105 research outputs found

    Low cost, formable, high T(sub c) superconducting wire

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    A ceramic superconductivity part such as a wire is produced through the partial oxidation of a specially formulated copper alloy in the core. The alloys contain low level quantities of rare earth and alkaline earth dopant elements. Upon oxidation at high temperature, superconducting oxide phases are formed as a thin film

    The effect of sulfur and zirconium Co-doping on the oxidation of NiCrAl

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    The adhesion behavior of Al2O3 scales formed on NiCrAl+Zr alloys was examined as a function of both sulfur and zirconium doping levels. In general, very high levels of zirconium were required to counteract the detrimental effects of sulfur. A sulfur-zirconium adherence map was constructed, as determined from the oxidation and spalling behavior in 1100 C cyclic tests. For low sulfur alloys, the amount of zirconium required for adherence at any given sulfur level can be described by Zr greater than 600 S sup 0.2 (in ppma). These results underscore the importance of sulfur to adhesion mechanisms and suggests that sulfur gettering is a first order effect of reactive element additions to MCrAl alloys

    The chemistry of Saudi Arabian sand: A deposition problem on helicopter turbine airfoils

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    Recent operations in the Persian Gulf have exposed military helicopter turbines to excessive amounts of ingested sand. Fine particles, less than 10 microns, are able to bypass the particle separators and enter the cooling and combustion systems. The initial sand chemistry varies by location, but is made up of a calcium aluminum silicate glass, SiO2 low quartz (Ca,Mg) CO3 dolomite, CaCO3 calcite, and occasionally CaCl rocksalt. The sand reacts in the hot combustion gases and deposits onto the turbine vanes as CaSO4, glass, and various crystalline silicates. Deposits up to 0.25 in. thick have been collected. Although cooling hole plugging is a considerable problem, excessive corrosion is not commonly observed due to the high melting point of GaSO4

    Method of forming low cost, formable High T(subc) superconducting wire

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    A ceramic superconductivity part, such as a wire, is produced through the partial oxidation of a specially formulated copper alloy in a core. The alloys contains low level of quantities of rare earth and alkaline earth dopant elements. Upon oxidation at high temperatures, and superconducting oxide phases are formed as a thin film

    Oxidation behavior of FeAl+Hf,Zr,B

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    The oxidation behavior of Fe-40Al-1Hf, Fe-40Al-1Hf-0.4B, and Fe-40Al-0.1Zr-0.4B (at. percent) alloys was characterized after 900, 1000, and 100 C exposures. Isothermal tests revealed parabolic kinetics after a period of transitional theta-alumina scale growth. The parabolic growth rates for the subsequent alpha-alumina scales were about five times higher than those for NiAl+0.1Zr alloys. The isothermally grown scales showed a propensity toward massive scale spallation due to both extensive rumpling from growth stresses and to an inner layer of HfO2. Cyclic oxidation for 200 1-hr cycles produced little degradation at 900 or 1000 C, but caused significant spallation at 1100 C in the form of small segments of the outer scale. The major difference in the cyclic oxidation of the three FeAl alloys was increased initial spallation for FeAl+Zr,B. Although these FeAl alloys showed many similarities to NiAl alloys, they were generally less oxidation resistant. It is believed that this resulted from nonoptimal levels of dopants and larger thermal expansion mismatch stresses

    Moisture-Induced TBC Spallation on Turbine Blade Samples

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    Delayed failure of TBCs is a widely observed laboratory phenomenon, although many of the early observations went unreported. "The weekend effect" or "DeskTop Spallation" (DTS) is characterized by initial survival of a TBC after accelerated laboratory thermal cycling, then failure by exposure to ambient humidity or water. Once initiated, failure can occur quite dramatically in less than a second. To this end, the water drop test and digital video recordings have become useful techniques in studies at NASA (Smialek, Zhu, Cuy), DECHMA (Rudolphi, Renusch, Schuetze), and CNRS Toulouse/SNECMA (Deneux, Cadoret, Hervier, Monceau). In the present study the results for a commercial turbine blade, with a standard EB-PVD 7YSZ TBC top coat and Pt-aluminide diffusion bond monitored by weight change and visual appearance. Failures were distributed widely over a 5-100 hr time range, depending on temperature. At some opportune times, failure was captured by video recording, documenting the appearance and speed of the moisture-induced spallation process. Failure interfaces exhibited alumina scale grains, decorated with Ta-rich oxide particles, and alumina inclusions as islands and streamers. The phenomenon is thus rooted in moisture-induced delayed spallation (MIDS) of the alumina scale formed on the bond coat. In that regard, many studies show the susceptibility of alumina scales to moisture, as long as high strain energy and a partially exposed interface exist. The latter conditions result from severe cyclic oxidation conditions, which produce a highly stressed and partially damaged scale. In one model, it has been proposed that moisture reacts with aluminum in the bond coat to release hydrogen atoms that 'embrittle' the interface. A negative synergistic effect with interfacial sulfur is also invoked

