Scales, titanium aluminides

There has been relatively little work published on the reaction of titanium aluminides in atmospheres other than air or oxygen. Niu et al. [96] studied the reaction of Ti-25Al-llNb in a simulated combustion atmosphere (N2+1%02+ 0.5%SO2) with and without surface deposits of Na2S04-t- NaCl at temperatures between 600 and 800°C. Exposures in the absence of surface deposits resulted in reaction rates similar to those described above for simple oxidation. The rates in the presence of the deposits at 600 and 700 °C were initially rapid and then slowed markedly after 25 to 50 hours exposure. The rate at 800°C remained rapid with the kinetics being essentially linear. The major difference in the corrosion morphology at 800 °C was the presence of copious amounts of sulfides below the oxide scales. The authors postulate a mechanism of attack involving a combination of sulfidation-oxidation and scale-fluxing. 

The selective oxidation of an alloy component, e.g., A1 or Si, requires the alumina or silica to be more stable than the oxides of the other components in the alloy. Figure 2.5 indicates this condition would be met for compounds such as nickel aluminides and molybdenum silicides. However, in the case of Nb- or Ti-base compounds the oxides of the base metal are nearly as stable as those of A1 or Si. This can result in conditions for which selective oxidation is impossible. This situation exists for titanium aluminides containing less than 50 at% A1 as illustrated in Figure 5.27. In this case a two-phase scale of intermixed AI2O3 and I1O2 is generally observed. It should be emphasized that the determination of which oxide is more stable must take into account the prevailing metal activities. 

Figure 6-20. Schematic Ti-Al phase diagram (after Kattner et al. (1992) and Zhang et al. (1997)). There is no overlap between protective AI2O3 scale formation in air and room-temperature ductility (with alloying additions) in the Ti-Al system. The intermixed AI2O3 + Ti02 scale that forms on titanium aluminides can provide adequate protection from scaling up to ca 750-800 C.

Of the titanium aluminide intermetallic phases, only TiAl2, TiAl3, and certain t phase compositions can form a protective AI2O3 scale over a wide range of temperatures in air. They will be treated first, followed by the y, oui, and orthorhombic phases. 

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