Development of Oxidation Protection Coatings for Gamma Titanium Aluminide Alloys
Metallic material systems play a key role in meeting the stringent weight and durability requirements for reusable launch vehicle (RLV) airframe hot structures. Gamma titanium aluminides (gamma-TiAl) have been identified as high-payoff materials for high-temperature applications. The low density and good elevated temperature mechanical properties of gamma-TiAl alloys make them attractive candidates for durable lightweight hot structure and thermal protection systems at temperatures as high as 871 C. However, oxidation significantly degrades gamma-TiAl alloys under the high-temperature service conditions associated with the RLV operating environment. This paper discusses ongoing efforts at NASA Langley Research Center to develop durable ultrathin coatings for protecting gamma-TiAl alloys from high-temperature oxidation environments. In addition to offering oxidation protection, these multifunctional coatings are being engineered to provide thermal control features to help minimize heat input into the hot structures. This paper describes the coating development effort and discusses the effects of long-term high-temperature exposures on the microstructure of coated and uncoated gamma-TiAl alloys. The alloy of primary consideration was the Plansee alloy gamma-Met, but limited studies of the newer alloy gamma-Met-PX were also included. The oxidation behavior of the uncoated materials was evaluated over the temperature range of 704 C to 871 C. Sol-gel-based coatings were applied to the gamma-TiAl samples by dipping and spraying, and the performance evaluated at 871 C. Results showed that the coatings improve the oxidation resistance, but that further development is necessary.
Oxidation protection of gamma-titanium aluminide using glass-ceramic coatings (vol 203, pg 1797, 2009)
Abstract γ-TiAl intermetallic alloys are presently considered an efficient structural material for advanced turbine blades and aero-engine components due to their various advantages compared to the traditionally used superalloys. However, their poor oxidation resistance at temperatures > 750 °C severely limits their wider application. The present study dealt with the improvement of oxidation resistance of this alloy by applying impervious glass–ceramic coatings by vitreous enameling technique. Results showed that MgO–SiO 2 –TiO 2 glass–ceramic coating could offer excellent oxidation resistance to γ-TiAl at 800 °C even up to 100 h with negligible weight gain (~ 0.10 mg/cm 2 ) compared to that of the bare alloy (~ 1.3 mg/cm 2 ). The coatings those were belonging from BaO–MgO–SiO 2 , ZnO–Al 2 O 3 –SiO 2 and BaO–SiO 2 systems also extend appreciable improvement in the oxidation resistance of the alloy at 800 °C up to 100 h. At further higher temperature such as at 1000 °C, the ABK-13 and ABK-103 glass–ceramic coatings offered significant protection to the alloy up to 25 h of exposure in air with minimum weight gain (~ 0.34 mg/cm 2 ). However, after that the coated layers started to peel off from the alloy surface.
Processing, Properties and Applications of Gamma Titanium Aluminide Sheet and Foil Materials
Intermetallic {gamma}-TiAl based alloys ({gamma}-alloys) have a great potential to become important materials for advanced applications in aerospace, automotive and related industries. Research and development on {gamma}-alloys have progressed significantly within the last decade. This research has led to a better understanding of the fundamental correlations between alloy composition and microstructure, processing behavior and mechanical properties. This paper describes the progress in sheet rolling of {gamma}-TiAl based alloys on industrial scale. Employing an advanced hot-rolling process sheets with lengths >1,000 mm have been rolled. Furthermore, first results of foil rolling are presented. The mechanical properties of {gamma}-TiAl sheet material with regard to processing route, alloy composition and microstructure are summarized and discussed. Sheet forming by means of superplastic forming and conventional metal forming techniques has successfully been conducted. Different joining techniques have been studied for {gamma}-alloys including solid-state diffusion bonding. The oxidation resistance of {gamma}-alloys is higher than that of Ti-alloys, however, for long-term applications at temperatures >700 C the need for reliable oxidation protective coatings is anticipated. Recent results of cyclic oxidation tests on coated {gamma}-TiAl sheet are presented. Finally, the results of a stability test conducted on a {gamma}-TiAl panel at 750 C are summarized. 60 refs., 11more » figs., 1 tab.« less