Karl E. Wiedemann

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.

Microhardness and lattice parameter calibrations of the oxygen solid solutions of unalloyed alpha-titanium and Ti-6Al-2Sn-4Zr-2Mo

Standards were prepared for calibrating microanalyses of dissolved oxygen in unalloyed α-Ti and Ti-6Al-2Sn-4Zr-2Mo. Foils of both these materials were homogenized for 120 hours in vacuum at 871 °C following short exposures to the ambient atmosphere at 854 °C that had partially oxidized the foils. The variation of Knoop microhardness with oxygen content was calibrated for both materials using 15 g and 5 g indentor loads. The unit-cell lattice parameters were calibrated for the unalloyed α-Ti. Example analyses demonstrate the usefulness of these calibrations and support an explanation of an anomaly in the lattice parameter variation. The results of the calibrations have been tabulated and summarized using predictive equations.

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.

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.

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