Due to their excellent thermo-mechanical properties at high temperatures such as high mechanical strength, good creep response and corrosion resistance, nickel-based superalloys currently dominate the field of turbine blades in aircraft engines. The main drawback of these superalloys, related to  environmental impact, is their high density, which is around 8 g/cm³. This fact has propelled the study and development of the $\gamma}$-TiAl intermetallics, with a density of 3.9-4.2 g/cm³, as an alternative. In this vein, General Electric (GE) implemented its GE 48-2-2 $\gamma}$-TiAl alloys, with a microstructure based primarily on $\gamma+\alpha_{2}$ ordered phases [1], in low-pressure turbine blades. Afterwards, a new beta-stabilized TiAl alloy with a microstructure composed of $\gamma+\alpha_{2}+\beta_{0}$ ordered phases, named TNM, was successfully developed and also used to manufacture low-pressure turbine blades [1,2]. The improved workability of the TNM alloy allows its use up to 750ºC; above that temperature, its employment is still impeded by insufficient oxidation resistance. The relationship between the thermo-mechanical properties and microstructure of those TiAl alloys has been extensively studied [3], but the optical properties in relation to its microstructure have hardly been investigated. In the current work, in addition to a detailed microstructural characterization, a thermo-optical analysis on these important TiAl alloys, General Electric´s (GE) TiAl and TNM has been carried out. This study consists on directional spectral emissivity measurements between 200ºC and their working temperature (750-850ºC) under vacuum. The results of directional spectral emissivity measurements have been integrated to calculate the total hemispherical emissivity, the key heat transfer parameter in the high-temperature high-vacuum environments such as additive manufacturing process. In addition, spectral emissivity measurements during isothermal oxidation in air at 850ºC and 750ºC of both alloys have been performed. Finally, the total hemispherical emissivity of the oxidized samples has been determined at different temperatures in air, including service temperatures.

References

[1]     B.P. Bewlay, S. Nag, A. Suzuki, M.J. Weimer, TiAl alloys in commercial aircraft engines, Mater. High. Temp. 33 (4-5) (2016) 549-559.
[2]    H. Clemens, S. Mayer, Adv. Eng. Mater. 15 (4) (2013) 191-215.
[3]     RR.  Boyer, JC . Williams. In: Zhou L, Chang H, Lu J, Xu D, editors. Titanium 2011
proc. Beijing: Science Press; 2012. pp. 10e9