Sankara N. Sankaran

Oxidation characteristics of Ti-25Al-10Nb-3V-1Mo

Static oxidation kinetics of Ti-25Al-10Nb-3V-1Mo (atomic percent) were investigated in air over the temperature range of 650–1000°C using thermogravimetric analysis. The oxidation kinetics were complex at all exposure temperatures and displayed up to two distinct stages of parabolic oxidation. Breakaway oxidation occurred after long exposure times at high temperatures. Oxidation products were determined using x-ray diffraction techniques, electron microprobe analysis, and energy dispersive x-ray analysis. Oxide scale morphology was examined using scanning electron microscopy of the surfaces and cross-sections of oxidation specimens. The oxides during the parabolic stages were compact and multilayered, consisting primarily of TiO2 doped with Nb, a top layer of Al2O3 and a thin bottom layer of TiN. The transition between the first and second parabolic stage is linked to the formation of a TiAl layer at the oxide-metal interface. Porosity also formed in the TiO2 layer during the second stage, causing degradation of the oxide and breakaway oxidation.

Detection of Hydrogen Leakage in a Composite Sandwich Structure at Cryogenic Temperature

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 significantly 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.

Simultaneous measurement of temperature and strain using a single Bragg grating

A fiber optic sensor using the first and the second order fiber Bragg grating spectra to simultaneously measure temperature and strain is investigated. A theoretical model for estimating the wavelength and the reflectivity of the second order Bragg resonance in a fiber grating is developed, and the results are compared with the experimental results in Corning SMF 28 fiber. Moreover, technical issues regarding the sensor design such as the spectral characteristics of the Bragg gratings, bending loss in optical fibers and the higher order propagation modes are investigated. Experiments are conducted to measure the strain and temperature response of the grating sensors and compared to conventional sensors with encouraging results.

Platinum substitutes and two-phase-glass overlayers as a low cost alternatives to platinum aluminide coatings

The feasibility of processing less-expensive alternative coatings to platinum aluminide was examined. Three approaches were followed: (1) enhancement of nickel-aluminide coatings by application of sol-gel derived two-phase-glass (TPG) overlayers, (2) evaluation of TPG coatings on bare IN 738LC, and (3) substitution of Pt with a less expensive platinum group metal (palladium). Accordingly, IN 738LC coupons were tested with several coatings including TPG, aluminide coatings (platinum aluminide, palladium aluminide, and conventional nickel aluminide), and TPG overlayers on the aluminide coatings. Isothermal-oxidation, cyclic-oxidation, and hot-corrosion tests were conducted at 900°C for 500 h to evaluate the coatings. The results showed that the TPG by itself provided superior protection compared to the platinum-aluminide coatings under both oxidation and hot-corrosion conditions. The TPG coating also showed promise as an overcoat on aluminide coatings.

Barrier-layer formation and its control during hydrogen permeation through Ti-24Al-11Nb alloy

Hydrogen transport through Ti-24Al-11Nb (at. pct) alloy has been measured using ultrahigh vacuum permeation techniques over the temperature range of 500 °C to 900 °C and a hydrogen pressure range of 20 to 1333 Pa. The hydrogen uptake behavior of the alloy is influenced by even very small amounts of contaminants in the hydrogen gas. It is shown that contaminants such as CO and H2O are released from the walls of the vacuum system by the incoming hydrogen gas due to collision-induced desorption and that these species form a barrier layer. In the presence of the barrier layer, the hydrogen-permeation behavior of the alloy has two apparent temperature regimes: a high-temperature regime where the barrier layer has minimal effect, and a low-temperature regime where the barrier layer inhibits the hydrogen uptake. A physical model to explain this behavior is presented. It is further shown that the effect of the barrier layer can be minimized by maintaining dynamic flow conditions in the upstream chamber. Under these conditions, the transport process is diffusion-limited: the permeability has a weak temperature dependence, but the diffusivity has a strong temperature dependence.

Hydrogen transport behavior of Timetal-21S alloy

Ti-15Mo-2.7Nb-3Al-0.2Si (Timetal-21S), a metastable [beta]-titanium alloy, is a candidate material for titanium matrix composite structures in hydrogen-fueled hypersonic planes because of its excellent formability and adequate mechanical properties in the 500--800 C temperature range. The alloy is strengthened through the precipitation of fine [alpha] particles in the [beta] matrix. The mechanical properties and microstructures are controlled by a solutionizing/aging heat treatment. A major concern in using titanium alloys in hydrogen service is the embrittlement caused by the precipitation of hydrides. It is believed that the large solubility of hydrogen in the [beta]-phase would preclude the precipitation of hydrides in Beta titanium alloys, especially at low hydrogen pressures. However, depending on the hydrogen content, a shift in the ductile/brittle transition temperatures to levels much higher than room temperature has been reported for Timetal-21S alloy. The objective of the present investigation was to determine the hydrogen transport characteristics of the Timetal-21S alloy at moderate pressures up to 6,667 Pa (50 torr) and at temperatures from 400 C to 800 C. A microbalance technique, also referred to as thermogravimetric analysis (TGA), for sorption measurements under ultrahigh vacuum (UHV) conditions was the primary method in this work.

Platinum Substitutes and Two-Phase-Glass Overlayers as Low Cost Alternatives to Platinum Aluminide Coatings

The feasibility of processing less-expensive alternative coatings to platinum aluminide was examined. Three approaches were followed: 1) enhancement of nickel-aluminide coatings by application of sol-gel derived two-phase-glass (TPG) overlayers, 2) evaluation of TPG coatings on bare IN 738LC, and 3) substitution of Pt with a less expensive platinum group metal (palladium). Accordingly, IN 738LC coupons were tested with several coatings including TPG, aluminide coatings (platinum aluminide, palladium aluminide, and conventional nickel aluminide), and TPG overlayers on the aluminide coatings. Isothermal-oxidation, cyclic-oxidation, and hot-corrosion tests were conducted at 900°C for 500 hours to evaluate the coatings. The results showed that the TPG by itself provided superior protection compared to the platinum-aluminide coatings under both oxidation and hot-corrosion conditions. The TPG coating also showed promise as an overcoat on aluminide coatings.Copyright