    Oxygen diffusivity in alumina scales grown on Al-MAX phases

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    Ti3AlC2, Ti2AlC, and Cr2AlC are oxidation resistant MAX phase compounds distinguished by the formation of protective Al2O3 scales with well controlled kinetics. A modified Wagner treatment was used to calculate interfacial grain boundary diffusivity from scale growth rates and corresponding interfacial grain size, based on the pressure dependence of oxygen vacancies and diffusivity. MAX phase data from the literature yielded grain boundary diffusivity nearly coincident with that for Zr-doped FeCrAl (and many other FeCrAl alloys), suggesting similar oxidation mechanisms. The consolidated body of diffusivity data was consistent with an activation energy of 375 ± 25 kJ/mol

    A deterministic interfacial cyclic oxidation spalling model

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    A series summation has been developed to model the iterative scale growth and spalling process of cyclic oxidation. Parabolic scale growth has been assumed. Interfacial spallation of a constant area fraction was stipulated to occur only at the thickest portions. Inputs are the parabolic growth rate constant, spall area fraction, oxide stoichiometry, and cycle duration. Outputs include the net weight change, amount of oxygen and metal consumed, and amount of oxide spalled. Classic weight change curves are produced with an initial maximum and final linear weight loss rate. This simplicity allowed for representation by explicit algebraic functions for all outputs and characteristic features. The maximum in weight change varies directly with the parabolic rate constant and cycle duration and inversely with the spall fraction, all to the ½ power. The ratio of the number of cycles to reach maximum and zero weight change is exactly 1:3, and these vary only with the inverse of the spall fraction. Many similarities to and some differences with previous cyclic models are identified

    Compiled furnace cyclic lives of EB-PVD thermal barrier coatings

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    Furnace cycling has been widely used to study the failure of EB-PVD thermal barrier coatings. This contribution compiles TBC furnace cyclic lives over a broad literature base to highlight optimum systems and generalized trends not always apparent in one study. Systems included typical bond coats (Pt-modified aluminides, diffused Pt-only γ/γ′, and NiCoCrAlY (±Pt, Hf) overlays) and superalloy substrates (1st, 2nd, 3rd generation single crystals, directionally solidified, or conventionally cast). Pretreatments included controlled low p(O2) bond coat pre-oxidation and grit blasting (or none). The aggregate lives (~70) suggest a general trend with temperature, ~10-fold decrease for every 100 °C increase. Measured alumina scale thicknesses (~30) were, on average, ~6.1 ± 1.8 μm at failure and independent of temperature for conventional systems. Most failures thus occurred in less time than that predicted to grow 7 μm of alumina scale (as estimated from separate TGA studies of a Ptmodified aluminide coated 2nd generation single crystal superalloy). A tentative activation energy indicated from the broad distribution of failure times was ~280 kJ/mol, while that from homogeneous TGA testing was ~380 kJ/mol, with regression coefficients of r2 = 0.57 and 0.98, respectively

    A deterministic interfacial cyclic oxidation spalling model

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    A series summation has been developed to model the iterative scale growth and spalling process of cyclic oxidation. Parabolic scale growth has been assumed. Interfacial spallation of a constant area fraction was stipulated to occur only at the thickest portions. Inputs are the parabolic growth rate constant, spall area fraction, oxide stoichiometry, and cycle duration. Outputs include the net weight change, amount of oxygen and metal consumed, and amount of oxide spalled. Classic weight change curves are produced with an initial maximum and final linear weight loss rate. This simplicity allowed for representation by explicit algebraic functions for all outputs and characteristic features. The maximum in weight change varies directly with the parabolic rate constant and cycle duration and inversely with the spall fraction, all to the ½ power. The ratio of the number of cycles to reach maximum and zero weight change is exactly 1:3, and these vary only with the inverse of the spall fraction. Many similarities to and some differences with previous cyclic models are identified
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