Academic literature on the topic 'Tungsten - Analysis'

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references (p. 31).<br>Nanocrystalline nickel-tungsten alloys are harder, stronger, more resistant to degradation, and safer to electrodeposit than chromium. Atomistic computer simulations have previously met with success in replicating the energetic and atomic conditions of physical systems with 2-4nm grain diameters. Here, a new model subjects a vertically thin unique volume containing 3nm or 10nm FCC grains with aligned z axes to a Monte Carlo-type minimization to investigate the segregation and ordering behavior of W atoms. Short-range order is also tracked with the Warren-Cowley parameter, and energetic results are explored as well. It was found that the Ni-W system has a very strong tendency toward SRO. The 10nm models exhibited more robust order at low concentrations, but ordering in the 3nm model was generally more pronounced. At the dilute limit atoms are driven to the grain boundaries, but as the boundaries are saturated intragranular ordered formations increase and may even perpetuate over low-angle grain boundaries. Ordering was also observed within the grain boundaries at all concentrations for both diameters. The 10nm models were saturated at lower concentration, and grain boundary energy was reduced by up to 93%. W atoms preferred to associate with each other as third-nearest neighbors, but at very high concentrations formations with W atoms as second nearest neighbors were also observed.<br>by Alison Michelle Engwall.<br>S.B.

The displacement damage induced by primary recoils of fusion neutrons in tungsten and alloys has been studied with self-ion irradiations, followed by damage characterization with electron microscopy. Tungsten and alloys (≤ 5 wt.% Re, Ta, V) were implanted with 2 MeV W+ ions over a dose range of 3.3×1017 - 2.5×1019 W+m-2 at temperatures ranging from 300 to 750°C. Dislocation loops of b = ½<111> (> 60%) and b = <100> were identified, and that ½<111> loops were found more thermally stable. Among loops that were large enough for nature determination, at least 50% were found to be of interstitial type, with larger fractions in high-temperature and high-dose conditions. The diameter of loops did not exceed 20 nm, with the majority being ≤ 5 nm. The loop number density varied between 1022 and 1023 m-3. The effects of ion dose, irradiation temperature, composition and grain orientation on damage microstructure were investigated. In-situ irradiations (150 keV W+ ions) were carried out as a complement to the bulk implantations. Qualitative trends in loop size, geometry and nature with irradiation dose and temperature were similar to bulk irradiated specimens. Also, the dynamics of defects and their effects on the damage evolution were explored. In-situ annealing of irradiated thin foils was performed to investigate the thermal stability of radiation damage in tungsten. The majority of microstructure transformations were completed within 15 min of annealing. However, extended durations did favour the increase of loop size and the fraction of ½<111> loops.

Welding is used extensively in aerospace, automotive, chemical, manufacturing, electronic and power-generation industries. Thermally-induced residual stresses due to welding can significantly impair the performance and reliability of welded structures. Numerical simulation of weld pool dynamics is important as experimental measurements of velocities and temperature profiles are difficult due to the small size of the weld pool and the presence of the arc. From a structural integrity perspective of welded structures, it is necessary to have an accurate spatial and temporal thermal distribution in the welded structure before stress analysis is performed. Existing research on weld pool dynamics simulation has ignored the effect of fluid flow in the weld pool on the temperature field of the welded joint. Previous research has established that the weld pool depth/width (D/W) ratio and Heat Affected Zone (HAZ) are significantly altered by the weld pool dynamics. Hence, for a more accurate estimation of the thermally-induced stresses it is desired to incorporate the weld pool dynamics into the analysis. Moreover, the effects of microstructure evolution in the HAZ on the mechanical behavior of the structure need to be included in the analysis for better mechanical response prediction. In this study, a three-dimensional model for the thermo-mechanical analysis of Gas Tungsten Arc (GTA) welding of thin stainless steel butt-joint plates has been developed. The model incorporates the effects of thermal energy redistribution through weld pool dynamics into the structural behavior calculations. Through material modeling the effects of microstructure change/phase transformation are indirectly included in the model. The developed weld pool dynamics model includes the effects of current, arc length, and electrode angle on the heat flux and current density distributions. All the major weld pool driving forces are included, namely surface tension gradient, plasma drag force, electromagnetic force, and buoyancy. The weld D/W predictions are validated with experimental results. They agree well. The effects of welding parameters (like welding speed, current, arc length, etc.) on the weld D/W ratio are documented. The workpiece deformation and stress distributions are also highlighted. The transverse and longitudinal residual stress distribution plots across the weld bead and their variations with welding speed and current are also provided. The mathematical framework developed here serves as a robust tool for better prediction of weld D/W ratio and thermally-induced stress evolution and distribution in a welded structure by coupling the different fields in a welding process.<br>Ph. D.

