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Oxidation performance of V-Cr-Ti alloys

Description: Vanadium-base alloys are being considered as candidates for the first wall in advanced V-Li blanket concepts in fusion reactor systems. However, a primary deterrent to the use of these alloys at elevated temperatures is their relatively high affinity for interstitial impurities, i.e., O, N, H, and C. The authors conducted a systematic study to determine the effects of time, temperature, and oxygen partial pressure (pO{sub 2}) in the exposure environment on O uptake, scaling kinetics, and scale microstructure in V-(4--5) wt.% Cr-(4--5) wt.% Ti alloys. Oxidation experiments were conducted on the alloys at pO{sub 2} in the range of 5 x 10{sup {minus}6}-760 torr (6.6 x 10{sup {minus}4}-1 x 10{sup 5} Pa) at several temperatures in the range of 350--700 C. Models that describe the oxidation kinetics, oxide type and thickness, alloy grain size, and depth of O diffusion in the substrate of the two alloys were determined and compared. Weight change data were correlated with time by a parabolic relationship. The parabolic rate constant was calculated for various exposure conditions and the temperature dependence of the constant was described by an Arrhenius relationship. The results showed that the activation energy for the oxidation process is fairly constant at pO{sub 2} levels in the range of 5 x 10{sup {minus}6}-0.1 torr. The activation energy calculated from data obtained in the air tests was significantly lower, whereas that obtained in pure-O tests (at 760 torr) was substantially higher than the energy obtained under low-pO{sub 2} conditions. The oxide VO{sub 2} was the predominant phase that formed in both alloys when exposed to pO{sub 2} levels of 6.6 x 10{sup {minus}4} to 0.1 torr. V{sub 2}O{sub 5} was the primary phase in specimens exposed to air and to pure O{sub 2} at 760 torr. The implications of the increased O concentration are ...
Date: April 3, 2000
Creator: Natesan, K. & Uz, M.
Partner: UNT Libraries Government Documents Department

Development of electrically insulating coatings for service in a lithium environment

Description: Several experiments were conducted to develop electrically insulating CaO coatings on a V-4Cr-4Ti alloy for application in an Li environment. The coatings were developed by vapor phase transport external to Li, and also in-situ in an Li-Ca environment at elevated temperature. In the vapor phase study, several geometrical arrangements were examined to obtain a uniform coating of Ca on the specimens, which were typically coupons measuring 5 to 10 x 5 x 1 mm. After Ca deposition from the vapor phase, the specimens were oxidized in a high-purity argon environment at 600 C to convert the deposited metal into oxide. The specimens exhibited insulating characteristics after this oxidation step. Several promising coated specimens were then exposed to high-purity Li at 500 C for 48--68 h to determine coating integrity. Microstructural characteristics of the coatings were evaluated by scanning electron microscopy and energy-dispersive X-ray analysis. Electrical resistances of the coatings were measured by a two-probe method between room temperature and 700 C before and after exposure to Li.
Date: May 17, 2000
Creator: Natesan, K.; Uz, M. & Wieder, S.
Partner: UNT Libraries Government Documents Department

Effects of thermomechanical processing on tensile and long-time creep behavior of Nb-1%Zr-0.1%C sheet

Description: Effects of thermomechanical processing on the mechanical properties of Nb-1 wt%Zr-0.1wt.%C, a candidate alloy for use in advanced space power systems, were investigated. Sheet bars were cold rolled into 1-mm thick sheets following single, double, or triple extrusion operations at 1,900 K. All the creep and tensile specimens were given a two-step heat treatment of 1 hr at 1,755 K + 2 hr at 1,475 K prior to testing. Tensile properties were determined at 300 as well as at 1,350 K. Microhardness measurements were made on cold rolled, heat treated, and crept samples. Creep tests were carried out at 1,350 K and 34.5 MPa for times of about 10,000 to 19,000 hr. The results show that the number of extrusions had some effects on both the microhardness and tensile properties. However, the long-time creep behavior of the samples were comparable, and all were found to have adequate properties to meet the design requirements of advanced power system regardless of thermomechanical history. The results are discussed in correlation with processing and microstructure, and further compared to the results obtained from the testing of Nb-1wt.%Zr and Nb-1wt.%Zr-0.06wt.%C alloys.
Date: July 1, 1994
Creator: Titran, R. H. & Uz, M.
Partner: UNT Libraries Government Documents Department

Effects of oxygen and oxidation on tensile behavior of V-(4-5)Cr(4-5)Ti alloys

Description: Vanadium-base alloys are potential candidates for applications such as the first wall and other structural components of fusion reactors, but a good understanding of the oxidation behavior of the alloys intended for elevated-temperature use is essential. The authors conducted a systematic study to determine the effects of time and temperature of air exposure on the oxidation behavior and microstructure of V-4Cr-4Ti and V-5Cr-5Ti alloys. Uniaxial tensile tests were conducted at room temperature and at 500 C on preoxidized specimens of the alloys to examine the effects of oxidation time and oxygen migration on maximum engineering stress and uniform and total elongation. The effect of preexposure of the specimens to environments with varying oxygen partial pressures on the tensile properties of both alloys was investigated. Extensive microstructural analyses of the oxygen-exposed/tensile-tested specimens were conducted to evaluate the cracking propensity for the alloys. In addition, tensile-property data for the alloys were correlated with oxygen pressure in the exposure environment, test temperature, and exposure time.
Date: October 1, 1997
Creator: Natesan, K.; Soppet, W.K. & Uz, M.
Partner: UNT Libraries Government Documents Department

Development of CaO coatings by thermal and chemical vapor deposition.

