Tungsten and Its Alloys

Electrodeposited nanocrystalline nickel-tungsten alloys are being investigated as an attractive alternative to electrodeposited nickel for applications involving fabrication of micro electro mechanical systems (MEMS). Ni-W alloys are also being considered as an environmentally friendly alternative to hard chrome plating in some cases. In applications involving sliding contacts such as in micro-gears in MEMS, mould inserts, etc., tribological properties of Ni-W alloys would be of relevance. In this work, the sliding friction and wear characteristics of Ni-W alloys with different tungsten contents were investigated and compared with that of nickel film deposited from sulphamate bath commonly used in microfabrication. For wear tests, Ni-W alloy films of about 5-7 μm, deposited from ammonia-citrate baths on copper substrates were employed. The alloy films possessed W contents in the range of 8.4-12.7 at.% and had an average grain size of about 20 nm. Wear tests were conducted in a pin-on-disc type tribometer under un-lubricated conditions. All the wear tests were carried out at room temperature in air with a controlled relative humidity of 50 ± 5% at a normal load and linear sliding speed of 1 N and 3 cm s-1, respectively. Hardened steel balls were used as the counter body. Friction force was recorded online during the wear test. Wear damage on Ni and Ni-W alloy films was estimated from the width of the wear track and the wear rate of the counter body was calculated from the worn volume. Results show that Ni-W alloys have somewhat lower friction coefficient against steel counter body as compared with that of the nickel-steel pair. Addition of tungsten to nickel is also seen to result in an improvement in wear resistance. Friction and wear mechanisms operative in Ni-W alloys sliding against steel are discussed.

We use a physically-based crystal plasticity model to predict the yield strength of body-centered cubic (bcc) tungsten single crystals subjected to uniaxial loading. Our model captures the thermally-activated character of screw dislocation motion and full non-Schmid effects, both of which are known to play critical roles in bcc plasticity. The model uses atomistic calculations as the sole source of constitutive information, with no parameter fitting of any kind to experimental data. Our results are in excellent agreement with experimental measurements of the yield stress as a function of temperature for a number of loading orientations. The validated methodology is employed to calculate the temperature and strain-rate dependence of the yield strength for 231 crystallographic orientations within the standard stereographic triangle. We extract the strain-rate sensitivity of W crystals at different temperatures, and finish with the calculation of yield surfaces under biaxial loading conditions that can be used to define effective yield criteria for engineering design models.

Full journal version available: doi: 10.1016/j.scriptamat.2018.08.045 This poster presents the preliminary results of our work on Cr-Mo-V-W high-entropy alloy published as "Cr-Mo-V-W: A new refractory and transition metal high-entropy alloy system". For further details, http://unsworks.unsw.edu.au/fapi/datastream/unsworks:57273/bin5cf5882d-409c-485b-b03d-3190d0c72699?view=true Abstract: Cr-Mo-V-W high-entropy alloy (HEA) is studied, with 2553 K equilibrium solidus and high Cr content to promote protective oxide scale formation, suggesting potential applications in hot, oxidising environments. Alloy Search and Predict (ASAP) and phase diagram calculations found a single-phase, body-centred cubic (BCC) solid solution at elevated temperatures, across the range of compositions present within the system - uncommon for a HEA of refractory and transition metals. Density functional theory identified solubility of 22 at.% Cr at solidus temperature, with composition-dependent drive for segregation during cooling. An as-cast, BCC single-phase with the composition 31.3Cr-23.6Mo-26.4 V-18.7 W exhibiting dendritic micro segregation was verified. DOI: 10.1016/j.scriptamat.2018.08.045

Tungsten (W) as a structural component has grown roots in many special applications owing to its radiation-shielding capabilities and its properties at elevated temperatures. The high ductile-brittle transition temperature (DBTT) and the very high melting point of tungsten however have limited its processability to certain technologies such as powder metallurgy. Laser powder bed fusion (LPBF) has been introduced in recent years as an alternative for manufacturing tungsten parts to overcome the design limitations posed by powder metallurgy technology. A review of the literature shows significant improvements in the quality of tungsten components produced by LPBF, implying a strong potential for manufacturing tungsten with this technology and a need for further research on this subject. This review paper presents the current state-of-the-art in LPBF of unalloyed tungsten, with a focus on the effect of process parameters on the developed structure/properties and identifies current knowledge gaps.

