The permeation of oxygen through high-purity, large-grain Ag membranes has been studied over the temperature range of 400–800 °C. The permeability was found to be linear and repeatable, but the magnitude was 3.2 times smaller than that determined by past research. This factor may be due to negligible grain boundary diffusion that exists in this work. Auger electron spectroscopy (AES) does, however, suggest the importance of grain boundaries since intragranular oxygen was virtually undetectable and since AES line scans show substantial oxygen signals around the grain boundaries. The diffusivity measurements were found to exhibit two distinct linear regions, one above and one below a critical temperature of 630 °C. The high-temperature data have an activation energy (11.1 kcal mol−1) similar to that reported by others, but the low-temperature data have a comparatively larger activation energy (15.3 kcal mol−1). Vacuum desorption of the oxygen-saturated Ag was found to occur at the critical temperature of 630 °C, which is consistent with the increased mobility of oxygen atoms in the higher temperature regime. The higher activation energy observed in the lower temperature regime is probably due to the higher efficiency of traps.

Abstractand used to measure hydrogen leakage through complex X-33 liquid-hydrogen tank structure sections.

Hydrogen tanks made of polymer-matrix composite material have been proposed as an enabling technology for reducing the weight of reusable launch vehicles and increasing their payload. A key development issue for these lightweight structures is the leakage of hydrogen through the composite material, which is generally a function of the tank material, manufacturing method, mechanical load, any internal damage states in the material, and the temperatures at which the tank must operate. A method for measuring leakage through a geometrically complex structure at cryogenic temperatures and under mechanical load that has been developed, calibrated, and used to measure helium and hydrogen leakage through the X-33 liquid hydrogen tank structure is presented. In particular, results from the calibration tests are presented that indicate that the measurement errors are less than 10% for leak rates ranging from 0.3 to 200 cm 3 /min at standard atmospheric conditions. In addition, both hydrogen and helium leak tests that were performed on two specimens taken from a discarded segment of the X-33 tank structure and results are compared. For both the hydrogen and helium tests leak rates varied with the applied mechanical load. The level of hydrogen leakage is shown to be signi cantly higher than the helium leakage and to exceed the acceptable leak rate for a vehicle like the X-33 liquid hydrogen tank by an order of magnitude.

No abstract

Direct manufacturing of metallic materials has gained widespread interest in the past decade. Of the methods that are currently under evaluation, wire-fed electron beam deposition holds the most promise for producing large-scale titanium parts for aerospace applications [1]. This method provides the cleanest processing environment as the deposition is performed under vacuum. While this environment is beneficial in preventing contamination of the deposit, there is the potential for preferential vaporization of high vapor pressure elements during the deposition process. This can lead to detrimental chemistry variations, which can have negative impacts on physical and mechanical properties. Past experience has shown that deposition of the alloy Ti-6Al-4V using electron beam direct manufacturing can produce material with aluminum content below the specification minimum [2]. As aluminum has a high vapor pressure with respect to titanium and vanadium, it preferentially vaporizes from the molten pool. This aluminum loss scales with the size of the molten pool and thus the chemical content can vary throughout the build. Compensating for this loss is necessary in order to achieve nominal chemistry in the deposited material. This paper examines established processing conditions for direct manufacturing of titanium, quantitatively determines deposited alloy chemistry changes under various conditions, and suggests a feedstock composition that will result in deposited material with nominal Ti-6Al-4V chemistry.