Description: We have developed CaO coatings that are applied by a thermal and chemical vapor deposition process. Several experiments were conducted to study how the deposition of Ca on a V-4Cr-4Ti substrate alloy is affected by variations in process temperature and time, specimen location, and surface preparation and pretreatment. Results showed that thick adherent coatings can be fabricated by thermal/chemical vapor deposition, especially if a double Ca treatment is applied. Extensive microstructural analysis of the coatings showed almost 100% CaO over a coating thickness of 20-30 {micro}m; electrical resistance (measured by the two-probe method) of the coatings was at least two orders of magnitude higher than the minimum required for blanket application. The results obtained in this study indicate that CaO is a viable coating for V-Li advanced blankets, but that significant additional effort is needed, especially from the standpoint of structure/composition relationship to its electrical resistance and the coating stability in a flowing Li environment. Furthermore, resistance must be measured in situ in Li to simultaneously evaluate coating integrity, resistance, and Li compatibility.
Date: April 26, 2002
Creator: Natesan, K.; Uz, M. & Smith, D. L.
Partner: UNT Libraries Government Documents Department

Electrically insulating coatings for V-Li self-cooled blanket in a fusion system

Description: The blanket system is one of the most important components in a fusion reactor because it has a major impact on both the economics and safety of fusion energy. The primary functions of the blanket in a deuterium/tritium-fueled fusion reactor are to convert the fusion energy into sensible heat and to breed tritium for the fuel cycle. The liquid-metal blanket concept requires an electrically insulating coating on the first-wall structural material to minimize the magnetohydrodynamic pressure drop that occurs during the flow of liquid metal in a magnetic field. Based on the thermodynamics of interactions between the coating and the liquid lithium on one side and the structural V-base alloy on the other side, several coating candidates are being examined to perform the insulating function over a wide range of temperatures and lithium chemistries.
Date: May 17, 2000
Creator: Natesan, K.; Reed, C. B.; Uz, M.; Park, J. H. & Smith, D. L.
Partner: UNT Libraries Government Documents Department

Processing and microstructure of Nb-1%Zr-0.1%C alloy sheet

Description: A systematic study was carried out to evaluate the effects of processing on the microstructure of a Nb-1wt.%Zr-0.1wt.%C alloy sheet. The samples were fabricated by cold rolling different sheet bars that were single-, double- or triple-extruded at 1900 K. Heat treatment consisted of one- or two-step annealing of different samples at temperatures ranging from 1350 K to 1850 K. The assessment of the effects of processing on microstructure involved characterization of the precipitates including the type, crystal structure, chemistry and distribution within the material as well as an examination of the grain structure. A combination of various analytical and metallographic techniques were used on both the sheet samples and the residue extracted from them. The results show that the relatively coarse orthorhombic Nb{sub 2}C carbides in the as-rolled samples transformed to rather fine cubic monocarbides of Nb and Zr with varying Zr/Nb ratios upon subsequent heat treatment. The relative amount of the cubic carbides and the Zr/Nb ratio increased with increasing number of extrusions prior to cold rolling. Furthermore, the size and the aspect ratio of the grains appear to be strong functions of the processing history of the material. These and other results obtained will be presented with the emphasis on a possible relationship between processing and microstructure.
Date: September 1, 1994
Creator: Uz, M. & Titran, R. H.
Partner: UNT Libraries Government Documents Department

Thermal stability of the microstructure of an aged Nb-Zr-C alloy

Description: The effects of thermally aging with and without an applied stress on the microstructure of a Nb-Zr-C alloy containing 0.9 wt % Zr and 0.06 wt % C were studied. Chemical analysis, metallographic examination, energy dispersive x-ray spectra of the bulk material, and chemical and x-ray analyses of the phase-extracted residue were used to characterize the microstructure. The samples examined were from a creep strength study involving hot and cold working, and various combinations of exposure to temperatures ranging from 1350 to 1755 K with and without applied load for times as long as 34,000 plus hours. The results showed that the initial microstructure consisted primarily of orthorhombic precipitates of Nb{sub 2}C which were partially or completely transformed to face-centered cubic carbides of Nb and Zr, (Zr,Nb)C, upon prolonged exposure to elevated temperatures. Furthermore, it was found that the microstructure of the alloy is extremely stable owing to the very finely distributed precipitates throughout its matrix and along the grain boundaries. The lattice parameters of the cubic carbides were determined to vary from 0.458 to 0.465 nm as the Zr/Nb ratio varied from 38/62 to 75/25. 25 refs., 5 figs., 4 tabs.
Date: January 1, 1991
Creator: Uz, M. (Lafayette Coll., Easton, PA (USA). Dept. of Chemical Engineering) & Titran, R.H. (National Aeronautics and Space Administration, Cleveland, OH (USA). Lewis Research Center)
Partner: UNT Libraries Government Documents Department