Measuring the fracture behavior of small specimens of semibrittle materials such as tungsten is often difficult due to the lack of crack stability and a high ratio of the crack tip plastic zone size to specimen dimensions. To overcome this, microcantilever bending tests were used with a stable chevron notch geometry coupled with elastic-plastic fracture mechanical (EPFM) analysis. The chevron notch geometry was first validated by measurement of the ð111Þ cleavage toughness in single-crystal Si, then fracture resistance curves (R-curves) were calculated via EPFM analysis of fracture data obtained from a semibrittle W-1%Ta alloy. The accuracy of the fracture resistance curves measured from W-1%Ta was evaluated by means of ASTM standard macro-scopic fracture tests. The conditional fracture toughness (K Qc) prior to crack instability was found to be five times larger than the macroscopic fracture toughness (K Ic), due to the combination of plastic tearing of ductile ligaments and the extensive microplasticity ahead of the crack tip. These results suggest that use of chevron-notched microcantilevers is suitable for evaluating the fracture toughness of brittle silicon but overestimates the fracture toughness value for semibrittle tungsten.

Tungsten is a widely used transition metal that has not been thoroughly investigated with regards to its ecotoxicological effects. Tungsten anions polymerize in environmental systems as well as under physiological conditions in living organisms. These polymerization/condensation reactions result in the development of several types of stable polyoxoanions. Certain chemical properties (in particular redox and acidic properties) differentiate these polyanions from monotungstates. However, our current state of knowledge on tungsten toxicology, biological and environmental effects is based entirely on experiments where monotungstates were used and assumed by the authors to be the form of tungsten that was present and that produced the observed effect. Recent discoveries indicate that tungsten speciation may be important to ecotoxicology. New results obtained by different research groups demonstrate that polytungstates develop and persist in environmental systems, and that polyox-otungstates are much more toxic than monotungstates. This paper reviews the available toxicological information from the standpoint of tungsten speciation and identifies knowledge gaps and pertinent future research directions.

With two regulations, 244/2009 and 245/2009, the European Commission recently put into practice the EuP Directive in the area of lighting devices, aiming to improve energy efficiency in the domestic lighting sector. This article presents a comprehensive life cycle assessment comparison of four different lighting technologies: the tungsten lamp, the halogen lamp, the conventional fluorescent lamp and the compact fluorescent lamp.

Nanocrystalline Ni-W alloys have good potential in LIGA fabricated MEMS. Thermal stability of Ni-12 at%W alloy prepared on silicon wafer using PMMA resist irradiated by synchrotron x-ray, as well as on copper is investigated. The grain size of Ni-12 at%W alloy increases from 21 nm to 31 nm upon heating at 873 K for 48 hours. Grain growth exponent is found to be 2.25, which indicates an insignificant role of grain boundary diffusion. The microhardness of the alloy decreases from 705 to 420 HV. Under the same heating condition, the hardness of sulphamate Ni decreases from 324 to 92 HV. The initial cross sectional banded microstructure of LIGA test microcomponent persists during heating at 873 K for 48 hours. A comparison with literature data on nanocrystalline Ni and Ni base alloys reveals that Ni-12 at%W alloy possesses a much more stable structure at 873 K.

http://www.scopus.com/inward/record.url?eid=2-s2.0-53849145039&partnerID=40&md5=25886d73c700cb519d2e56f3216a8945

Ammonia-citrate bath has been investigated for the deposition of nano crystalline Ni-W alloy micro components using the LIGA process. First the bath stability and deposit characteristics were studied. Fabrication of micro specimens were then carried out on silicon substrates covered with novolac as well as thick PMMA resist for LIGA. Effects of different parameters like current density, nickel ion and tungsten ion concentration in the bath, deposition time etc. on the deposit characteristics and current efficiency were studied. The deposited Ni-W samples were characterized by scanning electron microscopy, energy depressive X-ray spectroscopy, light optical microscopy and X-ray diffraction. Results show that during a few tens of hours of deposition, ammonia loss from the covered bath used is minimal and the bath remains stable. Selection of proper bath and deposition parameters allows a window for the deposition of crack free, thick, nano crystalline nickel-tungsten alloys. Using the optimum parameters, it has been possible to fabricate Ni-12 at% W micro tensile specimens with a nominal thickness of 120 μm by the LIGA process.

http://link.springer.com/article/10.1007/s00542-007-0447-5

The intensity-calibrated spectra of W II have been recorded in the spectral interval 23 000–51 300 cm−1 (1950–4350 Å), using the FT500 UV Fourier Transform Spectrometer at Lund Observatory. Combining the intensity data in this work with lifetimes previously measured using the time-resolved laser-induced-fluorescence (TR-LIF) technique resulted in transition probabilities and log gf values for 95 transitions in W II, originating from nine different upper levels with energies between 47 179 and 55 392 cm−1. Of these transitions, 85 have never been measured before. The new data are compared with theoretical calculations and with previously measured values when available.