Tungsten-rhenium Alloys Research Articles

Microstructural development of tungsten and tungsten–rhenium alloys due to neutron irradiation in HFIR

The microstructural development of pure tungsten (W) and tungsten–rhenium (Re) alloys due to neutron irradiation in the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory, TN, USA, was investigated in this work. The irradiation conditions were ∼1 displacements per atom (dpa) at 500 and 800°C. After the neutron irradiation, microstructural observations were performed using a transmission electron microscope (TEM). Large amounts of precipitates identified as sigma- and chi-phases were observed in not only the W–Re alloys but also in the pure W after the neutron irradiation. The precipitates observed in the pure W were coarse and larger than those in the W–Re alloys. This was considered to be caused by the transmutation products of W and Re, namely, Re and osmium (Os), respectively, under irradiation in the HFIR with a higher contents of thermal neutron flux.

On the reduction of deuterium retention in damaged Re-doped W

Modelling tritium retention in radiation-damaged pure-tungsten samples, the concentration of deuterium retained in the tungsten-ion-induced damage zone decreases with increasing exposure temperature; whereas for a similarly treated tungsten–rhenium alloy, it can drop significantly faster than in pure tungsten, given sufficiently high temperatures. In contrast, similar changes in retention behaviour were not observed in undamaged samples. Based on these findings, as well as on corresponding previous TEM results, it is concluded that the concentration of high-energy radiation-induced defects responsible for trapping of deuterium is lower in the alloy than in pure tungsten. Therefore, a small amount of transmutation rhenium in damaged tungsten should be able to keep tritium retention to a low level in ITER.

Rhenium Alloys as Ductile Substrates for Diamond Thin-Film Electrodes.

Molybdenum-rhenium (Mo/Re) and tungsten-rhenium (W/Re) alloys were investigated as substrates for thin-film, polycrystalline boron-doped diamond electrodes. Traditional, carbide-forming metal substrates adhere strongly to diamond but lose their ductility during exposure to the high-temperature (1000°C) diamond, chemical vapor deposition environment. Boron-doped semi-metallic diamond was selectively deposited for up to 20 hours on one end of Mo/Re (47.5/52.5 wt.%) and W/Re (75/25 wt.%) alloy wires. Conformal diamond films on the alloys displayed grain sizes and Raman signatures similar to films grown on tungsten; in all cases, the morphology and Raman spectra were consistent with well-faceted, microcrystalline diamond with minimal sp2 carbon content. Cyclic voltammograms of dopamine in phosphate-buffered saline (PBS) showed the wide window and low baseline current of high-quality diamond electrodes. In addition, the films showed consistently well-defined, dopamine electrochemical redox activity. The Mo/Re substrate regions that were uncoated but still exposed to the diamond-growth environment remained substantially more flexible than tungsten in a bend-to-fracture rotation test, bending to the test maximum of 90° and not fracturing. The W/Re substrates fractured after a 27° bend, and the tungsten fractured after a 21° bend. Brittle, transgranular cleavage fracture surfaces were observed for tungsten and W/Re. A tension-induced fracture of the Mo/Re after the prior bend test showed a dimple fracture with a visible ductile core. Overall, the Mo/Re and W/Re alloys were suitable substrates for diamond growth. The Mo/Re alloy remained significantly more ductile than traditional tungsten substrates after diamond growth, and thus may be an attractive metal substrate for more ductile, thin-film diamond electrodes.

Monte Carlo criticality analysis of simple geometries containing tungsten–rhenium alloys engrained with uranium dioxide and uranium mononitride

The critical mass and dimensions of simple geometries containing highly enriched uranium dioxide (UO 2) and uranium mononitride (UN) encapsulated in tungsten–rhenium alloys are determined using MCNP5 criticality calculations. Spheres as well as cylinders with length to radius ratios of 1.82 are computationally built to consist of 60 vol.% fuel and 40 vol.% metal matrix. Within the geometries, the uranium is enriched to 93 wt.% uranium-235 and the rhenium content within the metal alloy was modeled over the range of 0–30 at.%. The spheres containing UO 2 were determined to have a critical radius of 18.29–19.11 cm and a critical mass ranging from 366 kg to 424 kg. The cylinders containing UO 2 were found to have a critical radius ranging from 17.07 cm to 17.84 cm with a corresponding critical mass of 406–471 kg. Spheres engrained with UN were determined to have a critical radius ranging from 14.82 cm to 15.19 cm and a critical mass between 222 kg and 242 kg. Cylinders which were engrained with UN were determined to have a critical radius ranging from 13.81 cm to 14.15 cm and a corresponding critical mass of 245–267 kg. The critical geometries were also computationally submerged in a neutronically infinite medium of fresh water to determine the effects of rhenium addition on criticality accidents due to water submersion. The Monte Carlo analysis demonstrated that rhenium addition of up to 30 at.% can reduce the excess reactivity due to water submersion by up to $5.07 for UO 2 fueled cylinders, $3.87 for UO 2 fueled spheres and approximately $3.00 for UN fueled spheres and cylinders.