<p> The analysis of horse serum samples for copper is relevant to biomarkers for a variety of pathologies in humans. The inexpensive and compact nature of locally laboratory built tungsten filament electrothermal atomic absorption spectrometry instruments (WETAAS) makes them well suited to copper analysis in the clinical context; however, little research in this area exists. This project was undertaken to determine the experimental conditions necessary for determining copper in serum by WETAAS without resorting to sample digestion. Serum samples were prepared by 1:5 dilution in 0.2% nitric acid solution. The filament positioning with the light path plays an important role in the actual number of vaporized atoms of analyte being detected. In aqueous calibration standards, ammonium dihydrogen phosphate was found to be a critical matrix modifier, but in the presence of serum as an interference, the matrix modifier was not of much use. The results indicate that the serum matrix strongly suppresses the copper analytical signal. The merits of using integrated area for quantitation rather than peak height was found in the complex matrix system. Then the background concentration of copper in serum was determined by the standard additions method. Then the same samples were analyzed using a standard method for analysis of copper in serum through flame atomic absorption spectrometry, which is a matrix interference independent system.</p>

Plane strain transient finite thermomechanical deformations of heat-conducting particulate composites comprised of circular tungsten particulates in nickel-iron matrix are analyzed using the finite element method to delineate the initiation and propagation of brittle/ductile failures by the nodal release technique. Each constituent and composites are modeled as strain hardening, strain-rate-hardening and thermally softening microporous materials. Values of material parameters of composites are derived by analyzing deformations of a representative volume element whose minimum dimensions are determined through numerical experiments. These values are found to be independent of sizes and random distributions of particulates, and are close to those obtained from either the rule of mixtures or micromechanics models. Brittle and ductile failures of composites are first studied by homogenizing their material properties; subsequently their ductile failure is analyzed by considering the microstructure. It is found that the continuously varying volume fraction of tungsten particulates strongly influences when and where adiabatic shear bands (ASB) initiate and their paths. Furthermore, an ASB initiates sooner in the composite than in either one of its constituents. We have studied the initiation and propagation of a brittle crack in a precracked plate deformed in plane strain tension, and a ductile crack in an infinitely long thin plate with a rather strong defect at its center and deformed in shear. The crack may propagate from the tungsten-rich region to nickel-iron-rich region or vice-a-versa. It is found that at the nominal strain-rate of 2000/s the brittle crack speed approaches Rayleigh's wave speed in the tungsten-plate, the nickel-iron-plate shatters after a small extension of the crack, and the composite plate does not shatter; the minimum nominal strain-rate for the nickel-iron-plate to shatter is 1130/s. The ductile crack speed from tungsten-rich to tungsten-poor regions is nearly one-tenth of that in the two homogeneous plates. The maximum speed of a ductile crack in tungsten and nickel-iron is found to be about 1.5 km/s. Meso and multiscale analyses have revealed that microstructural details strongly influence when and where ASBs initiate and their paths. ASB initiation criteria for particulate composites and their homogenized counterparts are different.<br>Ph. D.

Summary Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to measure the concentrations of cobalt, tantalum, titanium, vanadium and chromium in solutions of tungsten carbide. The main advantage of the method described here lies in the speed, convenience and effectiveness of the dissolution procedure. Aliquots of powdered tungsten carbide were dissolved in a solution of 5% aqua regia in 30% hydrogen peroxide. Complete dissolution was usually achieved within 10 min. The accuracy of the method was assessed by the analysis of certified reference materials, secondary reference materials and matrix spiking. The method was successfully applied to commercial type samples with differing compositions. Slightly more emphasis was placed on the measurement of vanadium, since no information on the measurement of this element in solutions of tungsten carbide, by ICP-OES, has been published. Investigation of the interference effects of the elements in the sample matrix on each other was essential for accurate results comparable to other published analytical methods.

Fabrication des ensembles multicouches ayant pour role la protection et l'interconnexion des circuits integres. Diminution de la temperature de cofrittage de 1600 a 1400 non=c. Etude de cofrittage realisee sur des bandes d'alumine coulees par "tape casting" puis serigraphiees