Ronald K. Clark

Oxidation of commercial purity titanium

The oxidation kinetics of commercial purity Ti-A55 exposed to laboratory air in the 593–760°C temperature range were continuously monitored by thermogravimetric analysis. The oxide thickness was measured by microscopy and the substrate contamination was estimated from microhardness measurements. The microhardness depth profiles were converted to oxygen composition profiles using calibration data. The oxygen diffusion coefficient in alpha-Ti appears to be approximately concentration independent in the 1–10 at. % oxygen range. The combination of an “effective diffusion coefficient” and an “effective solubility” at the oxide-metal interface usefully describes the diffusion process over the entire composition range. A model for the total parabolic oxidation kinetics, accounting for the two individual components, oxide growth and solid solution formation, has been proposed. Diffusion coefficient for oxygen in TiO2 has been estimated as a function of temperature and is found to be about 50 times the value in alpha-Ti. The metallographically prepared cross-sections of the oxidized specimens revealed a “moving boundary” in the substrate, parallel to the oxide-metal interface. This boundary was associated with a specific oxygen level of 5.0±0.5 at.%. It occurred at a distance from the oxide-metal interface which was correlatable with temperature and time of exposure. The diffusion coefficient corresponding to the composition of this moving boundary is in excellent agreement with the effective diffusion coefficient for the substrate contamination.

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.

Effect of coatings on oxidation of Ti-6Al-2Sn-4Zr-2Mo foil

The viability of using coated Ti-6Al-2Sn-4Zr-2Mo foils at 620°C in air was established through mechanical and thermogravimetric testing. Weight-grained and oxygen embrittlement were significantly reduced by the coatings. The residual tensile elongation of coated specimens was 2.5 times that of uncoated specimens. Comparison depth-profiling with X-ray diffraction verified the reduction of oxygen solid-solution in the α-phase for a selection of coated specimens.

Oxidation of Ti−14Al−21Nb in air and oxygen from 700 to 1000°C

The oxidation behavior of Ti−14Al−21Nb in air and in oxygen was determined over the temperature range 700 to 1000°C. Weight gains in both atmospheres were measured using thermogravimetric analysis. The resulting oxidation products were identified using X-ray diffraction, and oxide morphology was evaluated using electron microscopy and wavelength-dispersive X-ray analysis. Total weight gains in oxygen were up to four times higher than in air, and a higher percentage of the weight gain in oxygen was due to oxygen dissolution into the metal. Based on metallurgical examination of the oxidized specimens, it was concluded that the lower oxidation weight gains in air are due to the formation of a thin layer of TiN and TiAl at the oxide-metal interface which inhibits the diffusion of oxygen into the metal.

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.

Titanium aluminides - Surface composition effects as a function of temperature

Results are presented from an AES investigation of the surface characteristics of pure Ti, Ti-14Al-21Nb (alpha-2 aluminide), and Ti-33Al-6Nb-1.4Ta (gamma aluminide) under ultrahigh vacuum conditions over the 20-1000 C temperature range. The observed variations in surface composition are noted to be dramatic and indicative of a direct relationship between the amount of retained oxygen and carbon on the surface and the quantity of Al in that surfaces vicinity. While TiO2 was the main surface oxide for pure Ti and the alpha-2 and gamma aluminides, only metallic peaks were observable at 600 C; at 1000 C, metallic Al and Ti were observed.

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.