Microstructure Development in Neutron Irradiated Tungsten Alloys

The aim of this work is to investigate the influence of neutron irradiation condition, especially temperature, on irradiation hardening and microstructure development in irradiated tungsten-rhenium (W-Re) alloys. Neutron irradiations were carried out in JOYO at the range of 400 to 750 � C, up to 1.54 dpa. Micro Vickers hardness tests and micro structural observations using a TEM were performed. Irradiation hardening of WRe alloys irradiated at 538 � C were clearly larger than those irradiated at other temperatures. Fine voids and fine needle- or plate-like precipitates were observed in pure W and W-Re alloys irradiated at 538 � C, respectively, with high number density. The fine complex microstructure seems to be the cause of the characteristic irradiation hardening. [doi:10.2320/matertrans.MBW201025]

Tungsten‐rhenium suture needles with improved properties for coronary artery bypass graft surgery

Suture needles are essential instruments for performing blood vessel anastomosis in coronary artery bypass graft (CABG) surgeries. Exceptional needle performance is needed now as never before because of the increasing prevalence of elderly patients with advanced stages of coronary disease and calcified tissue. The various properties that affect the performance of suture needles used in CABG surgery are reviewed and recent progress towards improving needle performance through the use of tungsten-rhenium alloys and novel lubricous needle coatings is presented. Substantial enhancement of properties beyond those exhibited by commercially available stainless steel suture needles has been made, including an approximate 40% increase in strength, 100% increase in stiffness, and superior multiple pass penetration performance in both synthetic rubber media and human cadaver carotid arteries (p < 0.01).

Fracture toughness of polycrystalline tungsten alloys

Tungsten and tungsten alloys show the typical change in fracture behavior from brittle at low temperatures to ductile at high temperatures. In order to improve the understanding of the effect of microstructure the fracture toughness of pure tungsten, potassium doped tungsten, tungsten with 1 wt.% La 2O 3 and tungsten rhenium alloys were investigated by means of 3-point bending, double cantilever beam and compact tension specimens. All these materials show the expected increase in fracture toughness with increasing temperature. The experiments demonstrate that grain size, texture, chemical composition, grain boundary segregation and dislocation density seem to have a large effect on fracture toughness below the DBTT. These influences can be seen in the fracture behavior and morphology, where two kinds of fracture occur: on the one hand transgranular and on the other hand intergranular fracture. Therefore, techniques like electron backscatter diffraction (EBSD), Auger electron spectroscopy (AES) and X-ray line profile analysis were used to improve the understanding of the parameters influencing fracture toughness.

Properties of tungsten-rhenium and tungsten-rhenium with hafnium carbide

Historically, tungsten-25wt.% rhenium alloy has been manufactured into wire for the thermocouple market, but recent demands for high-temperature structural components have forced the development of novel processing techniques for tungsten-rhenium and tungsten-rhenium with hafnium carbide. With a melting temperature of 3,050°C, and a recrystallization temperature near 1,900°C, tungsten-rhenium alloys are being used in aerospace, temperature measuring, and friction stir welding applications. The mechanical properties and microstructures of tungsten-25wt.% rhenium and tungsten-25wt.% rhenium with hafnium carbide are reported at ambient temperature, 1,371°C, and 1,926°C, after processing by three methods: hot isostatic pressing, swaging, and extrusion.

High temperature surface effects of He + implantation in ICF fusion first wall materials

The first wall armor of the inertial confinement fusion reactor chambers must withstand high temperatures and significant radiation damage from target debris and neutrons. The resilience of multiple materials to one component of the target debris has been investigated using energetic (20–40 keV) helium ions generated in the inertial electrostatic confinement device at the University of Wisconsin. The materials studied include: single-crystalline, and polycrystalline tungsten, tungsten-coated tantalum-carbide ‘foams’, tungsten–rhenium alloy, silicon carbide, carbon–carbon velvet, and tungsten-coated carbon–carbon velvet. Steady-state irradiation temperatures ranged from 750 to 1250 °C with helium fluences between 5 × 10 17 and 1 × 10 20 He +/cm 2. The crystalline, rhenium alloyed, carbide foam, and powder metallurgical tungsten specimens each experienced extensive pore formation after He + irradiation. Flaking and pore formation occurred on silicon carbide samples. Individual fibers of carbon–carbon velvet specimens sustained erosion and corrugation, in addition to the roughening and rupturing of tungsten coatings after helium ion implantation.

