Emissivity measurements conducted on intermetallic γ-TiAl-based alloys for aeronautical applications

The directional spectral emissivity of a Ti-48Al-2Nb-2Cr alloy (in at.%), 4822 alloy, and a Ti-43.5Al-4Nb-1Mo-0.1B alloy (in at.%),TNM alloy, used in the aeronautical industry, are measured between 150 and 850 ºC. The differences in the emissivity values between both alloys at the lowest temperatures, indicates that the βo phase, only present in TNM, exhibit higher emissivity values. By numerical integration of the measured data, the total directional and hemispherical emissivity have been calculated. At 850 ºC the total hemispherical emissivity in vacuum are nearly identical with 0.274±0.006 for the 4822 alloy and 0.273±0.007 for the TNM alloy. The lower emissivity change with temperature measured in TNM alloys is related with the deconvolution of βo phase by diffusion processes. Afterwards, near-normal spectral emissivity measurements are performed in both alloys during isothermal oxidation treatments at 750 ºC and 850 ºC for 120 hours. The emissivity data reveal that the TNM alloy exhibits higher oxidation resistance especially at 750 ºC. In parallel, microstructural characterization has been performed before the measurements, after the directional emissivity measurements prior to oxidation and after isothermal oxidations. The formed oxide scale is composed of four layers that coincide with those reported in the literature: an outer layer of TiO2 contiguous with a layer of Al2O3, followed by a TiO2/Al2O3 mixed layer and finally a thin layer of Nb-rich nitride. This mixed layer governs the interferential part of the alloys' emissivity spectra, which, in combination with the background, determines the overall radiative behavior of the alloys under service conditions.