Effects of impurities on doped tungsten wires: L. Uray. (Hungarian Academy of Sciences, Budapest, Hungary.) Int. J. Refract. Metals/Hard Mater., Vol 20, No 4, 2002, 319–326

Shape analysis — deformation of bubbles during drawing of doped tungsten wire

The relative terminal rising velocity of a single gas bubble, moving into a liquid phase, is determined by its size, by the interfacial tension, by the density and viscosity of the surrounding liquid. Both shape and velocity are strongly interacting. Along the years several methods have been presented for solving the problem of bubble deformation and relative rising velocity, at least in connection with some specific regimes of motion and/or shape of bubble. In this work, a new approach is proposed.

AKS doped tungsten wires-investigated by electrical measurements: II. Impurities in tungsten

The Al, K, Si doped tungsten wires contain several minor impurities (in the ppm range), which are usually measured by chemical analysis. The excess electrical resistivity measures only those impurities, which are present in solute form. However, this property enables the resistivity to follow solution–dissolution processes, and solute interactions with lattice defects. Combining the resistivity measurements e.g. by thermoelectric power, it helps separating several kinds of solute impurities from each other. Processes like segregation–desegregation (Fe, Co), oxidation–reduction (Al, Si), and the development of the stationary evaporation profile, can be studied as well.

Bubble evolution and effects during tungsten processing

The sintered microstructure of potassium doped P/M tungsten is besides the usual residual porosity characterized by small nanosized spherical potassium filled bubbles in the diameter range between 10 and 50 nm. During the deformation of the sintered ingots the spherical bubbles change their morphology into elongated bubble tubes or ellipsoids. At intermediate annealings these potassium tubes decay above critical dimensions due to Rayleigh instabilities into rows of spherical bubbles which determine the recrystallization and the high temperature–low stress creep behaviour of doped tungsten wires. For a correct description of the occurring shape changes the relative density of the tungsten ingots at the beginning of the deformation steps as well as the deformation temperature and the temperature–time regime of intermediate annealings must be considered. In addition, there are significant differences between bubble sizes and bubble numbers located in the grain volume and at grain boundaries. For the occurrence of single bubbles a mechanism of “in situ spheroidization” during deformation will be proposed. Further reasons for discrepancies between experimental and modeled bubble dimensions at high true strains of deformation will be discussed.

Inclusion Characterization and Bubble Evolution in Doped Molybdenum Wire

Abstract Previous studies on aluminum-potassium-silicon (AKS) doped tungsten wire have shown that the dispersion which provides the interlocking grain structure in recrystallized tungsten wire is bubbles of elemental potassium. However, there is little previous work on the K-containing dispersion in doped Mo. This paper will describe analyses of doped Mo. The creation of dopant inclusions in Mo powder and generation of K bubbles in doped Mo wire both prior to and following various heat treatments has been studied. The effect of doping Mo with K, Al and Si is to increase the recrystallization temperature of the Mo and to produce a microstructure of coarse, interlocking grains after recrystallization. Mo wire is produced in a similar manner to AKS doped tungsten, however lower processing temperatures are typically used in the production of Mo wire. The dopants are added to Mo oxide powder as potassium silicate and aluminum chloride.

Sintered materials studied by small-angle neutron scattering

Sintered single-phase materials with open or closed porosity are usually described by a two phase model. During sintering, the decrease of pore volume is accompanied with decreasing internal surface area measured by SANS. Such investigation can be extended to sintered WC–Co composites made from nano-crystalline powder having different kinds of internal surfaces. The deformation of remaining porosity after sintering can also be monitored by the 2D anisotropy of SANS distribution as the sintered body is processed through several steps of thermo-mechanical treatment. The macroscopic deformation of doped tungsten wire is followed by the elongation of the potassium gas filled bubbles and the annealing causes spheroidization with vanishing SANS anisotropy. The SANS method establishes a new description of mechanisms of thermomechanical processing.

Potassium bubble growth in doped tungsten

Potassium filled submicroscopic bubbles play a decisive role in determining the metallurgical properties of doped tungsten wires. Besides the influence of bubbles on the recrystallization behaviour and the high temperature creep strength, the bubbles may also form the nuclei for the generation of large cavities or voids which influence the fracture behaviour and lamp failure. The most accepted mechanism of bubble coarsening is stress assisted bubble growth; however, this needs as prerequisite a critical bubble size dependence on the stress system of the tungsten coil. It will be shown that a possible mechanism for the coarsening until the critical size is the diffusion controlled transport of potassium provided there are corresponding diffusion paths, as for example grain boundaries or dislocations. In addition, the new model also explains the formation of voids in doped tungsten wire and gives the basis for a material limited failure mechanism in halogen lamps.

Structure and mechanical properties of potassium doped tungsten wire : Y.V.Milman et al. (Inst. for Problems of Materials Science, Kiev, Ukraine.)

Structure and mechanical properties of potassium doped tungsten wire : Y.V.Milman et al. (Inst. for Problems of Materials Science, Kiev, Ukraine.)

Modeling the effect of recovery on the room temperature fracture strength of W-0.01% K (non-sag) wire

The room temperature (RT) strength and ductility of potassium doped tungsten wire (i.e. non-sag) varies significantly with annealing conditions. Wire, in an as-drawn condition, exhibits limited ductility ( 1,100 C) of drawn, doped tungsten wire. The result is of interest to those who specify doped tungsten wire for applications where strength retention, after thermal processing (i.e. brazing, annealing, metal-matrix/metal-fiber composites), is important.

Tensile behaviour of doped tungsten wire at elevated temperatures: K. Fujii et al. (Nippon Tungsten Co, Fukuoka, Japan). Nippon Tungsten Rev., vol 27, 1995, 1–9

Tensile behaviour of doped tungsten wire at elevated temperatures: K. Fujii et al. (Nippon Tungsten Co, Fukuoka, Japan). Nippon Tungsten Rev., vol 27, 1995, 1–9

High-Temperature Creep Resistance Resulting from Grain Morphologies in P/M Tungsten Fine Wires

The three-dimensional shape of secondary recrystallized grains grown in doped tungsten wires is constructed by an image analyzer. Some grain shape parameters characteristic of grain morphologies are then defined to clarify the morphological effect of grains on the minimum creep rate during diffusional creep without grain-boundary cavitation. While the creep rate decreases as the grain aspect ratio (GAR) increases in the range of GAR 30. The Raj and Ashby's model gives a satisfactory explanation to the former although it does not give the one to the latter. It is proved that their model must be modified using the new grain shape parameters to explain the latter behavior because of the different grain configurations.

The recrystallization mechanism of doped tungsten wire

In the earlier literature, studies on the nucleation and growth mechanism of primary recrystallization and the mechanism of secondary recrystallization were very few. This letter reports some of our work concerning these aspects. The samples were K-Al-Si-doped tungsten wire of diameter 0.39 mm

ドープW線のNi誘起再結晶とNiの浸透挙動

ドープW線のNi誘起再結晶とNiの浸透挙動