Thermal reaction of SiC films with tungsten and tungsten–rhenium alloys

Solid-state reactions between SiC films and W–xRe (x = 0, 5 and 25 at%) substrates on thermal annealing between 1673 K and 1873 K for various durations have been investigated. SiC coatings were deposited on metallic wires by hot filament chemical vapour deposition (HFCVD) from a gas mixture of tetramethylsilane (TMS) and hydrogen at 1373 K under normal pressure. The interface zones were characterized using scanning electron and optical microscopies, X-ray diffraction and electron microprobe microanalysis. All analyses reveal that SiC reacts with substrates. Various metal silicides and carbides were formed in layered reaction and the presence of these phases was confirmed by electron probe microanalysis. The effects of rhenium on the reactivity were established by the determination of growth kinetics deducted from the thicknesses of reaction zones as a function of annealing time. It has been found that an increase in the diffusion kinetics and activation energy with the quantity of rhenium in the tungsten wire.

Deuterium retention in rhenium-doped tungsten

Deuterium retention in rhenium doped tungsten (W–Re) was investigated by means of the thermodesorption technique. W–Re alloys of different structures and concentrations of Re were investigated. All tungsten rhenium alloys had a common structural feature – micron size voids in the bulk. The samples were irradiated by 200eV deuterium ions in the fluence range 1022–1024D/m2. The thermodesorption spectra of polycrystalline W measured with a linear ramp of 5K/s has two peaks at ∼450 and 650K. In TDS of W–Re alloys an additional high-temperature peak was found at 850K. The observed retention in W–Re alloys depends strongly on materials structure. In the range of 1–10%Re admixture the Re concentration in W–Re alloy does not influence noticeably the deuterium retention. Structure changes (filling of voids in material) was found to take place both at annealing and low-energy deuterium irradiation.

Kilovision: thermal modeling of a kilovoltage x-ray source integrated into a medical linear accelerator.

The thermal and thermo-mechanical (fatigue) properties of a stationary-anode kilovoltage x-ray source that can be integrated into the head of a medical linear accelerator have been modeled. A finite element program has been used to model two new target designs. The first design makes minor modifications to the existing target assembly of a Varian medical linear accelerator, while the second design adds an additional cooling tube, changes the target angle, and uses a tungsten-rhenium alloy rather than tungsten as the kilovoltage target material. The thermal calculations have been used to generate cyclic stress/strain values from which estimates of fatigue in the target designs have been made. Both kilovoltage and megavoltage operation have been studied. Analysis of the megavoltage operation shows that there are only small differences in the thermal and fatigue characteristics after the target assembly is modified to include a kilovoltage target. Thus, megavoltage operation should not be compromised. The first kilovoltage target design can handle a 900 W heat load (e.g., 120 kVp, 7.5 mA, 2 x 2 mm2 source size); the heat load being limited by the temperature at the surface of the cooling tubes and mechanical fatigue at the surface of the target. The second design can handle a 1250 W heat load (e.g., 120 kVp, approximately 10.4 mA, 2 x 2 mm2 source size). Our calculations show that installation of a kilovoltage x-ray target is practical from thermal and thermo-mechanical perspectives.

Phase diagram and lattice instability in tungsten–rhenium alloys

Tungsten has been suggested as a material in applications where it is irradiated by neutrons and undergoes transmutation to rhenium. Pure W has the bcc lattice structure. According to the equilibrium phase diagram about 30 at.% Re can go into a bcc W–Re solid solution. However, ab initio electronic structure calculations show that the bcc lattice becomes dynamically unstable at high Re concentrations. Through a detailed calculation of the phonon spectrum, we find that bcc W 1− c Re c does not become dynamically unstable until c≳0.7, i.e., well above the equilibrium solubility limit of Re in bcc W. Concentration fluctuations of Re in irradiated W will therefore not lead to any significant number of regions where the bcc lattice collapses due to a dynamical instability.

Use of powders of the tungsten—Rhenium alloys for the preparation of impregnated cathode skeletons. II. Emission capabilities of barium containing impregnated cathodes with tungsten—Rhenium alloy skeletons

The dependence of the emission properties of barium-containing impregnated cathodes on the rhenium content, phase composition, and pore structure of the metallic skeleton was studied. Skeletons were prepared from mixtures of elemental W and Re powders, as well as from W—Re alloy powders. The lowest work function (π(1300 K)=1.97–1.99 eV) was observed in cathodes containing 50–80 mass % Re. The use of W—Re alloy powders with additions of highly dispersed aluminum oxide for the preparation of skeletons increased the lifetime of impregnated cathodes by 2–2.5 times. Such cathodes possessed a stable and uniform grain structure which was resistant to recrystallization and grain growth, thus they had a stable skeleton structure.

