Oxidation behavior of titanium aluminides

Results from an ongoing study of the high temperature oxidation of intermetallic compounds in the Ti-Al system are presented. The oxidation behavior of alloys based on TiAl (γ) has been found to be extremely complex depending on temperature and atmosphere composition. Protective alumina scales are formed in pure O2 up to a critical temperature above which a mixed TiO2/Al2O3 scale forms and grows at rates which are orders-of-magnitude faster than that of alumina. This phenomenon is believed to result from the formation of Al-containing internal oxides which coupled with the closeness in stability of alumina and the titanium oxides, prevents the alumina from becoming continuous. Continuous alumina scales were not observed, even at temperatures below 1000 °C, when N2 was present in the oxidizing gas. The effect of the N2 has been shown to be involved with the initial development of the reaction products

Analysis of the Reactive Element Effect on the Oxidation of Ceria Doped Nickel

The effects of external doping with CeO2 on the oxidation of nickel have been evaluated. The materials studied were pure Ni and Ni with the surface doped with CeO2 by pulsed laser deposition. The oxidation kinetics were measured using thermogravimetric analysis. The oxidation microstructures were observed by scanning electron microscopy and cross-sectional transmission electron microscopy. Compositional analysis was performed with energy dispersive X-ray analysis and sputtering neutrals mass spectrometry. Phase identification was performed using X-ray diffraction. Doping with CeO2 resulted in a significant decrease in the NiO growth rate at intermediate temperatures, e.g. 800 A degrees C. The scales on doped Ni grew primarily inward whereas those on the undoped Ni grew primarily outward. Deposition of the CeO2 dopant onto Ni with a thin, preformed NiO layer produced a similar reduction in the subsequent NiO growth rate. The CeO2 dopant did not reduce the growth rate at high temperature (1,300 A degrees C). The results indicate that the CeO2 dopant influences grain boundary transport in the NiO. Mechanisms are presented to attempt to describe the above observations