Oxidation behavior of gamma alloys designed for high temperature applications

The oxidation behavior of gamma alloys was investigated in air and scale spallation was observed. The gamma alloys were superior in oxidation behavior to binary TiAl alloys in both isothermal and cyclic exposure. The mass gain of the alloy with high Nb content was smaller than low Nb alloys at 760 and 815 °C, the difference between them decreased at 870 °C. The improved oxidation resistance of the alloys is attributable to Nb addition resulting from reduced TiO2 growth through the so-called doping effect. Not only the alloys with low Nb but also the high Nb alloys suffered from scale spallation, especially after longer and/or higher temperature exposure. The density of the scale formed on the gamma alloys was found to be about one half of bulk oxide (Al2O3, TiO2) and was comparable to that formed on binary TiAl alloys. The coefficient of the thermal expansion (CTE) of the gamma alloys was larger than that of the oxides and the alloying addition reduced the CTE value of the alloys. Although the Nb addition suppressed the oxide growth and reduced the CTE difference between alloy and oxides, the prevention of the scale spallation could not be attained by single addition of Nb.