Use of powders of the tungsten—Rhenium alloys for the preparation of impregnated cathode skeletons I. Densification and formation of porous structure in sintering of powders of tungsten-rhenium alloys

The effect of phase composition, fineness, and morphology of W-Re powders with different rhenium content on compaction, and formation of porous structure has been studied. Fine particle powders with a specific surface of 5–8 m2/g, mechanical mixtures of tungsten and rhenium powders, and W-Re alloy powders were used. Local nonuniform compaction is observed during sintering of powder mixtures and alloy powders. It was established that alloy powders and intermetallic powders have lower sintering activity.

Key Points in Processing of Tungsten-lUienium Alloys

Tungsten-rhenium alloys play an important role in various modern technical applications. These alloys are primarily produced by powder metallurgy. Nevertheless, the usage of these alloys is sometimes limited due to difficulties encountered in processing. The common problems are as follows: 1. The inhomogeneity of the alloys composition. 2. The bubbling effect during sintering of billets. 3. Breaks occurring during deformation. 4. The formation of a brittle second phase in the alloy. With the purpose to improve the properties of these alloys, a systematic review of the above mentioned problems is presented.

Erosion lifetime of ITER divertor plates

For ITER divertor operation in the detached regime, erosion of divertor plates made of beryllium, carbon fibre composites and tungsten-rhenium alloys is evaluated. Erosion due to (a) physical and, for CFC, chemical sputtering (including surface temperature variation over the lifetime because of neutron irradiation), (b) disruption thermal quench using published data and (c) evaporation and melt layer loss (metals) for infrequent high-power transients (20 MW/m 2, up to 10 s) is evaluated both with and without the reduction of net incident power by radiation from the evaporated impurities (‘low density vapour shield’). The composite lifetime is calculated for normal operation (detached, 10% transients, 10% disruptions); the effect of an increase in transient frequency and in incident power (semi-attached operation) is determined. The resulting erosion lifetime excludes beryllium, whereas both CFC and W-3%Re yield acceptable lifetimes.

Sigma Phase in Tungsten-Rhenium Alloys. I

Abstract In developing tungsten alloys for service both above the recrystallization temperature and below room temperature the main task is to make an alloy, which in addition to having a low ductile to brittle transition temperature in the recrystallized condition can readily be manufactured into various configurations. Tungsten-rhenium alloys have been found to be useful for such applications, since they have a combination of high strength and good ductility. The best ductility is reached in alloys with a rhenium content close to the solubility limit in tungsten. During manufacturing of such alloys the forming properties are adversely affected by the precipitation of the intermetallic compound W2Re3 (σ-phase). The purpose of this work is to clarify the influence of sigma phase on the formability and product quality after high temperature densification and thermomechanical treatment (TMT) of powder metallurgy manufactured W-25%Re alloys using different processing parameters. The study consists of two parts. Part I deals with theoretical and practical aspects on the formation of sigma phase particles and their influence on metal flow and product quality. Different ways of interpreting the W-Re phase are discussed and the characteristics of the σ-phase are presented. Practical recommendations to control the negative influence of the σ-phase are also given. A more complete understanding of the effect of various processing parameters will be the subject of Part II of this paper.

Sigma Phase in Tungsten-Rhenium Alloys. II

Sigma Phase in Tungsten-Rhenium Alloys. II

Effect of rhenium addition on fracture toughness of tungsten at elevated temperatures

Fracture toughness tests of tungsten and tungsten-rhenium alloy specimens were carried out at elevated temperatures. Temperature dependence of fracture toughness and effect of rhenium content on fracture toughness were investigated. Although fracture toughnesses of three kinds of specimens with rhenium contents of 0, 5 and 10 wt% were almost identical at room temperature, fracture toughness at elevated temperatures increased with increasing rhenium content. The brittle-ductile transition, similar to steels, and subsequent transition of the fracture mode from ductile dimple to intergranular were observed for all three kinds of specimens. With increasing rhenium content, the transition temperatures increased. A significant grain growth was found, not for tungsten-rhenium alloy specimens, but for a tungsten specimen without rhenium in a temperature range higher than the recrystallizing temperature, which resulted in transition of the fracture mode from dimple to intergranular.